UNIT-I PN JUNCTION DEVICESPart-A

1.What is meant by N-type and P-type semiconductor?When a small amount of impurity (e.g. Antimony, Arsenic) is added to a pure semiconductor

crystal the resulting extrinsic semiconductor is N-type semiconductor. If trivalent impurity (e.g. Indium, Gallium) is added to a pure semiconductor then the resulting extrinsic semiconductor is known as P-type semiconductor.2. Define doping?

The process of adding impurity to pure semiconductor is known as doping. As a result of it the characteristics of semiconductor is changed and hence the conductivity increases.3. What are donor and acceptor impurities?

Pentavalent impurities (Antimony, Arsenic) have five valence electrons. They can donate one excess electron to adjacent atoms to complete lattice structure; therefore they are called donor impurities.Trivalent impurities (Indium, Gallium) have three valence electrons. They have tendency to accept one electron from adjacent atoms to complete lattice structure, therefore they are known as acceptor impurities.4. State mass action law.

Mass action law states that in a semiconductor the product of the number of holes and the number of electrons is constant and is independent of the amount of donor and acceptor impurity doping.

np=ni2

Where,n= free electron concentration p= hole concentration; nj = intrinsic concentration

5. Define the term the drift current.If a steady electric field is applied across a semiconductor, it causes the free electrons to move

towards the positive terminal and the holes move towards the negative terminal of the battery. This combined effect causes a current flow in the semiconductor. The current produced in this manner is known as drift current.

Drift current density due to electronsJn=qnμnE

Drift current density due to holes J p=qp μpE

Jn = Drift current density due to electrons,Jp= Drift current density due to holes q = Charge of the carrier, n = Mobility of electrons p = Mobility of holes, E = Applied electric field strength.

6. What is diffusion current?(Dec 2014)In a semiconductor it is possible to have a non-uniform distribution of carriers. A

concentration gradient exists if the number of either holes or electrons is greater in one region as compared to the rest of the region. The holes and electrons then tend to move from a region of higher concentration to lower concentration region.This process is known as diffusion and the electric current produced due this process is known as diffusion current.7. What is a PN junction diode?

A PN junction diode is a two terminal device consisting of a PN junction formed either of Germanium or Silicon crystal. A PN junction is formed by diffusing P type material to one half side and N type material to other half side.8. Draw the symbol of the following devices. (Nov 2015)(a) PN Diode (b) Zener Diode (c) LED (d) UJT

9. What is Depletion region in a PN junction diode?In a PN junction diode, the holes and the electrons combine to form electron-hole pair,

leaving the uncovered acceptor and donor ions at the vicinity of the junction. The region where the

charge carriers are depleted and has only immobile charges which are electrically charged is known as depletion region or space charge region.10. Define barrier potential.

Potential barrier is defined as an electric potential that is established across the junction, during the initial diffusion of charge carriers at the junction, which restricts further movement of charge carriers across the junction. 11. Explain the terms knee voltage and breakdown voltage?(Nov 2010)

Knee voltage: The forward voltage at which the current through the PN junction starts increasing rapidly is known as knee voltage. It is also called as cut-in voltage or threshold voltage.Breakdown voltage: It is the reverse voltage of a PN junction diode at which the junction breaks down with sudden rise in the reverse current.12. Write down and explain junction diode equation.

The equation which explains the forward and reverse characteristics of a semiconductor diode is known diode equation. The diode current is given by

I=I 0(evV T−1)

Where Io = reverse saturation current = 1 for Ge diodes, 2 for silicon diodes V- External voltage; VT = volt equivalent of temperature. = T/11,600

13. Define and explain Peak Inverse Voltage (PIV) (Nov 2010)Peak inverse voltage is the maximum reverse voltage that can be applied to the PN junction

without damage to the junction. If the reverse voltage across the junction exceeds to its peak inverse voltage, the junction may be destroyed due to excessive heat.14. Differentiate drift and diffusion current.

Drift current Diffusion current1. Developed due to potential gradient Developed due to concentration gradient2. Phenomenon found both in semiconductors and metals

Only in semiconductors

3. Jn = qnn EJp = q p p E

Jn = q Dndn / dxJp = q Dpdp / dx

15. State the relationship between diode capacitance and the reverse bias voltage.

Transition capacitance CT=∈ AW

The equation shows that the width of the depletion layer (W) is inversely proportional to the capacitance. Since the width increase in the reverse voltage, the width of the depletion region increases, the capacitance decreases with increase in reverse voltage.16. Define the term diffusion capacitance or storage capacitance. (May 2015) (Nov 2014)

The diffusion capacitance effect is found when the diode is forward biased and it is defined as the rate of change of injected charge with voltage and given by

Cd=τIV T

I = diode current, VT = volt equivalent temperature. VT = T /11,600 = constant= 1 for Ge diodes, 2 for silicon diodes; = mean life time.17. Calculate the diffusion capacitance for a silicon diode with a 15 mA forward current, if the charge carrier transit time is 70nsec. (Nov 2015)

For T = 250C = 298 K;VT = T /11,600 = 25.7mV= 2 for silicon diode; I = 15mA; τ = 70 nsec.

Cd=τIV T

=70×10−9×15×10−3

2×25. 7×10−3 =20.428nF

18. Define the term transition capacitance (Nov 2014)When P-N junction is reverse biased the depletion region act as an insulator or as a dielectric

medium and the p-type an N-type region have low resistance and act as the plates. Thus this P-N junction can be considered as a parallel plate capacitor. This junction capacitance is called as space charge capacitance or transition capacitance and is denoted as CT.

