Common Base Configuration: CB 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

As shown in the following circuit, the base is common to both input and output terminals of amplifier circuit. Thus the input signal is connected between emitter & base and the amplified output signal is taken between collector & base.

Mathematical Analysis

The small input voltage V_in is amplified by the transistor and we get output voltage V_o across the output resistor R_c. In this configuration, when input signal is positive going, output is also positive and when input is negative output is also negative, as shown in above wave diagram. So the input and output signals are IN PHASE i.e. input and output have 0° phase difference between them. Hence the circuit is called as non-inverting amplifier circuit.

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

Current gain = I_c / I_e since I_c > I_e current gain is always less than unity

Input resistance = V_in / I_e as I_e is very large, input resistance is small

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_e × R_e )

Exercise

1. In common base amplifier circuit, if current gain is 0.9 and emitter current is 1mA, then calculate base current. (Ans: I_b = 0.1mA)
2. In CB amplifier, collector current is 0.95mA and base current is 0.05mA. Then find the value of current gain α.  (Ans: α = 0.95)
3. Find input & output resistance and current gain (α)and (β) of the transistor in CB amplifier circuit if input voltage of 2V and output voltage of 6V with input current 20mA output current 18mA. (Ans: 100Ω, 333.33Ω, α = 0.9, β = 9)

CB, CE and CC Comparison