basic electronics EE.....pdf

BASIC ELECTRONICS (EE-3G-17321)

Prepared by: - Sudershan. Dolli Page 1

Topic 1) Semiconductor Diode Marks 12

Semiconductor Theory:-

Review of semiconductor theory (No questions to be set in Theory Paper) Concepts of intrinsic semiconductor, extrinsic semiconductor, doping, and

dopants.

Trivalent & pentavalent impurities. P- Type and N- Type semiconductor.

Semiconductor Diode:-

PN Junction theory: - Barrier voltage, Depletion region, Junction capacitance, Forward and reverse biased junction

V- I characteristics of P-N junction diode. Circuit diagram for characteristics (Forward & Reverse) Specifications of diode:-

o Forward Voltage Drop, Reverse Saturation Current, Maximum Forward Current, Power Dissipation.

Ideal Diode Model. Zener diode:-

Construction, symbol, and circuit diagram for characteristics of zener diode (Forward & Reverse).

Specification of zener diode: - o Zener voltage (VZ), Maximum power dissipation (PDmax), Break over

current (IZK), zener resistance.

Special purpose diodes:-

Schottky diode, Point-contact diode, Varacter diode (Construction, symbol, Characteristics and applications of each).

Optical diodes:-

LED, IRLED, Photodiode and LASER diode (Symbol, operating principle and applications of each)



Electronics:- o Electronics deals with electrical circuits that involve active electrical components such

as vacuum tubes, transistors, diodes and integrated circuits, and associated passive

interconnection technologies.

Applications of electronics:- o In instrumentation:-

To design robots. In PLC systems. In control systems. As an instrumentation amplifier. As transducers. Signal generators. CRO. Frequency counters.

o In defense:- Navigation. RADAR. Guided missiles. Global positioning system (GPS). Bomb detector. Security systems.

o In medical science:- Electro cardiograph (ECG): It is used to find the condition of the heart of a patient. X-Ray Machine: It is used for taking pictures of internal bone structures. Ultra Sound Scanner: It is used to take pictures and examine the functions of Brain,

Kidney etc.

Blood pressure measurement. Electrical Enceplograph (EEG): It is used for neurological investigations. Cathode Ray Oscilloscope: It is used for studying muscle actions within the body. Magnetic Resonance Imaging: It is used for investigating tumors in brain or other parts of

body.

o In industries:- To control quality. To measure weight. For atomization techniques. To design robots. In biometrics. To measure moisture. To amplify weak signals.



o In communication and entertainment:- Wired communication such as: -

Telegraph,

Telephone,

FAX,

Internet facility (data communication). Wireless communication such as: -

Radio broadcasting,

TV broadcasting,

Satellite communication. In audio amplifiers, public address (PA) system, and Dolby systems.

Active Components:- o Active components can amplify (increase the power of a signal) or processing the electrical

signal.

o They rely on a source of power for its working.

o Passive components Diode, switch, relay, transistor, FET, etc.

Passive Components:- o Passive components can't amplify (increase the power of a signal) or processing the electrical

signal.

o They also can't rely on a source of power, except for what is available from the circuit they are

connected to.

o Passive components include two-terminal components such as resistors, capacitors, and

inductors.

Resistance:- o It is the property of the material to oppose the flow of electricity through it.

o The unit of resistance is ohm ().

Resistor:- o Resistor is a component which opposes to the flow of electrons (current) through it.

o The current through a resistor is indirect proportion to the voltage across the resistor's terminals.

This relationship is represented by Ohm's law:

o The unit of resistance is ohm ().



Semiconductor Theory Certain substances like germanium, silicon, carbon etc. are neither good conductors like copper nor

insulators like glass.

In other words, the resistivity of these materials lies in between conductors and insulators.

Such substances are classified as semiconductors.

A semiconductor is a substance which has resistivity in between conductors and insulators e.g. germanium,

silicon, selenium, carbon etc.

Properties of Semiconductors:-

The resistivity of a semiconductor is less than an insulator but more than a conductor.

Semiconductors have negative temperature co-efficient of resistance i.e. the resistance of a

semiconductor decreases with the increase in temperature and vice-versa. For example, germanium

is actually an insulator at low temperatures but it becomes a good conductor at high temperatures.

When a suitable metallic impurity (e.g. arsenic, gallium etc.) is added to a semiconductor, its current

conducting properties change appreciably.

Intrinsic semiconductor:-

A semiconductor in an extremely purest form is known as an intrinsic semiconductor.

Extrinsic semiconductor:-

When an impurity (trivalent or pentavalent) is added to the purest form of semiconductor (intrinsic

semiconductor), then it is called extrinsic semiconductor.

Doping:-

The process of adding impurity (trivalent or pentavalent) to a pure semiconductor (intrinsic

semiconductor) is called as doping.

Dopants:-

The impurities (trivalent or pentavalent) that are added to a pure semiconductor (intrinsic

semiconductor) are called as dopants.



N-type of semiconductor:-

When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as n-

type semiconductor.

The addition of pentavalent impurity provides a large number of free electrons in the semiconductor.

Typical examples of pentavalent impurities are phosphor, arsenic, bismuth, and antimony.

Such impurities which produce n-type semiconductor are known as donor impurities because they

donate or provide free electrons to the semiconductor crystal.

P-type of semiconductor:-

When a small amount of trivalent impurity is added to a pure semiconductor, it is called p-type

semiconductor.

The addition of trivalent impurity provides a large number of holes in the semiconductor.

Typical examples of trivalent impurities are gallium and indium.

Such impurities which produce p-type semiconductor are known as acceptor impurities because the

holes created can accept the electrons.



Majority and Minority Carriers:-

It has already been discussed that due to the effect of impurity, n-type material has a large number of

free electrons whereas p-type material has a large number of holes.

However, at room temperature, some of the co-valent bonds break, thus releasing an equal number

of free electrons and holes.

An n-type material has its share of electron-hole pairs (released due to breaking of bonds at room

temperature) but in addition has a much larger quantity of free electrons due to the effect of

impurity.

These impurity-caused free electrons are not associated with holes.

Consequently, an n-type material has a large number of free electrons and a small number of holes as

shown in Fig. (i).

The free electrons in this case are considered majority carriers since the majority portion of

current in n-type material is by the flow of free electrons and the holes are the minority carriers.

Similarly, in a p-type material, holes outnumber the free electrons as shown in Fig. (ii). Therefore,

holes are the majority carriers and free electrons are the minority carriers.

PN Junction:-

When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is

called PN junction.

Most semiconductor devices contain one or more PN junctions.

Formation of depletion region:-



At the instant of PN-junction formation, the free electrons near the junction in the n region begin to

diffuse across the junction into the p region where they combine with holes near the junction.

The result is that N-region loses free electrons as they diffuse into the junction.

This creates a layer of positive charges (pentavalent ions) near the junction.

As the electrons move across the junction, the P-region loses holes as the electrons and holes

combine.

The result is that there is a layer of negative charges (trivalent ions) near the junction.

These two layers of positive and negative charges form the depletion region (or depletion layer).

The term depletion is due to the fact that near the junction, the region is depleted (i.e. emptied) of

charge carries (free electrons and holes) due to diffusion across the junction.

Once PN junction is formed and depletion layer created, the diffusion of free electrons stops.