CD=dQdV

Where dQ is the increase in charge and dV is the change or increase in voltage. The depletion region increases with the increase in reverse bias potential the resulting transition capacitance decreases. The

formula for transition capacitance is given as CT = Aε/W, where A is the cross sectional area of the region, and W is the width.19. What is avalanche breakdown in PN junction diode?

The avalanche breakdown takes place when both sides of the junction are lightly doped and due to this the depletion layer is large. When the reverse bias voltage is increased the accelerated free electrons collide with the semiconductor atoms in the depletion region Due to collision, the covalent bonds are broken and electron hole pairs are generated. These new charge carriers so produce acquire energy from applied potential and in turn produce additional carriers. This forms a cumulative process called avalanche multiplication,this causes the reverse current to increase rapidly. This leads to breakdown of the junction known as avalanche breakdown.20. Define Static resistance and Dynamic resistance? (May 2013)

The resistance offered by the diode to DC operating conditions is called “Static resistance” and the resistance offered by the diode to AC operating conditions is called “Dynamic resistance”.21. List the applications of PN junction diode:

1. Used as rectifier in DC power supplies.2. Used as signal diodes in communication circuits.3. Used in clipper and clamper circuits.

22. What is a rectifier and list its types? (May 2015) (Nov 2014)Rectifier is a circuit which converts a.c. to d.c. signal.

Half-wave rectifier: It is the simplest type of rectifier, which is made with just one diode. Full-wave rectifier: This rectifier is essentially made of two half-wave rectifiers, and can be made with two diodes and an earthed centre tap on the transformer. The centre tap allows the circuit to be completed because current cannot flow through the other diode.Bridge rectifier: A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave rectification. 23.Define: Ripple factor&transformer utilization factor.Ripple factor: It is the ratio of a.c voltage to d.c voltage or a.c. current to d.c current.Transformer Utilization Factor:It is the ratio of power delivered to the load to the volt ampere rating of transformer.24. Mention some characteristics of LASER diode.1. It is coherent i.e. there is no path difference between the waves comprising the beam.2. It is monochromatic i.e. it consists of one wavelength and hence one color only.3. It is collimated i.e. emitted light waves travel parallel to each other.25. Mention some applications of LASER diode.

Laser diodes used in variety of applications ranging from medical equipment used in surgery to consumer products like optical disk equipment, laser printers, and hologram scanners, etc. Laser diodes emitting visible light are used as pointers. Those emitting visible and infrared light are used to measure the distance.

Part – B1.(i) With necessary diagrams, explain the forward and reverse characteristics of PN junction diode.

(8) (Nov 2015) (ii) Draw the circuit diagram of a half wave rectifier for producing a positive output voltage. Explain the circuit operation and sketch the waveforms. (8) (Nov 2015)

2. Explain the construction and working of PN diode with a neat sketch. (16) (Nov 2013,2014)3. (i) Discuss about drift and diffusion current of PN diode. (8) (May 2013)

(ii)The reverse saturation current of a silicon PN diode is 10µA. Calculate the diode current for the forward bias voltage of 0.6V at 250C. (8) (May 2013)4. Explain the working of centre tapped full wave rectifier(with and without filter) with neat diagrams and derive the necessary equations. (16) (May 2015) (Nov 2013)5. (i) Discuss in detail about Zener shunt regulator. (10) (May 2015) (May 2013)

(ii) Explain the effect of temperature of a diode. (6) (May 2013)6. (i) Explain the construction and working of LED with its characteristics. (8) (May 2013)

(ii) Explain the principle and operation of Laser diode with neat diagram.(8) (May 2015)7. Draw the circuit diagram of half wave rectifier and explain its operation with necessary waveform.

Also derive the necessary equations. (16) (May 2012)8. With neat diagram, explain the operation of Zener diode and its forward and reverse

characteristics. Also distinguish between Avalanche and Zener breakdowns. (16) (Nov 2015,2013)9. (i) Discuss in detail about diffusion and transient capacitance. (8)

(ii) A germanium diode has a contact potential of .2volt while the concentration of accepted impurity atoms is 3x1020 /m3. Calculate for a reverse bias of .1 volt, the width of the depletion region. If the reverse bias is increased to 10volt, calculate the new width of the depletion region.

Assuming cross sectional area of the junction as 1mm2, calculate the transition capacitance values for both the cases. Assume r=16 for germanium. (8)

10. (i) Compare half wave and full wave rectifier. (8) (ii) An a.c. supply of 230V is applied to a half-wave rectifier circuit through transformer ofturns ration 5:1. Assume the diode is an ideal one. The load resistance is 300Ω.Find (a) dc output voltage (b) PIV (c) maximum and average values of powerdelivered to the load. (8)

UNIT II- TRANSISTORSPart-A

1. What is meant by biasing a transistor? (Nov 2014)Transistor biasing is the process of maintaining proper flow of zero signal collector current

and collector-emitter voltage during the passage of signal. Biasing keeps emitter-base junction forward biased and collector-base junction reverse biased during the passage of signal.2. Define the different operating regions of transistor.Active region: It is defined in which transistor collector junction is biased in reverse direction and emitter junction in forward direction.Cutoff region: The region in which the collector and emitter junctions are both reverse-biased Saturation region: The region in which both the collector and emitter junctions are forward biased.3. Define Base width modulation (Early effect).

