Common Emitter Configuration
In this configuration we use emitter as common terminal for both input and output. This common emitter configuration is an inverting amplifier circuit. Here the input is applied between base-emitter region and the output is taken between collector and emitter terminals. In this configuration the input parameters are VBE and IB and the output parameters are VCE and IC.This type of configuration is mostly used in the applications of transistor based amplifiers. In this configuration the emitter current is equal to the sum of small base current and the large collector current. i.e. IE = IC + IB. We know that the ratio between collector current and emitter current gives current gain alpha in Common Base configuration similarly the ratio between collector current and base current gives the current gain beta in common emitter configuration. This configuration is mostly used one among all the three configurations. It has medium input and output impedance values. It also has the medium current and voltage gains. But the output signal has a phase shift of 1800 i.e. both the input and output are inverse to each other.
Now let us see the relationship between these two current gains.
Current gain (α) = IC/IE
Current gain (β) = IC/IB
Collector current IC =α IE = βIB
The input characteristics of common emitter configuration are obtained between input current IB and input voltage VBE with constant output voltage VCE. Keep the output voltage VCE constant and vary the input voltage VBE for different points, now record the values of input current at each point. Now using these values we need to draw a graph between the values of IB and VBE at constant VCE. The equation to calculate the input resistance Rin is given below.
Rin = VBE/IB (when VCE is at constant)
The output characteristics of common emitter configuration are obtained between the output current IC and output voltage VCE with constant input current IB. Keep the base current IB constant and vary the value of output voltage VCE for different points, now note down the value of collector IC for each point. Plot the graph between the parameters IC and VCE in order to get the output characteristics of common emitter configuration. The equation to calculate the output resistance from this graph is given below.