Junction theory:-

Barrier voltage (knee voltage or threshold voltage):-

o In other words, the depletion region acts as a barrier to the further movement of free

electrons across the junction.

o The positive and negative charges set up an electric field. This is shown by in Fig.

o The electric field is a barrier to the free electrons in the N-region.

o There exists a potential difference across the depletion layer and is called barrier potential

(VB).

o The barrier potential of a PN junction depends upon several factors including the type of

semiconductor material, the amount of doping and temperature.

o The typical barrier potential is approximately:-

For silicon, VKNEE = 0.7 V ;

For germanium, VKNEE = 0.3 V



Biasing the PN junction:- In electronics, the term bias refers to the use of D.C voltage to establish certain operating conditions

for an electronic device.

In relation to a PN junction, there are following two bias conditions :

o Forward biasing.

o Reverse biasing.

1. Forward biasing:-

When external D.C. voltage applied to the junction is in such a direction that it cancels the

potential barrier, thus permitting current flow, it is called forward biasing.

To apply forward bias, connect positive terminal of the battery to P-type and negative terminal to N-

type as shown in Fig above.

The applied forward potential establishes an electric field which acts against the field due to

potential barrier.

As potential barrier voltage is very small (0.3 V for germanium), therefore, a small forward voltage

is sufficient to completely eliminate the barrier.

Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost

zero and a low resistance path is provided for current.

Therefore, current flows in the circuit. This is called forward current.

2. Reverse biasing:-

When the external D.C voltage applied to the junction is in such a direction that potential barrier is

increased, it is called reverse biasing.

To apply reverse bias, connect negative terminal of the battery to P-type and positive terminal to N-

type as shown in Fig. above.

It is clear that applied reverse voltage establishes an electric field which acts in the same direction as

the field due to potential barrier.

The increased potential barrier prevents the flow of charge carriers across the junction.

Thus, a high resistance path is established for the entire circuit and hence the current does not flow.



Important terms:-

(iv) Breakdown voltage:-

Reverse current.

(v) Knee voltage:-



(vi) Reverse current:-

Zener diode:- The breakdown or zener voltage depends upon the amount of doping.

If the diode is heavily doped, depletion layer will be thin and consequently the breakdown of the

junction will occur at a lower reverse voltage. On the other hand, a lightly doped diode has a higher

breakdown voltage.

When an ordinary crystal diode is properly doped so that it has a sharp breakdown voltage, it is

called a zener diode.

A zener diode is always reverse connected i.e. it is always reverse biased.

When forward biased, its characteristics are just those of ordinary diode.

It indicates that the forward current is very small for voltages below knee voltage and large for

voltages above knee (i.e. cut in) voltage.

The reverse characteristics curve indicates that negligible reverse saturation current flows until it

reaches the breakdown voltage (i.e. Zener voltage Vz).

A zener diode has sharp breakdown voltage, called zener voltage VZ.

The breakdown has a very sharp knee, followed by an almost vertical increase in reverse current.

The voltage across the zener diode is approximately constant and equal to Zener voltage VZ over

most of the zener breakdown region (region between IZMIN and IZMAX).

It will come out of the breakdown region, when the applied reverse voltage is reduced below the

Zener breakdown voltage.



Applications of Zener diode:- 1. Zener diode is used as a voltage regulator. 2. Zener diode is used in protection circuits. 3. Zener diode is used as clipper. 4. Zener diode is used as a reference element. 5. Zener diode is used in pulse amplifier. 6. Zener diode is used for switching operation.

List application of PN junction diode:-

1. Used in rectifier circuits.

2. Used in clipper and clamper circuit.

3. Used in AM detection circuits.

4. Used in voltage multiplier circuits.

5.

6.



Zener diode specification:- 1. Zener voltage:-

The voltage at which the Zener diode breaks down is called as Zener voltage (VZ).

From the bottom of the knee point the Zener voltage remains constant.

2. Power dissipation:-

The power dissipation of a Zener diode is the product of breakdown voltage (VZ) and reverse current (IZ).

Mathematically, the power dissipation is given by:-

The maximum value of power dissipation, which a Zener can dissipate, without failure, is called power rating (PZM).

3. Dynamic resistance (Zener resistance):-

The Zener resistance or dynamic resistance is defined as the ratio of the change in the values of Zener voltage ( VZ) to the change in the values of Zener current (IZ).

4. Maximum Zener current:-

It is the maximum current that can be carried through the Zener diode without exceeding its power rating.

If the current through the Zener diode rises above the value of maximum Zener current, then the diode may get damaged.



Special purpose diodes:-

Schottky diode:-

o As shown in figure, the Schottky diode has been lower forward voltage drop (VF) and reverse breakdown voltage (VBR) than those of PN junction diode.

o The metal region of a Schottky diode is heavily occupied with the conduction band electrons and the N-type region is lightly doped.

o There are no minority carriers as in other types of diodes, but there are only majority carriers as electrons. It operates only with majority carriers.

o When it is forward biased, higher energy electrons in the N regions are injected into the metal region where that gives up their excess energy very rapidly.

o The Schottky diode has little junction capacitance. Because of this, it can be operated at higher frequencies.

o The reduced junction capacitance also results in a much faster switching time. Because of this reason, Schottky devices are being used in digital switching applications.

Applications of Schottky diode:-

1. In digital computers. 2. To fabricate Low power Schottky TTL ICs. 3. Low voltage bridge rectifiers. 4. Used for rectification of higher frequency signals. 5. Digital switching applications. 6. In clipping and clamping circuits. 7. In low voltage power supply circuits (SMPS). 8. In mixing and detection circuits used in communication systems.



Point contact diode:-



Varactor diode:-

A varactor diode is basically a reverse biased PN junction, which utilizes capacitance of the depletion

layer.

It is also known as varicap, or voltcap.

When the reverse bias voltage increases, the depletion layer widens. This increases the dielectric

thickness, which in turn, reduces the capacitances.

When reverse bias voltage decreases, the depletion layer narrows down. This decreases the dielectric

thickness, which in turn, increases the capacitances.

Figure below shows the variation in the capacitance with respect to reverse voltage.

In varactor diode, the capacitance parameter is controlled by doping in depletion region, size and

geometry of the diode.

The range of capacitance variation in an abrupt junction diode is 4:1, and in hyper abrupt varactor diode

is 10:1.

Applications of varactor diode:-

1. As tuner in radio receiver.

2. As electronics tuner in TV receiver.

3. In tank circuits.

4. In commercial receivers.

5. In automatic frequency control device.

6. In adjustable band-pass filter.



Optical LEDs:-

Light Emitting Diode (LED):-

OR

Emission of light



Advantages of LED:-

1. Requires low voltage. 2. Longer life (more than 20 years). 3. Fast On-Off switching. 4. Less power consumption. 5. Uniform brightness in all direction.

Applications of LED:-

1. In7-segment display. 2. Alphanumeric displays. 3. For video displays. 4. As indicating power ON/OFF conditions. 5. In optical switching application. 6. In burglar alarm system. 7. For image sensing circuits. 8. In optocoupler. 9. In infrared remote control. 10. In traffic signal management.

Infrared light emitting diode (IRLED):-



This type of LED emits light in the infrared region of electromagnetic spectrum.

The IR-LED is a PN junction diode.

It is of Gallium Arsenide (GaAs) and is operated in the forward biased condition.

When the forward voltage is applied, the electrons from the N-side will recombine with the holes on P-side.

During every recombination some of energy is released which is radiated in the form of Infra-Red light.

Applications of IRLED:-

1. In remote control handsets. 2. In optocoupler. 3. As a light source in optical fibre communication system. 4. In burglar alarm system. 5. In optical switching application. 6. For data transmission (mobile phones, PCs).