In a CB configuration, an increase in collector voltage increases the width of the depletion region at the output junction diode. This will decrease the effective width of the base. This is known as early effect. Due to this effect recombination rate reduces at the base region and charge gradient is increased within the base.4. Explain the significance of Base width modulation (Early effect). a) It reduces the charges recombination of electrons with holes in the base region, hence the current gain increases with the increase in collector -base voltage b) The charge gradient is increased within base; hence the current due to minority carriers injected across emitter junction increases.5. Among CB, CE, CC which is most important?

The CE configuration is important. The reasonsi) High current gain ii) Output to input impedance ratio is moderate therefore easy coupling is possible

between various transistor stagesiii) It finds excellent usage in audio frequency applications hence used in receivers and

transmitters6. What is thermal runaway?

The reverse saturation current in a semiconductor doubles for every 100 C rise in temperature I as temperature increases the leakage current increases I and the collector current also increases. The increase in collector produces an increase in power dissipation at the collector - base junction. This I in turn further increase the temperature of the collector-base junction causing the collector current to further increase. This process may become cumulative and it is possible for the transistor to burn out. This process is known as Thermal runaway. It can be avoided using a stabilization or heat sink with the transistor.7. How a transistor is used as a switch?

A transistor should be operated in saturation and cutoff regions to use it as a switch I While operating in saturation region I transistor carry heavy current hence considered as ON state. In cutoff it doesn't carry current and it is equivalent to open switch.8. Define alpha and beta of a transistor.

Alpha (α ) is the current gain of a common base transistor.It can be defined as the ratio of change in collector current to the change in the emitter current at constant collector-base voltage VCB

α=∆ I c∆ IE

, at constant VCB

Beta () is the current gain of a common emitter transistor. It can be defined as the ratio of change in collector current to the change in the base current at constant collector-base voltage VCE

β=∆ I c∆ I B

, at constant VCE

9. What are the advantages of IGBTs?i) They have high input gate impedance.

ii) They have low conduction loss. iii)They have fast switching characteristics. iv)They have very high operating frequency.10. Which configuration is known as emitter follower and why it is named so?

CC configuration is known as emitter follower, whatever may be the signal applied at the input, may produce same signal at the output. In other words, the gain of the circuit is unity. So that the commoncollector circuit - the so called emitter follower is named as emitter follower.(Output follows the input)11. Compare BJT and JFET. (May 2015) (May 2010 /Nov 2014)

BJT JFETLow input impedance High input impedanceHigh Output impedance Low output impedance

Bipolar device Unipolar device

Noise is more Less noise

Cheaper Costlier

Gain is more Gain is less

Current controlled device Voltage controlled device

12. Calculate IC and IE for a transistor that has αdc = 0.99 and IB = 150 µA. Determine the value of βdc for the transistor. (Nov 2015)

αdc= ICI E

; IC = αdc IE

IE = IB + IC;IB = IE - αdcIE; IB = (1 - αdc) IE =>IE = IB

1−α dc = 150 µA

1−0.99 = 15mA

IC = αdc IE = 0.99×15 mA = 14.85 mA

βdc = α

1−α = 0.991−0.99 = 99

13. Mention the advantages of FET over BJT? (Nov 2013)i) The noise level is very low in FET since there are no junctions.ii) FET has very high power gain iii)Offers perfect isolation between input and output since it has very high input impedance.iv) FET is a negative temperature coefficient device hence avoids thermal runaway.14. Define transconductance of JFET.

Transconductance (gm) is defined as the ratio of small change in drain current (Id) to the corresponding change to gate source (Vgs) at constant drain to source voltage (Vds).

gm=∆ I d∆V gs

at constant Vds

15. Define amplification factor of JFET?(May 2010)Amplification factor () is defined as the ratio of small change in drain to source voltage

(Vds) to the corresponding change in gate to source voltage (Vgs) at a constant drain current Id.

μ=∆V ds

∆V gsat constant Id

16. Give the Shockley's equation for FET.The Shockley's equation gives the relation between drain current (ld) in the pinch of region

and the gate to source voltage Vgs

I d=I dss¿)Where Idss = maximum value of drain current when Vgs = 0,Vp = Pinch off voltage.17. What is meant by Gate -Source threshold voltage and pinch off voltage of a JFET?

The voltage at which the channel is completely cut-off and the drain current becomes zero is called as Gate -Source threshold voltage.

In the output characteristics of FET, the drain current rises rapidly with drain source voltage. After reaching some value, it becomes constant. The drain source voltage above which drain current becomes constant is known as pinch off voltage. The corresponding Vgs is called Gate -Source threshold voltage.18. What is MOSFET? (May 2013)

The MOSFET is an abbreviation of Metal Oxide Semiconductor Field Effect Transistor.

It is a three terminal semiconductor device similar to a FET with gate insulated from the channel. Therefore it is a also known as insulated Gate (IGFET)19. What is a thyristor? Mention two of them. (May 2015)

A thyristor is normally four layer three-terminal device. Four layers are formed by alternating n – type and p – type semiconductor materials. Consequently there are three p – n junctions formed in the device. It is a bistable device. The three terminals of this device are called anode (A), cathode (K) and gate (G) respectively. The gate (G) terminal is control terminal of the device. That means, the current flowing through the device is controlled by electrical signal applied to the gate (G) terminal. The anode (A) and cathode (K) are the power terminals of the device handle the large applied voltage and conduct the major current through the thyristor. They are SCR, DIAC, TRIAC.20. Why E-MOSFET is normally called as OFF – MOSFET?

