In common collector amplifier, we know that the collector is commons since it is connected to AC ground. Now, the characteristics of a common collector amplifier is that it has unity voltage gain, to be exact, it is less than 1 due to internal resistance of the transistor, and has high input impedance and low output impedance. Now, the problem is, whenever i try to add a 8 ohm load (my speaker), the voltage gain drops drastically. I thought common collector amplifier is immune to loading effects since it is use as a buffer. What is happening? (It appears to me that it is suffering the same fate as common emitter (which drops voltage when a load is connected to it directly))
Electronic – common collector amplifier
amplifiertransistors
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Best Answer
A CC is not immune to loading since it has a low, but non-zero, output impedance.
The small signal output impedance of a CC amplifier is roughly:
\$\dfrac{V_T}{I_E} + \dfrac{R_{tb}}{\beta + 1}\$
where
\$V_T \approx 25mV \$ (assuming room temperature)
and \$R_{tb}\$ is the equivalent AC resistance connected to the base.
For example, if \$I_E = 1mA\$, the output impedance is at least 25 ohms.
So, while the output impedance of a CC amplifier is much lower than a CE amplifier, it is not immune to loading effects.