Photo Diodes:- Figure shows the structure, symbol and VI characteristics of photo diode.

It has a small transparent window, which allows the light to strike the PN junction.



Applications of Photodiode:- 1. Photo detection. 2. Demodulation. 3. Logic circuits. 4. Switching. 5. Optical communication system. 6. Encoders. 7. Alarm circuits. 8. Object counter circuits.

Laser (Light Amplification by Stimulated Emission of Radiation):- Figure below shows the structure of an edge emitter laser diode.

Like LEDs, laser diodes are typical PN junction devices used under a forward-bias.

The word LASER in an acronym for Light Amplification by Stimulated Emission of Radiation.

The laser diode emits light in a direction parallel to the PN junction plane.

A P-N junction is formed by two layers of doped gallium arsenide (GaAs).

When the P-N junction is forward-biased by an external voltage source, electrons move across the junction and usual recombination occurs in the depletion region which results in the production of

photons.

As forward current is increased, more photons are produced which drift at random in the depletion region. Some of these photons strike the reflective surface perpendicularly.

These reflected photons move back and forth between the two reflective surfaces.

The photon activity becomes so intense that at some point, a strong beam of laser light comes out of the partially reflective surface of the diode.

Applications of Laser:- 1. Laser diodes are used in medical equipment. 2. Laser printers. 3. Optical disk equipments. 4. Scanners. 5. Laser diodes are also used to measure distance (range). 6. Laser diodes are used in optical communication system.



Compare LED and LASER:-

Sr.

No

Parameters LED LASER

1 Full form LED means Light Emitting Diode LASER means Light Amplification

by Stimulated Emission of Radiation.

2 Principle operation Spontaneous emission Stimulated emission

3 Output beam Non-coherent Coherent

4 Transmission distance Smaller Longer

5 Temperature sensitivity Less sensitive More sensitive

6 Lifetime Shorter lifetime as compared to

LASER

Longer lifetime as compared to LED

7 Cost Low cost High cost

8 Wavelength available Wide range of wavelength is available Small range of wavelength is

available

9 Size LED are smaller in size LASER are larger in size

10 Response Response is slower Response is faster



7. Symbol Symbol

8. VI characteristics VI characteristics



Two marks question bank:- 1. Define:-

a. Intrinsic semiconductor and extrinsic semiconductor. b. Doping. c. Barrier voltage. d. Depletion region. e. Static resistance of a PN junction diode and dynamic resistance of a PN junction diode. f. Forward voltage drop. g. Power dissipation. h. Knee voltage i. Forward voltage. j. Reverse saturation current. k. Maximum forward current. l. Zener voltage m. Breakdown voltage n. Peak inverse voltage (PIV)

2. State the values for knee voltage for silicon diode and germanium diode. 3. Draw symbols of:-

a. PN junction diode. D. Zener diode. b. Schottky diode. E. LED c. Varactor diode.

4. What is avalanche breakdown? 5. What is zener breakdown? 6. List applications of PN junction diode. 7. List applications of Zener diode. 8. List applications of Schottky diode. 9. List applications of point contact diode. 10. List applications of varactor diode. 11. List applications of LED. 12. List applications of IRLED. 13. List applications of photo diode. 14. List applications of LASER diode. 15. 16. 17. 18. 19. 20.



Four marks question bank 1. Compare PN junction diode and Zener diode. 2. Compare PN junction diode and LED. 3. Compare ideal diode and practical diode. 4. Compare avalanche breakdown and zener breakdown. 5. Compare LED and LASER 6. Compare PN junction diode and Varactor diode 7. Explain formation of depletion region in PN junction diode. 8. Explain the concept of hole, majority and minority charge carriers in PN junction. 9. With neat circuit diagram of forward biased PN junction diode, explain its working. Also draw the VI

characteristics (forward characteristics).

10. With neat circuit diagram of reverse biased PN junction diode, explain its working. Also draw the VI characteristics (reverse characteristics).

11. Explain working of Zener diode in reverse biased condition. Also draw its VI characteristics. 12. Describe the operating principle of Schottky diode with neat sketch. 13. Describe the operating principle of point contact diode with neat sketch. 14. Describe the operating principle of varactor diode with neat sketch. 15. Draw and describe the working principle of LED. 16. Draw and describe the working principle of IRLED. 17. Draw and describe the working principle of photo diode. 18. Describe the operating principle of LASER with neat sketch. 19. Describe the operating principle of Tunnel diode with neat sketch.



Topic 2) Rectifiers, and Filters Marks 08

Rectifier: -

Definition ,

Need for rectification

Types of Rectifiers:-

o Half wave Rectifier,

o Full Wave Rectifier (Centre Tapped and Bridge Circuit diagram , Operation and input- output

waveforms ( No derivations).

Definition of Ripple Factor, Efficiency, PIV Comparison of Rectifiers

Comparisons of rectifier.

Filters:-

Definition ,

Need for Filters

Types of Filters:-

o L: - Circuit Diagram, Principle of working, Input- Output Waveform.

o C: - Circuit Diagram, Principle of working, Input- Output Waveform.

o LC: - Circuit Diagram, Principle of working, Input- Output Waveform.

o CLC: - Circuit Diagram, Principle of working, Input- Output Waveform.

Comparison of Filters.

Advantages and Disadvantages of all filter circuits.



Need of Rectifiers:- The electrical power is generated, transmitted and distributed in the form of AC because of economical

consideration.

But there are many applications where DC supply is needed.

So almost all electronic equipments include a circuit that converts AC into DC, and that circuit is called as Rectifier.

Rectifier is a device which converts AC voltage into pulsating DC voltage.

Types of Rectifier:- 1. Half wave rectifier. 2. Full wave Rectifiers.

a. Center tap full wave Rectifier. b. Bridge type full wave Rectifier.

Half wave Rectifier (HWR): - Figure below shows a half wave rectifier circuit, and its waveforms.

It consists of a single diode in series with a load resistor.

The input to the half wave rectifier circuit is supplied from the 50 HZ AC supply.

The use of transformer provides two advantages:- o It allows us to step-up or step-down the AC voltage. o The transformer isolates the rectifier circuit from the power line and reduces the risk of getting

the electric shock.

Working / operation:-

During positive half cycle of the AC input, the diode is forward biased and conducts.

Therefore, the diode acts as a short circuit, so the current flows through a diode and produces a voltage across the load resistor (RL).

The voltage produced across the load resistor (RL) has the same shape as that of the positive half cycle of an AC input voltage.

During the negative half cycle, the diode is reverse biased and hence it does not conducts.

Thus there is no current flow or voltage drop across load resistor (RL).

Thus we get a pulsating DC at the output across a load resistor (RL).



Advantages of HWR:-

1. Simple circuitry. 2. Low cost. 3. Only 1 diode is used.

Disadvantage of HWR:- 1. As the output of a half wave rectifier is pulsating DC, therefore filtering is required to produce pure DC. 2. The AC supply delivers power only half the time. Therefore output is low. 3. Ripple factor is more. 4. Rectification efficiency is less.

Applications of HWR:- 1. Low cost power supplies. 2. Battery chargers. 3. As a clipper circuit.

Full wave rectifier (FWR):- A full wave rectifier is a circuit, which allows a unidirectional current to flow through a load resistor

(RL) during the entire AC input cycle. Therefore, a full wave rectifier utilizes both the half cycles (Full

cycle) of input AC voltage to produce DC output.

