Amplifier is a circuit which gives power gain.
||Common base amplifier
||Common emitter amplifier
||Common collector amplifier
||Medium high high
|Current gain (Ai)
||Ai = α < 1
||Ai β > 1
||Ai = (β + 1) > 1
|Voltage gain (AV)
||Av = α RL / Ri >1 = Ai RL/re >1
||Av = βRL / Ri > 1 = Ai RL / re > 1
||Av = (β +1) RL / Ri < 1 = Ai RL / Rb < 1
|Power gain Ap
||Ap = Av. Ai = α2 Rl / re > 1 nil
||Ap = β2 RL / rb > 1 1800 or π - rad
||Ap = (β + 1)2 RL / rb > 1 nil
|Input and output
It is clear from the above table that common base amplifier is a voltage amplifier as it amplifies only voltage.
Common collector is a current amplifier as it amplifies only current.
Common emitter amplifies both current and voltage. It gives a phase shift of 1800 or π - radian between input and output signal. Common collector amplifier is also known as emitter follower or a buffer amplifier.
Amplifiers may be of four types
(i) Class A
(ii) Class B
(iii) Class AB
(iv) Class C
Class A amplifier complete signal O – 3600 is amplified therefore. It is used when the signal is small that is at the input stage or first stage.
Class B amplifier class B amplifiers amplify half the signal O – 1800 or 1800 to 3600.
Normally output stage of an audio amplifier is class B. rather, class – B push – pull amplifier.
Class AB amplifier class AB amplifiers amplify more than 1800 but less than 3600 of the signal.
Class C amplifiers amplify only pulses when the signal is large. Class c amplifiers are used in transmitters.
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