Common Collector Amplifier

COMMON COLLECTOR AMPLIFIER

The collector terminal is common between the input and output terminals. The input is applied at the base and the output is taken across the emitter terminal.

Construction

i. The collector base junction acts as input and emitter base junction acts as output.

ii. The output voltage exactly follows the input voltage variations. Hence it is called "Emitter follower" amplifier.

iii. The load resistor R_L is capacitor coupled to the emitter terminal of the transistor.

iv. The circuit employs emitter current bias through the voltage divider resistors R₁ and R₂.

The capacitor C₁ and C₂ act as input and output coupling capacitors.

Operation

i. When an ac signal is applied to transistor base via C₁, V_B is increased and decreased as the signal varies from positive to negative voltage variations.

V_B – V_BE

ii. Thus the output voltage from a common collector circuit is same as its input voltage. So the CC amplifier has unity voltage gain.

Analysis of Emitter Follower Amplifier

Input Impedance

From Fig.2.22, Applying KVL

The Input voltage

Substitute

The input impedance is given by

Output Impedance

The input signal is assumed to be zero

The output impedance

Voltage Gain

Current Gain

Current gain 

Applying current division rule,

Substitute I_B value in I_E

for CC amplifier R_B >> Z_B

Power Gain

Characteristics of CC Amplifier

i. Provides current gain and power gain

ii. high input impedance and very low output impedance

iii. No voltage gain.

Applications

1. As buffer amplifier since voltage gain is unity