Center tapped full wave Rectifier:- Figure below shows a center tapped full wave rectifier circuit, and its waveforms.

The circuit uses two diodes, which are connected to center tapped secondary winding of the transformer.

The input is applied to the primary winding of a transformer.

The center tap on the secondary winding of a transformer is considered as the ground or zero voltage reference point.




During positive half cycle of the AC input, the voltage at the end A of the secondary winding becomes positive and end B becomes negative.

This forward biases diode D1 and reverse biases diode D2.

Thus diode D1 conducts (ON) and while diode D2 remains non-conducting (OFF).

The current flows through the diode D1, load resistor RL and the upper half of the secondary winding as shown by the arrow marks.

During negative half cycle of the AC input, the voltage at the end A of the secondary winding becomes negative and end B becomes positive.

This forward biases diode D2 and reverse biases diode D1.

Thus diode D2 conducts (ON) and while diode D1 remains non-conducting (OFF).

The current flows through the diode D2, load resistor RL and the lower half of the secondary winding as shown by the arrow marks.

It may be seen that current in the load resistor (RL) is in the same direction for both half cycles (positive and negative) of the AC voltage.

Therefore, the output voltage across a load resistor (RL) is a full wave rectified DC voltage.

Advantages of center tapped FWR:- 1. The DC output voltage and load current values are twice than those that of HWR. 2. The ripple factor is much less than that of HWR. 3. The efficiency is twice that of the HWR. 4. Ripple factor is less as compared to HWR.

Disadvantage of center tapped FWR:- 1. The output voltage is half of the secondary voltage. 2. The peak inverse voltage (PIV) of a diode is twice that of the diode used in HWR. 3. It is expensive to manufacture center tapped transformer, which produce equal voltages on each half of

the secondary windings.

4. It is difficult to locate the center tap on the secondary winding.

Applications of centre tapped rectifier:- 1. DC power supplies. 2. Battery chargers. 3. As AC to DC converters. 4. In voltage regulators.



Full wave bridge type Rectifier:- Figure below shows a full wave bridge type rectifier circuit, and its waveforms.

It uses 4 diodes (D1, D2, D3, D4) connected across the mains supply via transformer, to form a full

wave bridge type rectifier.


During the positive half cycle of the secondary voltage, the end P becomes positive and the end Q becomes negative.

This makes diodes D1 and D3 forward biased while diodes D2 and D4 reverse biased.

Therefore only diodes D1 and D3 conduct (ON), and the current flows as shown by dotted lines in the figure.

During the negative half cycle of the secondary voltage, the end P becomes negative and the end Q becomes positive.

This makes diodes D2 and D4 forward biased while diodes D1 and D3 reverse biased.

Therefore only diodes D2 and D4 conduct (ON), and the current flows as shown by solid lines in the figure.

It may be seen that current in the load resistor (RL) is in the same direction for both half cycles (positive and negative) of the AC voltage.

Therefore, the output voltage across a load resistor (RL) is a full wave rectified DC voltage.

Advantages of full wave bridge type rectifier:- 1. No output terminal is grounded. 2. The transformer used in the bridge type full wave rectifier is less costly. 3. No center tap is required on the transformer. 4. The output is twice as compared to center tapped full wave rectifier. 5. The PIV is one half as compared to center tapped full wave rectifier. 6. Ripple factor is less as compared to HWR. 7. Rectification efficiency is more.

Disadvantage of full wave bridge type rectifier:- 1. 4 diodes are used. 2. Voltage drop in the internal resistance of the rectifying unit will be twice as compared to center tapped

full wave rectifier.



Applications of Bridge type FWR:- 1. In DC power supplies.

2. Battery chargers.

3. As AC to DC converters.

4. In voltage regulators.

5. In electronics devices requiring DC signal from AC.

Compare HWR, centre tapped FWR and bridge type FWR:-



Definitions:- 1. Ripple:-

The ripples are the AC components present in the rectifiers output.

Ripples are also called as pulsating components.

2. Ripple factor:-

Mathematically ripple factor is defined as the ratio of RMS value of AC components in the output to the DC components present in the output.

Smaller the ripple closer is the output to a pure DC.

3. Ripple frequency:-

The frequency of AC components in the rectifiers output is called as ripple frequency.

The ripple frequency for HWR is 50HZ and for FWR is 100HZ, if input frequency is 50 HZ.

4. Peak inverse voltage (PIV) of diode:-

PIV is the maximum reverse voltage that can be applied across the reverse biased PN junction diode, without being damaged.

PIV for HWR and bridge type FWR is VM and for center tapped FWR it is 2VM.

5. Transformer utilization factor (TUF):-

TUF is defined as the ratio of DC power delivered to the load to the AC power rating of the transformer secondary.

6. Efficiency of rectifier:-

The rectifier efficiency is defined as the ratio of output d.c. power to the input a.c. power.



Need of filters: - The output of a rectifier circuit is Pulsating DC, i.e. the output of rectifier contains D.C. component as

well as an A.C. component (ripple).

The presence of an A.C. component is most undesirable and therefore must be removed from the rectifier output.

This can be done by a circuit called filter.

Filter:- Filter is a circuit which removes the AC components of the rectifier output but allows the DC to reach

the load.

A filter circuit consists of passive circuit elements such as inductor, capacitor, resistor and their combinations.

Types of Filters:- 1. Capacitor filter. 2. Inductor filter (choke input filter). 3. Inductor-capacitor filter (LC filter). 4. (CLC) filter.



Advantages of Capacitor filter:- 1. Easy to design. 2. Reduction in ripple content of the output voltage. 3. Increase in the average load voltage. 4. Small in size. 5. Low cost.

Disadvantages of Capacitor filter:- 1. Ripple factor is dependent on the load resistor. 2. Regulation is relatively poor. 3. Diodes have to handle high peak current.

(2) Inductor filter (choke input filter):-

It is also called as choke filter.

It consists of an inductor (L), which is suitably inserted between the rectifier and the load resistance (RL), as shown in figure.

The pulsating output of a rectifier is applied at the input of the Inductor filter.

When the signal passes through the inductor, it offers high reactance to the AC components, and no reactance to the DC components.

Therefore an AC component of the rectified output is blocked and only the DC component reaches to the load.

Theoretically the output of the inductor filter should consist of only DC voltage, but in practice it contains small AC components as well.

The inductor filter is more effective only for heavy load currents i.e. when the load resistance (RL) is small.

Advantages of Inductor filter (choke input filter):- 1. Low ripple factor for heavy load current. 2. No surge current through the diodes. 3. Reduced ripple content in the output.



Disadvantages of Inductor filter (choke input filter):- 1. It is bulky. 2. It is more costly. 3. Ripple factor is poor at light loads (small load current). 4. Audible noise (Hum) is produced.

(3) Inductor-capacitor filter (LC filter):-

Advantages of Inductor-capacitor filter (LC filter):- 1. Very good load regulation. 2. Ripple factor is low and does not depend on the load. 3. This filter is suitable for light as well as heavy loads. 4. Diodes do not have to carry surge currents.

Disadvantages of Inductor-capacitor filter (LC filter):- 1. Audible noise (Hum) is produced. 2. Due to the use of inductor and capacitor the circuit becomes bulky. 3. Due to the use of inductor and capacitor the circuit becomes more costly.



(IV) (CLC) filter:- The combination of two shunt capacitors and a series inductor connected in between these shunt

capacitors, a CLC filter or a capacitor input filter.

It is also called a - type filter.

It is used wherever a low O/P current and a high d.c O/P voltage are required.