WhenVgs = 0, the biasing supply VDD this to force the force the free electron to move source to drain. But the P substrate has only few generated conduction band electrons. Aside from this minority carriers some surface leakage, the current between source and drain is zero hence E- MOSFET is called as OFF MOSFET. 21. Differentiate JFET and MOSFET?

JFET MOSFETReverse bias for gate Positive or negative gate voltageGate is formed as a diode Gate is made as a capacitorOperated only in depletion mode Can be operated either in depletion mode or

in enhancement modeHigh input impedance Very high input impedance due to capacitive

effect22. What is intrinsic standoff ratio of a UJT? (Nov 2013)

The ratio of voltage between emitter and base 1 to VBB is called as intrinsic stand of ratio.The value lies between 0.51 to 0.82.

η=RB1

(RB1+RB 2)23. Show how an SCR can be triggered ON by the application of a pulse to the gate terminal. (Nov 2015)

This is the most commonly used method to trigger the SCR. In gate triggering method, the SCR is operated with an anode voltage slightly less than the rated forward break over voltage and is triggered into conduction by a low power gate pulse. It may be noted that once the SCR is switched ON, the gate has no further control on the device current. The gate pulse signals can be supplied either from a d.c source or an a.c source.24. Draw the two transistor model of SCR. (Dec 2011)

25. What is meant by latching current & holding current? (Dec 2012) Latching current is the minimum anode current required to maintain the thyristor in the on State

immediately after a thyristor has been turned on and gate signal has been removed. Holding current is the minimum anode current to maintain the thyristor in the ON state.

Part - B1. Explain the input and output characteristics of a CE transistor configuration. List out the comparisons between CE, CB and CC configurations. (Nov 2013)2. (i) Describe the static input and output characteristics of a CB transistor with neat circuit diagram. (8)(Nov 2010)

(ii) Explain the two transistor model of a thyristor in detail. (8) 3. (i) Draw and explain the characteristics of CC configuration.(8) (May 2011)

(ii) Explain the working of UJT as relaxation oscillator in detail.4. (i) Draw the cross section diagram for N type enhancement mode MOSFET. Briefly explain its operation. (8) (May 2015) (Nov 2015) ( Nov 2013)

(ii) Explain the selection of Q point for a transistor bias circuit and discuss the limitations on the output voltage swing. (8) (Nov 2015)5. With a neat sketch explain the construction and characteristics of DEMOSFET.( Nov 2013)6. Explain the construction of N channel JFET and also explain the drain and transfer characteristics of the same. (Nov 2010)7. (i) Draw the basic construction and equivalent circuit of a UJT. Briefly explain the device operation. (8) (May 2015) (Nov 2015) (May 2013)

(ii) Sketch the four layers construction of an SCR and the two transistor equivalent circuit. Explain the device operation. (8) (Nov 2015)8. Discuss in detail about the construction and operation characteristics of DIAC & TRIAC. (16)9. With a neat sketch explain the construction and working characteristics of IGBT. (16)10. Explain the construction and operation of NPN transistor with neat sketch. Also comment on the

characteristics of NPN transistor.(16) (Nov 2014)

UNIT – III AMPLIFIERSPart– A

1. Write the procedure to draw the a.c. equivalent of a network. i) Setting all the dc sources to zero and replacing them by a short circuit equivalent. ii) Replacing all capacitors by a short circuit. iii) Removing all elements bypassed by the short circuit equivalents introduced by step 1 &step 2. iv) Redraw the n/w in a more convenient & logical form.2. Draw the hybrid model of CE amplifier. (May 2015)

V b=hie ib+hℜV c

I c=h feib+hoeV c

3. What are amplifiers? Wrote its uses. (May 2015)An electronic device that is used to increase the magnitude of an electrical signal. Such a

device used for the amplification of audio frequency signals in a radio.4. Write the hybrid equations of CE amplifier.

V b=hie ib+hr eV c

I c=h feib+hoeV c

5. Write the hybrid equations of CB amplifier.V e=h ib ie+hrbV c

I c=h fb ie+hobV c

6. Write the hybrid equations of CCamplifier.V b=hic ib+hrcV e

I c=h feib+hocV e

7. What is meant by hybrid parameters?(May 2011 / Nov 2014)The parameters which have a combination of units are called hybrid parameters. The hybrid

parameters are input impedance, output impedance, current gain and voltage gain.8. Give the condition for approximate analysis of small signal model.

Product of hoe And RL must be < 0.1.is the basiccondition for Approximate analysis of small signal model.9. What are the high frequency effects?

At high frequencies, the internal capacitance of the transistors will reduce output voltage as well as reduces the circuit gain.

10. What is meant by gain bandwidth product?The product of gain and bandwidth of an amplifier is always constant. It defines, if gain

increases bandwidth decreases and vice versa.11. Draw the Eber moll model of a transistor.

12. Give the significance of coupling and bypass capacitor on BW of amplifiers. (Nov 2015)At low frequency the bypass capacitor makes gain to lower value and at higher frequency the

coupling capacitor decreases the gain so at low and high frequencies response curves is varied and mid-range of frequencies the gain is nearly constant.13. State the reason for fall in gain at low and high frequencies.

The coupling capacitance has very high reactance at low frequency, therefore it will allow only a small part of signal from one stage and in addition to that the bypass capacitor cannot bypass the emitter resistor effectively. As a result of these factors the voltage gain rolls off at low frequency.

At high frequency the reactance of coupling capacitor is very low, therefore it behaves like a short circuit. As a result of this the loading effect of the next stage increases which reduces the voltage gain. Hence the voltage gain rolls off at high frequency.14. Write short note on effects of coupling capacitor.

