Common Emitter configuration of transistor, characteristics, application

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

Application:-

 It is used for as an amplification also same as common base configuration but, it is used in voltage amplification, current amplification, power amplification. Mostly it is used in current and power amplification. Therefore, this configuration is used the most.

Properties: -

1) The input impedance is the ratio of change in base emitter voltage (VBE) to change in base current when collector emitter voltage (VCE) is constant.

 R_i  = (ΔVBE / ΔIB)_VCE     

2) The output impedance is the ratio of change in collector emitter voltage (VCE) to change in collector current when base current (IB) is constant.

 Ro  ₌(ΔVCE / ΔIC)_IB   

3) Its voltage gain is high.

4) Its current gain is high.

5) Its power gain is high.

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

β =  ΔIC / ΔIB

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 base terminal of transistor is connected to the one side of base resistance (RB). Other side of base resistance (RB) is connected to the positive terminal of VBB.

CE configuration of NPN 

 Negative terminal of VBB is directly connected to the ground terminal. Emitter 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.

CE configuration of PNP

Working:-

The base emitter of the transistor is forward bias junction which we call VBE. And its collector emitter of the transistor is reverse bias junction which we call VCE.

Expression:-

 We had two equations in common base configuration.

IE = IB + IC ------------- (i)

IC = α.IE + I_CBO -------- (ii)

Put the value of IE from equation (i) in equation (ii) 

IC = α.(IB + IC) + I_CBO 

IC = α.IB + α.IC + I_CBO 

IC - α.IC = α.IB  + I_CBO

IC(1 - α) = α.IB  + I_CBO

 Dividing both side by (1 - α).

IC(1 - α) / (1 - α) = α.IB / (1 - α)  + I_CBO / (1 - α) 

IC = α.IB / (1 - α)  + I_CBO / (1 - α) --------- (iii)

We know that 

α = β / 1+β

Put the value of α in equation (iii)

 IC = β / 1+β .IB / (1 - β / 1+β )  + I_CBO / (1 - β / 1+β)

 IC = β / 1+β .IB / (1 - β + β / 1+β )  + I_CBO / (1 - β + β / 1+β)

 IC = β / 1+β .IB / (1 / 1+β )  + I_CBO / (1 / 1+β)

 IC = β / 1+β . (1+β / 1). IB  + I_CBO .1+β / 1

IC = β. IB  + I_CBO .(1+β) 

We know that what is I_CBO. It is the leakage current for common base configuration. But in case of common emitter configuration, the leakage current is the combination of  I_CBO .(1+β). We can say that 

I_CBO .(1+β) = I_CEO

I_CEO is the leakage current of common emitter configuration. It is the current between collector and emitter terminal when base terminal of the transistor is open ie IB =0, So 

IC = β. IB  + I_CEO 

This leakage current I_CEO  we can neglect because it much smaller than IB 

So    IC = β. IB

        β = IC / IB                                   

 :-  β =  Current amplification factor for common emitter

The value of β is lies between 20 to 500.

Input characteristics of common emitter configuration:-

Input characteristics of CE configuration

It is the curve between base current (IB) and voltage of base and emitter (VBE) at constant voltage of collector and emitter (VCE). 

The first thing to do is to keep the VCE value constant. Then we have to increase the value of VBB gradually, as given in fig.

Input characteristics Chart of CE configuration

As we increase the value of VBB, the value of IB and VBE also increases with it. Similarly, by keeping the constant value of VCE three times or as you can do for many times than we get the reading of IB and VBE as shown in fig.

Output characteristics of common emitter configuration:-

Output characteristics of CE configuration

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

Output characteristics chart of CE configuration

As soon as we increase VCC, IC is gradually increase and IC much be larger than IB. The value of VCE also increase slowly. Similarly, by keeping the constant value of IE three times or as you can do for many times than we get the reading of IC and VCE as shown in fig.

Waveform:-

Its input waveform has a low signal and in its output waveform we get a high signal. But its input and output waveform invert each other.

Click to know about Common Base configuration

SHARE

Ishan Kumar