It consist of a filter capacitor C1 connected across the rectifier output, a choke L in series and another filter capacitor C2 connected across the load resistor RL.

Advantages of (CLC) filter:- 1. Ripple factor is very low. 2. High DC voltage. 3. Increase in the average load voltage. 4. Reduced ripple content in the output.

Disadvantages of (CLC) filter:- 1. Due to the use of inductor and capacitor the circuit becomes bulky. 2. Due to the use of inductor and capacitor the circuit becomes more costly. 3. Current rating of inductor needs to be high. 4. Power loss is more. 5. High peak diode current.



Question: - Explain the working of HWR with capacitive filter. Draw necessary diagrams. Answer:-

Working:-

Connect the capacitor C in shunt with the load RL (output of rectifier).

The capacitance offers a lower reactance (opposition) path to the AC signal. To DC, this acts like an

open circuit. Thus all the DC current passes through the load and appears at the output.

When the rectifier output is increasing, the capacitor charges to the peak voltage, i.e. point A in the

waveforms.

As the rectifier output falls, the diode is now reverse biased and the capacitor now discharges through

the load resistance from A to B.

When the source voltage becomes more than the capacitor voltage, the capacitor quickly charges to the

peak voltage from B to C.

Thus the voltage across load resistance remains nearly constant.

A small part of the AC component passes through the load (output) reducing a small ripple voltage.

Question: - Explain the working of center taped FWR with LC filter. Also draw necessary diagrams.

Answer:-



Working:-

A centre- tap full wave rectifier produces the rectified voltage (full wave rectification), and it drives the

LC filter.

We know that the series inductor filter is preferred for low values of load resistance (i.e. heavy loads)

and the shunt capacitor filter is preferred for high values of load resistance (i.e. light loads)

Therefore, the LC filter can give low ripple factor irrespective of the load current as it is combination of

these two filters.

The series connected inductor (L) offers a high reactance to the AC components (ripple) in the output

and attenuate (remove) them.

The parallel capacitor (C) now provides a low reactance by- pass for the ripple and these ripples are

grounded.

Thus, this reduces the ripple further because they are not allowed to pass through the load RL.

The output voltage waveforms are similar to that as shown in figure above.

Question: - Draw the circuit diagram of bridge rectifier with filter. Describe its working and draw its

input/output waveforms.

Answer:-



Working:-

A full wave bridge rectifier produces the rectified voltage (full wave rectification), and it drives the pie

() filter or CLC filter.

As C1 comes first, looking from the rectifier side, the filter behaves in a very similar manner as the

capacitor filter.

The rectifier converts the AC input into pulsating DC waveform and since the bridge rectifier is used the

efficiency is more.

Due to use of three filtering components (C1, L, and C2), the ripple factor of the filter is very low as

compared to the other filters.

The capacitors C1 and C2 provide a low reactance path for the ripple whereas the series inductor L

provides a high reactance to the AC ripple.

The combined effect of this is the reduction in ripple and improvement in the output waveform which is

shown below.



Numericals:-



Compare C, L, LC, and CLC () filters:- Sr.

No

Parameters C filter L filter LC filter CLC () filter

1 Position of

filter

Across the load In series with the

load

Across the load Across the load

2 Suitable for Light (loads with

larger value of

resistances) load

applications

Heavy load (loads

with smaller value of

resistances)

applications

Light as well as

heavy load

applications

Low output

current and high

DC output

voltage.

3 Expression

for Ripple

factor

4 Ripple factor

depends on

Values of capacitance

and load resistor

Values of inductance

and load resistor

Valves of

inductance and

capacitance, but

independent on the

value of load

resistance.

Valves of

inductance,

capacitance, and

load resistance.

5 Size of filter Small Bulky Bulky Bulky

6 Cost Low cost Moderate cost High cost Highest cost

7 Voltage

regulation

Poor Moderate Good Very good

8 Applications Cell chargers, small

eliminators, etc

High current DC

supplies

DC power supplies DC power

supplies

9 Circuit

diagram

10 Waveforms



Two marks question bank 1. State need of rectifier. 2. Classify the rectifiers.

3. Define:- a. Rectifier. b. Ripple c. Ripple frequency d. Ripple factor e. PIV f. TUF g. Rectification efficiency.

4. Write the values of PIV, TUF, ripple factor, and rectification efficiency for HWR, center tapped FWR and bridge type FWR.

5. Define filter 6. State need of filter circuit. 7. List the application of the following:-

a. HWR b. Center tapped FWR c. Bridge type FWR

8. List the advantages of following:- a. HWR b. Center tapped FWR c. Bridge type FWR

9. List the disadvantages of following:- a. HWR b. Center tapped FWR c. Bridge type FWR

10. List the advantages of following:- a. C filter b. L filter c. LC filter d. CLC filter

11. List the disadvantages of following:- a. C filter b. L filter c. LC filter d. CLC filter



Four marks question bank

1. Draw the circuit diagram of half wave rectifier and explain its working along with the waveform.

2. Draw the circuit diagram of center tapped rectifier and explain its working along with the waveform.

3. Draw the circuit diagram of bridge type rectifier and explain its working along with the waveform.

4. Draw the circuit diagram of HWR along with capacitive filter and explain its working. Also draw its waveforms.

5. Draw the circuit diagram of center tapped FWR along with inductive filter. Draw the suitable waveform. Write the equation of ripple factor of inductive filter.

6. Draw the circuit diagram of bridge type FWR along with LC filter. Draw the suitable waveform. Write the equation of ripple factor of inductive filter.



Regulated DC power supply:-

Working:-

Transformer:-

o 230 AC mains supply is given to step down transformer it will reduce the AC voltage as

per requirement and that step down AC voltage is given to rectifier.

Rectifier:-

o The rectifier will convert AC voltage to pulsating DC signal. This impure DC signal

contains AC components as well as DC component, so this signal is applied to filter.

Filter:-

o The pulsating dc (or rectified ac) voltage contains large ripple. This voltage is applied to

the Filter circuit and it removes the ripple. The function of a filter is to remove the ripples

to provide pure DC voltage at its output.

Regulator:-

o The unregulated DC voltage is applied to a voltage regulator makes this DC Voltage

steady and independent of variation in load and mains AC voltage



Topic 3) Bipolar Junction Transistor Marks 24

Bipolar junction transistor:-

1. Introduction to Unipolar and Bipolar junction Transistors. 2. Bipolar junction Transistors Definition, Types (PNP, NPN) Symbol, Working

Principle of NPN transistor.

3. Types of Transistor Configuration CE, CB, and CC (Only circuit Diagrams). 4. Characteristics of CE configuration Input /Output Characteristics: - Identification of

Cut off, Active and Saturation Region.

5. Transistors characteristics parameters: - Input resistance, Output resistance. 6. Current gain ( and ), Relation between and . 7. Transistor Biasing- Need for biasing, DC load line, Q- point, Types of biasing

Voltage divider bias

Field effect transistor:-

1. Field Effect Transistor- Types ( JFET and MOSFET) 2. JFET- N Channel and P channel Symbol, Construction, and working principle. 3. Characteristics of JFET Drain and Transfer Characteristics 4. FET parameters DC Drain Resistance, AC drain Resistance, Transconductance,

Amplication Factor, and Input Resistance.