The coupling capacitor transmits a.c. signal but blocks d.c. This prevents d.c. interference between various stages and the shifting of operating point. It prevents the loading effect between adjacent stages.

15. Draw the small signal equivalent circuit of a CSJFET. (Nov 2015)

16. Define frequency response. Draw its curve of an amplifier.Frequency response can be defined as measure of output parameter variation with respect to

variation of input frequency.

17. What are 3 dB frequencies?The frequency at which we have 70.7% of fall from the maximum gain is called 3db

frequency

18. In an amplifier the maximum voltage gain is 2000, occurs at 2 KHz. It falls to 1414 at 10Hz and 50Hz.Find i) B.W ii) Lower and upper cut off frequency.

BW=50Hz−10Hz=40HzF1=10Hz&F2=50Hz19. Define upper and lower cut off frequencies of an amplifier.

The frequency at which the voltage gain of the amplifier is exactly 70.7% of the maximum gain is known as lower cut off frequency.

The frequency at which the voltage gain of the amplifier, is exactly 70.7% o0f the maximum gain is known as upper cutoff frequency.20. Define the term bandwidth and gain bandwidth product.

Bandwidth is defined as the range of frequency over which the gain remains constant.The product of midland gain and bandwidth is called gain bandwidthproduct.21. What are the causes of occurrence of upper cutoff frequency in BJT?

The internal capacitors are the main element for decrease of gain as well as occurrence of upper cut off frequency.22. List out the application and characteristics of CE amplifier.

It is used as voltage amplifier, among the three basic transistor amplifiers.Characteristics of CE amplifier:

1. It has good voltage gain with phase inversions. i.e. the output voltage is 180°out of phase with input2. It also has good current, power gain and relatively high input and output impedance.23. Mention some application of CC amplifier.

1. It is used as buffer amplifier as it has unity voltage gain.2. It is used as impedance matching network.

24. What is millers theorem?Millers theorem states that the capacitor connected between the input and output can be split

in to two networks such that one network appears as the mirror image of the other one.The impedance of such network can be taken by open circuiting or short circuiting the common connections exist between the two networks. 25. State the reason for choosing 3 dbpoint to determine the bandwidth.

The reason for choosing 3 db point to determine the bandwidth is that, above this level, larger the frequency variation (i.e. output delivers the constant output below this level even for lower frequency variation), the gain variation is large i.e. the output is not constant. Thus 3 dbpoint is selected as reference to find the bandwidth.

Part-B1. The hybrid parameters of a transistor used as an amplifier in the CE configuration are h ie = 800, hfe= 46, hoe = 80 x 10-6 and hre= 5.4x 10-4. If RL = 5K and Rs = 500. Calculate Ai, Ri, Av,Pi. (16)2. Explain about CE amplifier and derive the expression for h parameters of the same. Also derive the expression for gain, input impedance and output impedance of CE amplifier. (16) (Nov 2013) (Nov 2014) (May 2015)3. (i) Discuss the factors involved in the selection of IC, RC and RE for a single stage common emitter BJT amplifier circuit, using voltage divider bias. (8) (Nov 2015) (ii) A CC amplifier shown in the below figure has VCC = 15V, RB = 75KΩ and RE = 910Ω, β of the silicon transistor is 100 and the load resistor is 600Ω Find rin and Av. (8) (Nov 2015)

4. Explain about CB amplifier and derive the expression for h parameters of the same. Also derive the expression for gain, input impedance and output impedance of CB amplifier. (Nov 2013)5. Explain about CC amplifier and derive the expression for h parameters of the same. Also derive the expression for gain, input impedance and output impedance of CC amplifier.

6. Explain about CD amplifier and derive the expression for gain, input impedance and output Impedance.7. Explain about CS amplifier and derive the expression for gain, input impedance and output impedance and also draw its small signal equivalent circuit. (May 2013) (Nov 2014) (Apr 2015)8. Explain about the high frequency response of FET and derive the expression for lower cut off frequency and upper cut off frequency.9. (i) With neat circuit diagram, perform ac analysis for common source using equivalent circuit NMOSFET amplifier. (8) (Nov 2015) (ii) The MOSFET shown in the below figure has the following parameters. VT = 2V, β = 0.5×10-3, rd = 75KΩ and it is biased at ID = 1.93mA. Determine the input impedance and voltage gain. (8) (Nov 2015)

10. A common base transistor amplifier is driven by a voltage source VS and internal resistance RS = 1200Ω. The load impedance is a resistor RL of 1000Ω. The ‘h’ parameters are given below: h ib = 220Ω, hrb= 3 x 10-4, hfb= -0.98, hob = 0.5 µA / V. Compute current gain, input impedance, voltage gain, output impedance and power gain. (16) (Nov 2012)

UNIT – IV MULTISTAGE AMPLIFIERS AND DIFFERENTIAL AMPLIFIERSPart - A

1. What is a differential amplifier? What are its advantages? An amplifier that has two inputs and produces an output signal that is a function of the

difference between the two inputs.Advantages:(i) It can compare any two signals and detect any difference. (ii).It gives higher gain than two cascaded stages of ordinary direct coupling. (iii) It provides very uniform amplification of signal from dc up to very high frequencies. 2. What are the applications of differential amplifier? (Nov 2011)

1. In the medial electronics field 2. As a input stage in the measuring instruments 3. In analog computation 4. In linear integrated circuits

3. Define CMRR. What is its ideal value? (Nov 2015) (Nov 2014) (May 2010)The common mode rejection ratio (CMRR) serves as a figure of merit of a differential

amplifier and is defined as the ratio of the differential voltage gain (A d) to the common mode voltage gain (Acm).Its ideal value is infinity.A typical CMRR is 80-100dB at low frequency.

