The emitter diode of a transistor has an equivalent resistance for ac signals. The ac resistance of the emitter diode, designated = 25 mV/IE.
In a common-emitter amplifier, the input signal is applied to the base and the output signal is taken from the collector.
A common-emitter amplifier provides a large voltage gain, a large current gain, and a very high power gain.
The input and output ac voltages in a common-emitter amplifier are 180° out of phase.
In the ac equivalent circuit of a transistor amplifier, the coupling and bypass capacitors and dc voltage sources appear as ac shorts.
In the ac equivalent circuit of a transistor amplifier, the emitter diode is replaced with its equivalent ac resistance, r'e.
The voltage gain of a common-emitter amplifier equals rL/r'e when the dc emitter resistance is completely bypassed. The ac load resistance, designated rL, equals the equivalent resistance of RC and RL in parallel.
A swamping resistor is an unbypassed resistance in the emitter circuit of a common-emitter amplifier. The swamping resistor stabilizes the voltage gain and reduces distortion.
The input impedance of a common-emitter amplifier equals Zin(base) || R1 || R2. The output impedance equals RC.
A common-collector amplifier provides a large current gain and a large power gain, but its voltage gain is approximately one or unity.
In a common-collector amplifier, the input signal is applied to the base, and the output signal is taken from the emitter. Because the ac signal at the emitter follows, or is in phase with, the ac signal at the base, the common-collector amplifier is also referred to as an emitter follower.
An emitter follower has high input impedance and low output impedance. This makes it ideal for impedance-matching applications.
In a common-base amplifier, the input signal is applied to the emitter, and the output signal is taken from the collector.
A common-base amplifier has high voltage and power gain, but its current gain is slightly less than one.
The main drawback of the common-base amplifier is its extremely low input impedance which is approximately equal to the low value of r'e.
