The Common Collector Configuration - GATE EE Analog and Digital Electronics

When no signal is applied, then the ratio of emitter current to base current is called as ϒ_dc of the transistor. As the collector is common to both input and output circuits, hence the name common collector configuration.

The current amplification factor (β) is given by I_C//I_B. When no signal is applied, then the ratio of collector current to the base current is called current amplification factor of a transistor. β is an ac base amplification factor. α is called as current amplification factor. The relation of I_C and I_B change as I_C= βI_B+ (1+ β) I_CBO.

It has a high input resistance and very low output resistance so the voltage gain is always less one. It is used for driving a low impedance load from a high impedance source.

The most important use of CC transistor is an impedance matching device. It is seldom used for amplification purposes. The current gain is same as that of CE configured transistor.

The CC transistor has a very low value of output resistance of 25 Ω. The voltage gain is always less one. It is used for driving a low impedance load from a high impedance source.

 r_o=∆V_CE/∆I_C
=3/0.3m=10kΩ.

 r_o=∆V_BE/∆I_B
=300m/100µ=30kΩ.

The point which represents the values of I_C and V_CE that exist in a transistor circuit when no signal is applied is called as operating point. This is also called as working point or quiescent point.

When V_CE is very low, the transistor said to be saturated and it operates in saturated region of characteristic. The change in base current I_B does not produce a corresponding change in the collector voltage I_C.

The ratio of change in base emitter voltage (∆V_BE) to resulting change in base current (∆I_B) at constant collector emitter voltage (V_CE) is defined as input resistance. This is denoted by r_i.