CMRR=Ad

Ac

4. Explain the need for constant current source for difference amplifier. The necessity for constant current source for differential amplifier is to increase the common

mode rejection ratio without changing the quiescent current (operating point) and without lowering the forward current gain. 5. Why RE is replaced by a constant current bias in a differential amplifier?

The emitter supply VEEused for biasing purpose must become larger as RE is increased in order to maintain the quiescent current at its proper value. If the operating currents of the transistors are allowed to decrease, this will lead to higher Hie values and lower values of Hfe. Both these effects

will tend to decrease CMRR. To overcome this practical limitations R E is replaced by a constant current bias. 6. What is the input impedance of a differential amplifier with RE at its emitter junction?

The input impedance of a differential amplifier with RE at its emitter junction is: Ri=2hie+(1+h fe)RE

7. Draw the ideal tuned circuit and write the expression for its resonant frequency. (May 2015)

8. Explain why constant current source biasing is preferred for differential amplifier.

The constant current source biasing is preferred for differential amplifier inorder to increase the input resistance and to make the common mode gain zero. 9. Distinguish between common mod signal and differential mode signal.

If the same input is applied to both the inputs, the operation is called common mode signal. It is theaverage value of the input signals.

V c=(V 1+V 2 )

2If the two opposite polarity input signals are applied, the operation is referred to as difference mode. The difference between the input voltages in called difference mode signal. 10. Explain how the differential amplifier can be used as an emitter coupled phase inverter.

A differential amplifier may be used as an emitter coupled phase inverter. In this case, the signal is applied at one base only leaving the second base unexcited but with proper bias. The output voltage is then picked up from the collectors. These two voltages are equal in magnitude and opposite in phase, thus the differential amplifier acts as a phase inverter or phase amplifier. 11. What is the current gain for a darlington pair?

The current gain for a darlington pair is given by,

Ai=(1+hfe)

2

(1+hoeh feRe)Where hfe - forward current gain for common emitter amplifier.

hfc - forward current gain for common collector amplifier. hoe - output resistance for common emitter amplifier.

12. Explain the difference between voltage and power amplifier.Voltage Amplifier: The input given to the transistor is in millivolts. The transistor used is a

small signal transistor. Power Amplifier: The input given to the transistor is in volts. The transistor used is a power transistor.13. What is a stagger tuned amplifier?

It is a circuit in which two single tuned cascaded amplifiers having certain bandwidth are taken and their resonant frequencies are adjusted that they are separated by an amountequal to the bandwidth of each stage. Since resonant frequencies are displaced it is called stagger tuned amplifier.Advantages:

The advantage of stagger tuned amplifier is to have better flat, wideband characteristics.14. Write down the need of cascading the amplifiers.(May 2015) (Nov 2014)

Cascading means connecting the output of one amplifier to the input of another to form a multistage amplifier. The overall gain of cascaded amplifiers depends on that of each stage and the total number of stages. The purpose of cascading amplifiers is to reach the desired signal power with a minimum amount of distortion, by providing equal overall gain characteristics to all frequencies in the signal.15. Define class A, B, C, ABpower amplifier.Class A:

It is an amplifier in which the input signals and the biasing is such that the output current flows for full cycle of the input signal.Class B:

It is an amplifier in which the input signal and the biasing is such that the output current flows for half cycle of the input signalClass C:

It is an amplifier in which the input signal and the biasing is such that the output currentflows for less than half cycle of the input signal.Class AB:

It is an amplifier in which the input signal and the biasing is such that the output Current flows for more than half cycle but less than full cycle of the input signal.16. Which power amplifier gives minimum distortion?

Class C power amplifier gives minimum distortion.17. Give two applications of class C power amplifier.1. It is used to generate pulses.2. It is used to trigger other devices. 18. What is a push pull amplifier?

Class B amplifier is used as a push pull amplifier which uses two transistors. Both the transistors work as a push pull arrangement.i.e. one transistor will be on at a time.Advantages:The harmonic distortions are removed.The efficiency is increased.19. What is cross over distortion? How it can be eliminated?

There is a 0.7V delay in between every half cycle. Due to this the sine wave will not be a continuous wave. This is called cross over distortion. It can be eliminated by class AB amplifier. 20.What is complementary symmetry amplifier?

The class B amplifier which uses both NPN and PNP transistor also both the halves are symmetrical is called complementary symmetry amplifier.21.What is second harmonics in power amplifiers?

When nonlinear distortion is present at the output waveform, it contains lots of harmonic components that is second, third and higher harmonic components. Out of these, the second harmonic components contribute more to the distortion.22. What is the need for neutralization? (Nov 2015)

The effect of collector to base capacitance of the transistor is neutralized by introducing a signal that cancels the signal coupled through collector base capacitance. This process is called neutralization.23. What are the different types of neutralization?1. Hazeltine neutralization2. Rice neutralization3. Neutrodyne neutralization24. What is a single tuned amplifier?

An amplifier circuit that uses a single parallel tuned circuit as a load is called single tuned amplifier.25. What are the advantages disadvantages of tuned amplifiers?Advantages:(i) They amplify defined frequencies.(ii) Signal to noise ratio at output is good(iii) They are suited for radio transmitters and receivers.Disadvantages:(i) The circuit is bulky and costly. (ii) The circuit design is complex.(iii) They are not suited to amplify audio frequencies.

