Question

Explain the working of a CC amplifier?

Answer 1

The circuit of a single stage common collector amplifier using npn transistor is shown below:

The ac is applied between base and collector terminals collector circuit. The collector of the transistor is at ac ground as R_c = 0. V_cc, R₁, R₂, R_E provide voltage divider DC bias with constant operating point in the active region. The output is taken across the emitter resistor R_E. C₁ and C₂ are the coupling Capacitors which block DC and allow ac signal.

During positive half cycle of input signal, forward bias on emitter base junction increases. Hence the base current emitter current and I_ER_E increases which is output voltage. Hence we get positive half cycle at the output.

During negative half cycle, forward bias between emitter base junction decreases. Hence, I_B, I_E and I_eR_e decrease which is the output voltage. Thus negative half cycle is obtained. The input and out signals are in phase. CC amplifier is also called emitter follower because emitter voltage follows the base voltage.
V₀ = I_ER_E = V_in - V_BP
V₀ = V_in - V_BP  is very small
∴ V₀ ≈ V_in
Voltage gain
\(A_V=\frac{V_0}{V_{in}}=1\)
CC amplifier is used as impedance matching device.