Common Emitter Configuration: CE Mode

The transistor is used as an amplifier. But in amplifier circuit there two input terminals and two output terminals. However, the transistor has only three terminals. So any one terminal of the transistor is KEPT COMMON to the input and output terminals of amplifier circuit. Thus there are three possible configurations of transistor, common base (CB), common emitter (CE) and common collector (CC) configurations.

Typical Circuit

In this circuit, the emitter is common to both input and output terminals of amplifier circuit. Thus, the input signal is connected between base & emitter and the amplified output signal is taken between collector & emitter, so that emitter is common.

Mathematical Analysis

The small input voltage V_in is amplified by the transistor and we get output voltage V_o across collector-emitter of the transistor. In this configuration, when input signal is positive going, output is negative going and when input is negative output is positive going, as shown in above wave diagram. So the input and output signals are OUT OF PHASE i.e. input and output are 180° phase difference between them. Hence the circuit is called as inverting amplifier circuit.

In this circuit, input current is I_b and output current is I_c. So the current gain of the circuit is the ratio of collector current to its base current.

Current gain = I_c / I_b since I_c > I_b so current gain is large

Input resistance = V_in / I_b as I_b is very small, input resistance is very high

Output resistance = V_o / I_c as I_c is very large, output resistance is low

Hence finally we have –

Voltage gain = V_o / V_in = ( I_c × R_c ) / ( I_b × R_b )

Exercise

1. In common emitter amplifier circuit, find the value of β, if α = 0.9, α = 0.98 and α = 0.99.
   (Ans: For α = 0.9, β = 9; α = 0.98, β = 49 and α = 0.99, β = 99)
2. Calculate the emitter current in a CE amplifier if the value of β = 50 and base current is equal to 20µA. (Ans: 1.02mA)
3. For a transistor β = 45 and voltage drop across 1kW resistor connected in output circuit is 1V. Then find the value of base current for CE configuration amplifier circuit. (Ans:  0.022mA)

CB, CE and CC Comparison