Part– B1. Draw the circuit diagram of an emitter coupled BJT differential amplifier and derive expressions for differential gain, common mode gain, CMRR, input impedance and output impedance. Hoe CMRR can be improved. (16) (May 2015)(Nov 2015)(May 2013) (Nov 2014) (Apr 2015)2. Explain about Class A transformer coupled amplifier and derive the expression for efficiency of the same.3. (i) Explain with circuit diagram Class B power amplifier and derive the expression for its efficiency. (8) (Nov 2015)

(ii) With neat circuit, explain and derive the gain and bandwidth of a single tuned amplifier. (8) (Nov 2015)4. Explain with a neat sketch the working of single tuned voltage amplifier using FET.5. (i) Derive the frequency response of single tuned voltage amplifier and also give its limitations. (10)

(ii) Compare power amplifiers. (6)

6. Explain Class AB amplifier in detail. Give its advantages and disadvantages. (16)7. (i) Explain Class C amplifier in detail. (12)

(ii) Discuss the advantages and limitations of multistage amplifier. (4)8. Explain about RC coupled Class A amplifier and derive the expression for efficiency of the same. (16)9. (i) Explain the different types of neutralization technique used in tuning amplifier. (10)(May 2015)

(ii) Discuss the effects of cascading of amplifiers. (6)10. Differential amplifier has the following values RC = 50 K, Re = 100K and Rs = 10K. The transistor parameters are rin= hie= 50K, hfe = Vo = 2 x103, ro = 400K. Determine Ad, Ac and CMRR in dB. (16)

UNIT-V FEEDBACK AMPLIFIERS AND OSCILLATORSPart–A

1. Mention the effects on bandwidth and output impedance due to various types of feedback.

Types of feedback Bandwidth Output impedanceVoltage series feedback Increases DecreasesVoltage shunt feedback Increases DecreasesCurrent series feedback Increases DecreasesCurrent shunt feedback Increases Decreases

2. Distinguish between series and shunt feedback.Series Feedback Shunt FeedbackIn series feedback amplifier, the feedback signal is connected in series with the input signal.

It increases the input resistance.

In shunt feedback amplifiers, the feedback signal is connected in parallel with the input signal.

It decreases the input resistance.3. State the effect of feedback on noise.

When feedback is employed in amplifiers, the noise is reduced. Let A - Voltage gain without feedback

- Feedback factor N - Noise without feedback Nf - Noise with feedback

The noise with feedback is given by the following relation.

NF= N(1+Aβ )

Thus, when feedback is employed, the noise is reduced by a factor (1+A)4. Define positive and negative feedback?

Positive feedback: If the feedback voltage (or current) is so applied as to increase the input voltage (i.e it is in phase with it), then it is called positive feedback.Negative feedback: If the feedback voltage (or current) is so applied as to reduce the input voltage(i.e it is 180out of phase with it), then it is called negative feedback.5. Mention the advantages of negative feedback? (Nov 2015)

The advantages of negative feedback are higher fidelity and stabilized gain, increased bandwidth; less distortion and reduced noise and input & output impedances can be modified as desired. 6. Negative feedback is preferred to other methods of modifying Amplifiercharacteristics. Why?

Negative feedback is preferred to other methods of modifying AmplifierCharacteristics because it has the advantages of reduction in distortion, stability in gain, increased bandwidth etc.7. State the Bharkausen’s criterion for oscillation. (Nov 2013 / 2014)

The two important and necessary conditions areThe feedback must be positive.Feedback factor must be unity i.e. A = 1 8. State Nyquist’s stability criteria for feedback amplifiers.

Nyquist 's stability criterion states that in a complex S plane if A +1 represents a circle of unit radius with itscentre at the point –1+ j0 and if A lies within the circle then 1+ A< 1, feedback is positive. Even with this positive feedback the system will not oscillate unless Nyquistcriterion is satisfied.9. What is the difference between an oscillator and amplifier?

S.No

Amplifier Oscillator

1.

2.

3.

An amplifier is a circuit used to magnify the given input signal.

Amplifiers have finite gain.

In an amplifier circuit, the frequency, waveform and magnitude of ac power output are controlled by as signal voltage applied at the input.

An Oscillator is a circuit used to generate the signals. It has no input.

The gain of the oscillator is infinity.

In an oscillator, the frequency waveform and magnitude of ac power generated are controlled by the circuit itself.

10. What is damped&sustained oscillation? The electrical oscillation in which the amplitude decreases with time are called as damped

oscillations. The electrical oscillations in which amplitude does not change with time are called as

sustained oscillations. It is also called as undamped oscillation. 11. What are the factors needed to choose type of oscillators?The factors needed to choose type of oscillators are (i) The nature of generated wave form (ii) The frequency of generated signals (iii) The type of associated circuit of components. (iv) The fundamental mechanism involved.12. In a feedback amplifier explain how oscillation takes place by deriving Af (Gain with

feedback). The function of the feedback circuit is to transfer a part of the output energy to the input in

proper phase. When the feedback is positive, the overall gain f the amplifier is written as

A f=A

(1−Aβ )Where A is feedback factor or loop gain. If A = 1, Af= . Thus the gain becomes infinity i.e., there is output without any input. Thus the amplifier works as an oscillator and the oscillation will take place. 13.Which oscillator uses both positive and negative feedback? Why?

