What is the value of the DC current gain in common collector configuration?

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ALP CBT 2 Electronic Mechanic Previous Paper: Held on 21 Jan 2019 Shift 1

Answer 1

Common collector configuration, also known as emitter follower provides high input impedance and low output impedance. So they are used for the purpose of impedance matching.
In common collector configuration, the collector terminal is common to both input and output terminals.

The DC current gain is therefore given by the ratio of emitter current to the base current, i.e.

\(\gamma=\frac{I_E}{I_B}\)

IE = Emitter Current

IB = Base Current

Also, IE = IB + IC

DC current gain will be:

\(\gamma=\frac{I_C+I_B}{I_B}=\frac{I_C}{I_B}+1\) ---(1)

The DC current gain for a common emitter configuration is defined as:

\(\beta=\frac{I_C}{I_B}\)

Equation (1) now becomes:

\(\gamma=\beta +1\)

26 June 1

Important Differences between different transistor configuration is as shown:

Parameter	Common-Base	Common-Emitter	Common-Collector
Input Current	IE	IB	IB
Output Current	IC	IC	IE
Current Gain	\({α _{dc}} = \frac{{{I_C}}}{{{I_E}}}\)	\({\beta _{dc}} = \frac{{{I_C}}}{{{I_B}}}\)	\(\gamma = \frac{{{I_E}}}{{{I_B}}} = \left( {1 + {\beta _{dc}}} \right)\)
Voltage Gain	Medium	Medium	Less Than 1