UNIT 3 : BIPOLAR JUNCTION TRANSISTORS

(BJT)

INTRODUCTION – BASIC BJTA Transistor is often called a Bipolar Junction

(BJT) or bipolar transistor. The name bipolar is adopted from its structure whereby it has ‘two junction’ and consist o three doped region either ‘n-p-n’ or ‘p-n-p’ combination.

BIPOLAR JUNCTION TRANSISTOR The bipolar junction transistor is a

semiconductor device constructed with three doped regions.

These regions essentially form two ‘back- to-back’ p-n junctionsin the same block of semiconductor material (silicon).

The most common use of the BJT is in linear amplifier circuits (linear means that the output is proportional to input). It can also be used as a switched (in, for example, logic circuits).

STRUCTURE AND SYMBOLS FOR BJTa. 3 Layer semiconductor device consisting:

- 2 n – and 1 p-type layers of material >NPN transistor

- 2 p- and 1 n-type layers of material > PNP transistor

b. The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material.

c. A single PN junction has two different types of bias: - forward bias - reverse bias

STRUCTURE AND SYMBOLS FOR BJTNPN

PNP

PHYSICAL STRUCTURE FOR BJTConsist of 3 alternate layers of n- and p-type

semiconductor is called emitter (E) , base (B) and collector (C).

Majority of current enters collectors , crosses base region and exits through emitter. A small current also enters base terminal, crosses base emitter junction and exits through emitter.

Carrier transport in the active base region directly beneath the heavily doped(n) emitter dominates i-v characteristics of BJT.

TRANSISTOR OPERATIONThe basic operation will be described using the

pnp transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged.

One p-n junction of a transistor is reverse- biased,whereas the other is forward-biased.

TRNSISTOR OPERATION Majority carriers can cross the reverse-

biased junction because the injected majority carriers in the n-type material.

Applying KCL to the transistor : IE = IC + IB

The comprises of two component – the majority carriers

IC = Icmajority + ICOminority

ICO – IC current with emitter terminal open and is called leakage.

EXAMPLE CONNECTION

TRANSISTOR OPERATIONWe will consider npn transistorsPnp devices are similar but with different polarity

of voltage and currents.When using npn transistors.

Collecter is normally more positive than the emitterVCE might be a few voltsDevices resembles two back – to – back diodes – but has

very different characteristicsWith the base open – circuit negligilbe current flows from

the collector to the emitter.

BJT OPERATES AN AMPLIFIER AND SWITCHA simple transistor amplifier - Rb is used to ‘bias’ the transistor by

injecting an appropriate base currentC is a coupling capacitor and is used to

couple the AC signal while preventing external circuits from affecting the bias

AC-coupled amplifier - Vb is set by the conduction voltage of the base-

emitter junction and so is about 0.7V - voltage across Rb is thus Vcc- 0.7 - this voltage divided by by Rb gives the base

current Ib - the collector current is then given by Ic= hFE

Ib - the voltage drop across Rc is given by IcRc - the quiescent output voltage is therefore Vo= Voc- IcRc

BJT OPERATES AS AN AMPLIFIER AND SWITCH

Base current is small, so

Vb= Vcc R2/R1+R2= 10 10kΩ/27kΩ+10kΩ =2.7V

Emitter voltage Ve= Vb- Vbe =2.7-0.7 =2.0VSince Ib is small,collector current Ic=Ie=

2mAOutput voltage =Vcc-IcRc = 10- 2mA x

2.2kΩ= 5.6V

A common-collector amplifier - unity gain - high input resistance - low output resistance - a very good buffer amplifier

Regions of BJT operations - Cut-off region: The transistor is off. There is no conduction

between the collector and the emitter. (Ib= 0 therefore Ic=0) - Active region: The transistor is on. The output current(Ic)

is relatively insentive to Vce. In this region the transistor can be an amplifier.

- Saturation region: The transistor is on. The collector

current varies very little with a change in the base current in the saturation region. The collector crrent is strongly dependt on Vce unlike the active region. It is desirable to operate transistor switches in or near the saturation region when in their state.

A common –emitter configuration - emitter is a common or reference to both input

and output terminals - emitter is usually the terminal closest to or at

ground potentialalmost amplifier design is using connection of CE

due to the high gain for current and voltageTwo set of characteristics are necessary to

describe the behaviour for CE: input(base terminal) and ouptut (collector terminal) parameters.

Common-emitter configurationActive region Saturation region Cut-off region

• B-E junction is forward bias•C-B junction is reverse bias•Can be employed for voltage, current, and power amplification

•B-E and C-B junction is forward bias, thus the value of Ib and Ic is too big.•The value of Vce is so small•Suitable region when the transistor as a logic switch•NOT and avoid this region when the transistor as an amplifier.

•Region below Ib= 0µA is to be avoided if an undistorted o/p signal is required•B-E junction and C-B junction is reverse bias•Ib= Iceo where is this current flow when B-E is reverse bias.

Common-base configurationActive region Saturation region Cut-off region

•Ie increased,Ic increased•BE junction forward bias and CB junction reverse bias•Ic is not depends on Vcb•Suitable region for the transistor working as amplifier

•BE and CB junction is forward bias•Small changes in Vcb will cause big different to Ic•The allogation for this region is to the left of Vcb= 0V

•Region below the line of Ie= 0A•BE and CB is reverse bias•No current flow at collector,only leakage current

Common-collector configurationalso called emitter-follower (EF)Is called common-emitter configuration since

both signal source and the load share the collector terminal as a common connection point

Output voltage is obtained at emitter terminalInput characteristic is similar with common-

emitter configurationProvided with the load resistor connected from

emitter to groundPrimarily for impendance-matching purpose since

it has high impedance an low output impedance

DEFINE – β DC AND β ACRatio of dc collector current(IC) to the dc base

current (IB) is dc beta(βdc) which is dc current gain where IC and IB are determined at a particular operating point, Q-point(quiescent point).

30< βdc<300 2N3904

On data sheet, βDC = hFE with h is derived from ac hybrid equivalent cct.FE are derived from forward-current amplification and common-emitter configuration respectively.

βdc IB

Ic

For ac conditions an ac beta has been defined as the changes of collector current(Ic) compared to the changes of base current6(IB).

On data sheet, βac=hFE

Beta(β) In the dc mode the level of Ic and Ib are related by

a quantity called beta and defined by the following equation:

βdc =Ic/IB

For the ac situation an ac beta has been defined as follows:

βac=∆ Ic/∆IB

β indicates the amplification factor of transistor. (β is sometimes refer as hfe,a term used in transistor modeling calculations)

Relationship between β and α

Both indicate an amplification factor.

α= β/ β +1

β=α/α-1

β provides a Relationship between Currents

Ic= β*IB

IE=(β+1)IB

Voltage-divider bias Because the base current is small, the

approximatioon VB=[R2/R1+R2] Vcc is useful for calculating the base voltage.

After calculating VB, you can find VE by subtracting 0.7V for VBE.

Next, calculate IE by applying Ohm’s law to RE :

IE=VE/RE

Then apply the approximation Ic=IE

Finally, you can find the collector voltage from Vc=Vcc-IcRc