Wien bridge oscillator uses both positive and negative feedback. The positive feedback ensures sustained oscillation, the negative feedback ensures constant output i.e. any increase or decrease in the oscillator output is taken care of by this negative feedback. 14. Name two low & high frequency oscillators.(Nov 2010) Low frequency oscillators:(i) RC phase shift oscillator. (ii) Wein bridge oscillator. High frequency oscillators:(i) Hartley oscillator (ii) Colpitts oscillator.15.Why crystal oscillators are superior to other oscillators?

Crystal oscillators are superior to other oscillators because of their great mechanical strength, simplicity of manufacture and it obeys the piezoelectric effect accurately. 16. How oscillations occur in a crystal oscillator?

When an alternating voltage is applied, the crystal starts vibrating with a frequency of applied voltage. If the frequency of applied voltage is made equal to the natural frequency of crystal, resonance takes place and crystal vibrates and the oscillations occur with maximum amplitude.17.What are the advantages of crystal oscillator?(Nov 2012)

Simple circuit since no tuned circuit is needed other than the crystal itself. Different frequencies of oscillations can be obtained by simply replacing one crystal by

another. Hence it makes it easy for a radio transistor to work at different frequencies. Since the frequency of oscillation is set by the crystal, changes in the supply and transistor parameters does not affect the frequency of oscillation. 18. Define: Piezo electric effect. (May 2013)

When an ac voltage is applied across a quartz material crystal, it vibrates at the frequency of the applied voltage. Also, if a mechanical force is applied to vibrate a quartz crystal, it generates an ac voltage.19. Why L-C oscillators are not preferred to generate low frequency signals even though they have higher frequency stability compared to R-C phase shift oscillator?

For the generation of low frequency signals, the LC circuits become impracticable and the RC phase shift oscillators are more suitable. With the advantage IC technology RC network is the only feasible solution. It is very difficult to make an inductance that too of very high value in IC. Therefore RC oscillators are increasingly popular.20. What is the expression for frequency of oscillation of a wein-bridge oscillator? (May 2015)

fr =1/2πRC21. Write the disadvantages of negative feedback amplifiers and it can be overcome? (May 2015)

(i) The main disadvantage of negative feedback is decrease in overall gain. The gain and feedback factor in an amplifier are often functions of frequency, so the feedback may lead to positive feedback. (ii) May lead to instability if not designed carefully. (iii) The input and output impedances of the amplifier with feedback (the closed-loop amplifier) become sensitive to the gain of the amplifier without feedback (the open-loop amplifier); that exposes these impedances to variations in the open loop gain.22. What is a resonant circuit oscillator?

The Oscillators using resonant LC tank circuits are most often used for sources of radio frequency(RF) energy are called as resonant circuit oscillator.23. What is meant by un-sustained oscillation? When it will occur?

The electrical oscillations whose amplitude goes on decreasing with time are called un-sustained oscillation. The electrical system in which these oscillations are generated has losses and some energy is lost during each oscillation. Further, no means are provided to compensate for the losses and consequently the amplitude of the generated wave decreases gradually. 24. What is the advantage of a colpitt’s oscillator compared to a phase shift oscillator? (Nov 2015)

(i) Colpitt’s oscillator generates high frequencies ranges between 1 MHz and 500 MHz.(ii) Good stability at high frequency.

25. Write down the general applications of oscillators.a) As a local oscillator in radio receivers. b) In T.V receivers. c) In signal generators. d) As

clock generation for logic circuits. e) AM and FM transmitters. f) In phase lock loops.

Part-B1. Draw and describe the four types of topology for feedback of an amplifier. Derive the expression for gain with feedback. Mention the advantages of negative feedback amplifier. (16)(Nov 2013)2. With a neat diagram explain about Colpitt’s oscillator & derive the expression for frequency of oscillation and condition of oscillation. (16) (May 2015)3. With a neat diagram explain about Hartley oscillator & derive the expression for frequency of oscillation and condition of oscillation. (16) (Nov 2013,2014)4. (i) Explain the operation of crystal oscillator with neat diagram and write the expression for its frequency of oscillation. (10)(May 2012)

(ii) Acolpitt’s oscillator is designed with C1 = 100pf and C2 = 7500pf. The inductance is variable. Determine the range of inductance values, if the frequency of oscillation is to vary between 950 KHz and 2050 KHz. (6)5. (i) Explain the operation voltage shunt feedback amplifier and Deduce necessary expressions to design it. (10) (ii) Voltage shunt negative feedback amplifier has a voltage gain without feedback of A=500, input resistances (Ri)=3KΏ, output resistance Ro=20KΏ, and feedback ratio of β= 0.01. Calculate the voltage gain AF, input resistance (Rif) and output resistance (Rof) of amplifier with feedback. (6)6. With a neat circuit diagram, describe the working of a Wien bridge oscillator. Derive an expression for the resonant frequency. Give its advantages and disadvantages? (16)7. With neat circuit diagram explain the operation of an RC phase shift oscillator and derive the condition for oscillation and resonant frequency with BJT. (16) (Nov 2012) (Nov2014)8. Sketch the circuit diagram of a two stage capacitor coupled BJT amplifier that uses series voltage negative feedback. Briefly explain how the feedback operates. (16) (Nov 2015)9. Describe and explain the operation of the following Oscillators. (Nov 2015) (i) Wien bridge oscillator (5) (ii) Design a Wien bridge oscillator circuit to oscillate at a frequency of 20 kHz. (5) (iii) Crystal oscillator. (6) 10. Determine the Rif,Rof,,AvandAvffor the following. (Nov 2014) (i) Voltage shunt feedback amplifier. (8) (ii) Current series feedback amplifier. (8)(May 2015)