5. Comparison of JFET and BJT 6. MOSFET: - Types, Symbol, working principle. 7. Applications of BJT, FET and MOSFET.



Basic concept:- The transistoran entirely new type of electronic device is capable of achieving amplification of weak

signals in a fashion comparable and often superior to that realized by vacuum tubes. Transistors are far

smaller than vacuum tubes, have no filament and hence need no heating power and may be operated in

any position.

They are mechanically strong, have practically unlimited life and can do some jobs better than vacuum tubes.

A transistor consists of two PN junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types. Accordingly ; there are two types of transistors, namely;

o n-p-n transistor o p-n-p transistor

An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type as shown in Fig. (i). However, a p-n-p transistor is formed by two p-sections separated by a thin section of

n-type as shown in Fig. (ii).

In each type of transistor, the following points may be noted :

o These are two PN junctions. Therefore, a transistor may be regarded as a combination of two diodes connected back to back.

o There are three terminals, one taken from each type of semiconductor. o The middle section is a very thin layer. This is the most important factor in the function of a

transistor.

Why named as Transistor? A transistor has two PN junctions.

One junction is forward biased and the other is reverse biased.

The forward biased junction has a low resistance path whereas a reverse biased junction has a high resistance path.

The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore, a transistor transfers a signal from a low resistance to high resistance.

The prefix trans means the signal transfer property of the device while istor classifies it as a solid element in the same general family with resistors.



Naming the transistor terminals:- A transistor (PNP or NPN) has three sections of doped semiconductors.

The section on one side is the emitter and the section on the opposite side is the collector. The middle section is called the base and forms two junctions between the emitter and collector.

o Emitter:- The section on one side that supplies charge carriers (electrons or holes) is called the

emitter. The emitter is always forward biased w.r.t. base so that it can supply a large

number of majority carriers.

In Fig. (i), the emitter (p-type) of PNP transistor is forward biased and supplies hole charges to its junction with the base. Similarly, in Fig. (ii), the emitter (n-type) of NPN

transistor has a forward bias and supplies free electrons to its junction with the base.

o Collector:- The section on the other side that collects the charges is called the collector. The collector

is always reverse biased. Its function is to remove charges from its junction with the base.

In Fig. (i), the collector (p-type) of PNP transistor has a reverse bias and receives hole charges that flow in the output circuit. Similarly, in Fig. (ii), the collector (n-type) of

NPN transistor has reverse bias and receives electrons.

o Base:-

The middle section which forms two PN-junctions between the emitter and collector is called the base.

The base-emitter junction is forward biased, allowing low resistance for the emitter circuit.

The base-collector junction is reverse biased and provides high resistance in the collector circuit.



Facts about transistor:-

The transistor has three regions, namely; emitter, base and collector.

1. The base is much thinner than the emitter while collector is wider than both as shown in Fig. 2. The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or

holes) into the base. The base is lightly doped and very thin; it passes most of the emitter injected

charge carriers to the collector. The collector is moderately doped.

3. The transistor has two PN junctions i.e. it is like two diodes. The junction between emitter and base may be called emitter-base diode or simply the emitter diode. The junction between the base

and collector may be called collector-base diode or simply collector diode.

4. The emitter diode is always forward biased whereas collector diode is always reverse biased. 5. The resistance of emitter diode (forward biased) is very small as compared to collector diode

(reverse biased). Therefore, forward bias applied to the emitter diode is generally very small

whereas reverse bias on the collector diode is much higher.

Transistor symbols:-

However, for the sake of convenience, the transistors are represented by schematic diagrams. The

symbols used for NPN and PNP transistors are shown in Fig.

Note that emitter is shown by an arrow which indicates the direction of conventional current flow with forward bias. For NPN connection, it is clear that conventional current flows out of the emitter as

indicated by the outgoing arrow in Fig. (i). similarly, for PNP connection, the conventional current flows

into the emitter as indicated by inward arrow in Fig. (ii).



Operating principle of PNP transistor:-

Fig. shows the basic connection of a PNP transistor.

The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current IE.

As these holes cross into n-type base, they tend to combine with the electrons.

As the base is lightly doped and very thin, therefore, only a few holes (less than 5%) combine with the electrons. The remainder (more than 95%) cross into the collector region to constitute collector

current IC.

In this way, almost the entire emitter current flows in the collector circuit.

It may be noted that current conduction within PNP transistor is by holes. However, in the external connecting wires, the current is still by electrons.

Operating principle of NPN transistor:-

Fig. shows the NPN transistor with forward bias to emitter base junction and reverse bias to collector-base junction.

The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current IE.

As these electrons flow through the p-type base, they tend to combine with holes.



As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with holes to constitute base current IB. The remainder (more than 95%) cross over into the collector region to

constitute collector current IC.

In this way, almost the entire emitter current flows in the collector circuit. It is clear that emitter current is the sum of collector and base currents i.e.

IE = IB + IC

Transistor Biasing (operating regions):- Applying DC voltages across the transistors terminals is called biasing.

I. Active region:- o In this mode, emitter base junction of a transistor is forward biased and the collector base

junction is reversed biased as shown in figure.

o The region between cut off and saturation is known as active region. In the active region, collector-base junction remains reverse biased while base-emitter junction remains forward

biased. Consequently, the transistor will function normally in this region.

II. Saturation region:- o In this mode, both emitter base junction and collector base junction is forward biased. o Here the transistor has very large value of current. o The transistor operated in this region is used as a closed switch (ON).

III. Cut-off region:- o In this mode, both emitter base and collector base junction is reverse biased. o Here the transistor practically has zero current. o The transistor is operated in this mode, when it is used as open switch (OFF).

Sr.No Operating mode Emitter base junction Collector base junction Application

1. Active Forward Reverse Amplifier

2. Saturation Forward Forward Closed switch (ON)

3. Cut-off Reverse Reverse Open switch (OFF)

Transistors configurations:- There are three leads in a transistor viz., emitter, base and collector terminals.

However, when a transistor is to be connected in a circuit, we require four terminals; two for the input and two for the output.

This difficulty is overcome by making one terminal of the transistor common to both input and output terminals.

The input is fed between this common terminal and one of the other two terminals.

The output is obtained between the common terminal and the remaining terminal.



Accordingly; a transistor can be connected in a circuit in the following three ways : 1. Common base configuration. 2. Common emitter configuration. 3. Common collector configuration.

I. Common Base configuration:-

In this circuit arrangement, input is applied between emitter and base and output is taken from collector and base.

Here, base of the transistor is common to both input and output circuits and hence the name common base connection.

The ratio of change in collector current to the change in emitter current at constant collector-base voltage VCB is known as current amplification factor i.e.

II. Common Emitter configuration:-

In this circuit arrangement, input is applied between base and emitter and output is taken from the collector and emitter.

Here, emitter of the transistor is common to both input and output circuits and hence the name common emitter connection.

The ratio of change in collector current (IC) to the change in base current (IB) is known as base current amplification factor i.e.



Input characteristic:-

o It is the curve between base current IB and base-emitter voltage VBE at constant collector-

emitter voltage VCE.

o The input characteristics of a CE connection can be determined by the circuit shown in Fig. o Keeping VCE constant (say at 10 V), note the base current IB for various values of VBE. Then

plot the readings obtained on the graph, taking IB along y-axis and VBE along x-axis.

o The following points may be noted from the characteristics :- The characteristic is similar to that of a forward biased diode curve. This is expected since the base-emitter section of transistor is a diode and it is

forward biased.

As compared to CB arrangement, IB increases less rapidly with VBE. Therefore, input resistance of a CE circuit is higher than that of CB circuit.

o Input resistance:- It is the ratio of change in base-emitter voltage (VBE) to the change in base current (IB) at constant VCE i.e.

