Common Base configuration of transistor, characteristics, application

Friends, today we will read about the common base configuration of BJT transistor. In this lecture, we will know its diagram, its characteristics and its application. We call it common base, because the base terminal is the common terminal for input and output. In this configuration, the emitter is input side and the collector is output.

Application:-

 It is used for only as a voltage amplification. Signal in input is low and get high signal from output. It is mostly used in RF amplifier and high frequency also. 

Properties: -

1) Its input impedance is very low.

 R_{i  = ΔVBE / ΔIE}        

2) Its output impedance is very high.

 Ro  _{= ΔVCB / ΔIC}

3) Its voltage gain is high.We can say that, we got high voltage gain in the output.

4) Its current gain is unity. That is, we get output equal to input.

5) It also has a current amplification factor, which we call α. α is known as the ratio of output current to input current.

α =  ΔIC / ΔIE

Construction Diagram:-

 As you know very well that for amplification we use transistor in active region. So we will understand the diagram of NPN and PNP transistor in active region very well. As shown in the diagram, the emitter terminal of transistor is connected to the one side of emitter resistance (RE). Other side of emitter resistance (RE) is connected to the negative terminal of VEE.

CB configuration of NPN

Positive terminal of VEE is directly connected to the ground terminal. Base of Transistor is also connected to the ground terminal. Collector of transistor is connected to the one side of collector resistance (RC). Other  side of collector resistance (RC) is connected to the positive terminal of VCC.  Negative terminal of VCC  is directly connected to the ground terminal.

CB configuration of PNP

Working:-

The base emitter of the transistor is the junction forward bias which we call VBE. And its collector base is a junction reverse bias which we call VCB. If we apply KCL then

IE = IB + IC 

i) When part of emitter current reaches to the collector current is α.IE. We can say that IC = α.IE

ii) In reverse bias mode of collector base junction, leakage current is created due to movement of minority charge carrier.

  IC = α.IE + I _leakage

We call this leakage current  I_{CBO .}  I_CBO   means current between collector and base terminal when emitter terminal is open ie IE =0, So 

  IC = α.IE + I_CBO 

Whatever I_CBO   is in it, we can ignore it because IE is much greater than I_CBO 

_So        IC = α.IE

           α =  IC / IE                                                :- α =  Current amplification factor

 The value of α is always smaller than 1. Its value varies between 0.95 to 0.98.

Input characteristics of common base configuration:-

It is the curve between emitter current (IE) and voltage of base and emitter (VBE) at constant voltage of collector and base (VCB). The first thing to do is to keep the VCB value constant. Then we have to increase the value of VEE gradually, as given in fig.

Input characteristics of CB configuration

Chart of input characteristics of CB configuration

As we increase VEE, the value of IE and VBE also increases with it. Similarly, by keeping the constant value of VCB three times, we get the reading of IE and VBE as shown in fig.

Output characteristics of common base configuration:-

It is the curve between collector current (IC) and voltage of collector and base (VCB) at constant emitter current (IE). The first thing to do is to keep the value of IE constant. Then we have to increase the value of VCC slowly, as given in fig.

Output characteristics of CB configuration

Chart of output characteristics of CB configuration

As soon as we increase VCC, IC is smaller than IE till some reading and later becomes equal to IE. But the value of VCB changes slowly. Similarly, by keeping the constant value of IE three times, we get the reading of IC and VCB as shown in fig.

Waveform:-

Its input waveform has a low signal and in its output waveform we get a high signal.

Click to know about Common Emitter configuration

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Ishan Kumar