Input resistance (ri) =

o The value of input resistance for a CE circuit is of the order of a few hundred ohms.

Output characteristic:-



o It is the curve between collector current IC and collector-emitter voltage VCE at constant base

current IB.

o The output characteristics of a CE circuit can be drawn with the help of the circuit shown in Fig.

o The following points may be noted from the characteristics:- The collector current IC varies with VCE, for VCE between 0 and 1V only. After this,

collector current becomes almost constant and independent of VCE.

This value of VCE up to which collector current IC changes with VCE is called the knee voltage (Vknee). The transistors are always operated in the region above knee

voltage.

Above knee voltage, IC is almost constant. However, a small increase in IC with increasing VCE is caused by the collector depletion layer getting wider and capturing a

few more majority carriers before electron-hole combinations occur in the base area.

o Output resistance:- It is the ratio of change in collector-emitter voltage (VCE) to the change in collector current (IC) at constant IB i.e.

Output resistance (ro) =

o It may be noted that whereas the output characteristics of CB circuit are horizontal, they have noticeable slope for the CE circuit.

o Therefore, the output resistance of a CE circuit is less than that of CB circuit. Its value is of the order of 50 k.

III. Common collector configuration:- o In this circuit arrangement, input is applied between base and collector, and output is taken from

collector and emitter.

o Here, collector of the transistor is common to both input and output circuits and hence the name common collector connection.

o The ratio of change in emitter current to the change in base current at constant base-collector voltage VCB is known as current amplification factor i.e.



Sr.No Parameters Common Base Common Emitter Common Collector

1. Common terminal to input

and output

Base Emitter Collector

2. Input impedance Very Low (about 100 ) Low(about 800 ) Very high (about 500 k)

3. Output impedance

Very high(about 1M ) High(about 50 k) Low(about 50 )

4. Voltage gain Medium High Less than 1

5. Current gain Less than unity (= Ic/IE) High (=Ic/IB) Greater than CE (=IE/IB)

6. Application Pre amplifier audio amplifier For impedance matching

7. Output signal In phase with input Out of phase with input In phase with input

Current gain:-

Current gain of common base configuration ():- The ratio of change in collector current to the change in emitter current at constant collector-base voltage VCB

is known as current amplification factor i.e.

Current gain of common emitter configuration ():-

The ratio of change in collector current (IC) to the change in base current (IB) is known as base current amplification factor i.e.

Relation between and :-



Numericals:-



Transistor biasing:-

Need of biasing:- The need of transistor biasing is:-

This increase in magnitude of the signal without any change in shape is known as faithful amplification. In order to achieve this, transistor biasing is done.

The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction properly reverse biased during the application of signal.

For deciding the location of Q point (operating point), as per the application (amplifier or switch), biasing is used.

The value of stability factor (S) should be as small as possible.

The Q-point should not be affected due to temperature changes or device to device variation.

Stabilization:- The process of making operating point independent of temperature changes or variations in transistor

parameters is known as stabilization.

State the need of stabilization:- Stabilization of the operating point is necessary due to the following reasons :

(i) Temperature dependence of collector current (IC).

(ii) Replacement of transistor may cause change in location of Q-point.

(iii) Thermal runaway may cause transistor to be damaged.

DC load line:-

Line AB is DC load line.

The DC word indicates that the line is drawn under dc operating conditions, i.e. without any ac signal at the input.

The word load line is used because the slope of this line is -1/RC, where RC is load resistance.



The straight line drawn on the output characteristics of a transistor amplifier circuit which gives the values of collector current IC and collector to emitter voltage VCE corresponding to either D.C. or A.C.

input conditions is called a load line.

The load line intersects horizontal axis at a point marked VCC, this point is called transistor cut-off point.

The load line intersects vertical axis at a point marked IC = (VCC/RC), this point is called transistor saturation point.

These values of currents & voltages define the point on a DC load line, at which transistor operates. This point is called operating point. It is also known as quiescent point or Q point.

Q-point (quiescent point or operating point):- For proper operation of transistor, in any application, we set fixed levels of certain voltages (VCE and

VBE) & currents in a transistor (IC and IB).

These values of currents & voltages define the point on a DC load line, at which transistor operates. This point is called operating point. It is also known as quiescent point or Q point.

Types of biasing:- 1. Base bias (fixed bias). 2. Base bias with emitter feedback. 3. Base bias with collector feedback. 4. Voltage divider biasing (self bias).

Voltage divider biasing network:-

In other biasing techniques the values of DC bias current and voltage of the collector depends upon the current gain of the transistor.

But is temperature sensitive especially for silicon transistors.

Fig. above shows the voltage divider circuit.

The name voltage divider is derived from the fact that resistors R1 and R2 form a potential divider across the VCC supply.

The voltage drop across resistor R2 forward biases the base emitter junction of a transistor.

The resistor RE provides the DC stability.



Derivation of IC and VCE for voltage divider biasing network:-

Field effect transistor:- The field effect transistor (FET) has, by virtue of its construction and biasing, its input impedance may

be more than 100 mega ohms.

The FET is generally much less noisy than the ordinary or bipolar transistor.

The rapidly expanding FET market has led many semiconductor marketing managers to believe that this device will soon become the most important electronic device, primarily because of its integrated-circuit

applications.

Types:-

There are two basic types of field effect transistors:- o Junction field effect transistor (JFET) o Metal oxide semiconductor field effect transistor (MOSFET)



Junction field effect transistor:- A junction field effect transistor is a three terminal semiconductor device in which current conduction

is by one type of carrier i.e., electrons or holes, and is controlled by means of an electric field between

the gate terminal and the conducting channel of the device..

Figure (1) Figure (2)

A JFET consists of a p-type or n-type silicon bar containing two PN junctions at the sides as shown in Fig (1).

The bar forms the conducting channel for the charge carriers.

If the bar is of n-type, it is called n-channel JFET as shown in Fig (i) and if the bar is of p-type, it is called a p-channel JFET as shown in Fig (ii).

The two PN junctions forming diodes are connected internally and a common terminal called gate is taken out. Other terminals are source and drain taken out from the bar as shown.

Thus a JFET has essentially three terminals i.e. gate (G), source (S) and drain (D).

Figure (2) shows the schematic symbol of JFET. The vertical line in the symbol may be thought as channel and source (S) and drain (D) connected to this line.

If the channel is n-type, the arrow on the gate points towards the channel as shown in Fig (i). However, for p-type channel, the arrow on the gate points from channel to gate as shown in Fig (ii).



N-channel JFET:-

Working principle:-

The current conduction by charge carriers (i.e. free electrons in this case) is through the channel between the two depletion layers and out of the drain.

The width and hence resistance of this channel can be controlled by changing the input voltage VGS.

Thus JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage VGS. In other words, the magnitude of drain current (ID) can be

changed by altering VGS.

Working:-

The working of JFET is as under : (i) When voltage on the gate is zero:-

The two PN junctions at the sides of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion

layers.

The size of these layers determines the width of the channel and hence the current conduction through the bar.

Since IDSS is measured under shorted gate conditions, it is the maximum drain current that you can get with normal operation of JFET.

(ii) When VGS is applied:- When a reverse voltage VGS is applied between the gate and source the width of the

depletion layers is increased.

This reduces the width of conducting channel, thereby increasing the resistance of n-type bar. Consequently, the current from source to drain is decreased.

Thus current from source to drain can be controlled by the application of potential (i.e. electric field) on the gate. For this reason, the device is called field effect transistor.

The curve between drain current (ID) and drain-source voltage (VDS) of a JFET at constant gate to source voltage (VGS) is known as output characteristics of JFET.



Drain characteristics or output characteristics:-

The following points may be noted from the characteristics:-

Ohmic region:- o In this region the drain current increases linearly with the increase in drain to source voltage,

obeying ohms law. o Hence it acts like a resistor in this region, so called as Ohmic region. o This region is shown as a curve OA in the figure. o Since IDSS is measured under shorted gate conditions, it is the maximum drain current that you

can get with normal operation of JFET.

Active region:- o The region between VP and VDS (max) (breakdown voltage) is called constant-current region or

active region.

o As long as VDS is kept within this range, ID will remain constant for a constant value of VGS. In other words, in the active region, JFET behaves as a constantcurrent device.

o For proper working of JFET, it must be operated in the active region. o At first, the drain current ID rises rapidly with drain-source voltage VDS but then becomes

constant. The drain-source voltage above which drain current becomes constant is known as

pinch off voltage VP.

o After pinch off voltage, the channel width becomes so narrow that depletion layers almost touch each other. Consequently, drain current remains constant.

Breakdown region:- o In this region, the drain current increase rapidly as the drain to source voltage is also increased. o This happens because of breakdown of gate to source junction. o The drain to source voltage at which the junction breaks down is called breakdown voltage.



o There is a maximum drain voltage [VDS (max)] that can be applied to a JFET. If the drain voltage exceeds VDS (max), JFET would breakdown as shown in Fig.

Terms in JFET:- Active region:-

o The region between VP and VDS (max) (breakdown voltage) is called constant-current region or active region. As long as VDS is kept within this range, ID will remain constant.

Pinch-off voltage (VP):- o It is the minimum drain-source voltage at which the drain current essentially becomes constant.

Gate to source cut-off voltage (VGS(OFF)):- o It is the gate-source voltage where the channel is completely cut off and the drain current

becomes zero.

Shorted-gate drain current (IDSS):- o It is the drain current with source short-circuited to gate (i.e. VGS = 0) and drain voltage (VDS)

equal to pinch off voltage.

DC drain resistance (static resistance):- o It is defined as the ratio of drain to source voltage (VDS) to the drain current (ID).

AC drain resistance:-

Transconductance:-



Amplification factor:-

Input resistance:-

o It is defined as the ratio of gate to source voltage (VGS) to the gate current (IG).

Relation between amplification factor, drain resistance and Transconductance:-

Advantages of JFET:-

1. It has very high input impedance (of the order of 100 M). 2. As JFET has no junction, so there is less noise. 3. A JFET has a negative temperature co-efficient of resistance. This avoids the risk of thermal runaway. 4. A JFET has a very high power gain. 5. A JFET has a smaller size, longer life and high efficiency. 6. JFET requires less space in case of IC fabrication. 7. High power handling capacity.

Compare BJT and FET:-

Sr.

No

Parameters BJT FET

1 Types of carriers Bipolar device (both charge

carriers i.e. electrons as well as

holes)

Unipolar device (either holes or

electrons)

2 Switching speed lower switching speed Faster switching speed

3 Thermal stability Less thermal stability More thermal stability



4 Space in case of IC fabrication More space required Less space required

5 Control parameter Current controlled device Voltage controlled device

6 Input impedance Less input impedance More input impedance

7 Output impedance More output impedance Less output impedance

8 Power gain Low power gain High power gain

9 Offset voltage Offset voltage present No offset voltage

10 Noise level More noise level Less noise level

11 Input current Base current in microamperes Zero gate current

12 Symbol

13 Applications As: - Amplifier, Switch, etc As: - Amplifier, Switch, etc

14 Advantages Linear device, higher gain-

bandwidth product, less cost as

compared to FET, etc

High power handling capability,

Smaller size, easy to fabricate,

etc

15 Thermal runaway Exist in BJT Doesnt exist in FET

Metal oxide semiconductor field effect transistor (MOSFET):-

There are two basic types of MOSFETs viz. 1. Depletion-type MOSFET or D-MOSFET:-

o The D-MOSFET can be operated in both the depletion-mode and the enhancement-mode. o For this reason, a D-MOSFET is sometimes called depletion/enhancement MOSFET.

2. Enhancement-type MOSFET or E-MOSFET:- o The E-MOSFET can be operated only in enhancement-Mode

Depletion type MOSFET:-

Symbols of N-channel D-MOSFET

Working:-

I. Depletion mode:-



Circuit diagram of N-channel D-MOSFET

Fig. above shows depletion-mode operation of n-channel D-MOSFET.

Since gate is negative, it repels the free electrons in the n-channel, in other words, we have depleted (i.e. emptied) the n-channel of some of its free electrons.

Therefore, lesser number of free electrons is made available for current conduction through the n-channel.

The greater the negative voltage on the gate, the lesser is the current from source to drain.

Thus by changing the negative voltage on the gate, we can vary the resistance of the n-channel and hence the current from source to drain.

II. Enhancement mode:-

Circuit diagram of N-channel D-MOSFET

Fig. above shows enhancement-mode operation of n-channel DMOSFET.

Since the gate is positive, it induces negative charges in the n-channel, these negative charges are the free electrons drawn into the channel.

Because these free electrons are added to those already in the channel, the total number of free electrons in the channel is increased.

Thus a positive gate voltage enhances or increases the conductivity of the channel. The greater the positive voltage on the gate, greater the conduction from source to drain.

Because the action with a positive gate depends upon enhancing the conductivity of the channel, the positive gate operation is called enhancement mode.

Characteristics:-



Enhancement type MOSFET:-

Symbols of N-channel E-MOSFET

Working:-

Circuit diagram of N-channel E-MOSFET

Fig. above shows the circuit of n-channel E-MOSFET. (i) When VGS = 0V:-

There is no channel connecting the source and drain.

The p substrate has only a few thermally produced free electrons (minority carriers) so that drain current is essentially zero.

For this reason, E-MOSFET is normally OFF when VGS = 0 V. (ii) When VGS is positive:-

When gate is made positive (i.e. VGS is positive), it attracts free electrons into the p region.



If VGS is positive enough, free electrons begin to flow from the source to drain.

The effect is the same as creating a thin layer of n-type material (i.e. inducing a thin n-channel) adjacent to the SiO2 layer.

Thus the E-MOSFET is turned ON and drain current ID starts flowing from the source to the drain.

The minimum value of VGS that turns the E-MOSFET ON is called threshold voltage [VGS (th)].

When VGS is less than VGS (th), there is no induced channel and the drain current ID is zero.

When VGS is equal to VGS (th), the E-MOSFET is turned ON and the induced channel conducts drain current from the source to the drain.

Applications of BJT:-

1. Bipolar transistor (BJT) can be used as Amplifier. 2. BJT can be used as switch. 3. BJT is used in digital electronics 4. BJT is used in operational Op-Amps 5. BJT is used in oscillators. 6.

Applications of FET:-

1. FET is used as Buffer amplifier. 2. In phase shift oscillators 3. As RF amplifiers 4. FET can also be used as voltage variable resistor (VVR). 5. 6.

Applications of MOSFET:-

1. Metal oxide semiconductor field effect transistor (MOSFET) can be used as Amplifier. 2. MOSFET can be used as switch. 3. MOSFET is used in digital electronics 4. As RF amplifiers

Compare JFET and D-MOSFET:-

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