The correct answer is A. Miller effect.
The Miller effect is a phenomenon that occurs in transistor amplifiers when the output signal is fed back to the input through the capacitance between the collector and base terminals. This feedback can cause the amplifier gain to vary with frequency, resulting in a phenomenon known as frequency distortion.
The Miller effect is caused by the fact that the collector-base capacitance is effectively in parallel with the load capacitance. This means that the effective load capacitance seen by the transistor is larger than the actual load capacitance, and this can cause the amplifier gain to decrease at higher frequencies.
The Miller effect can be reduced by using a smaller collector-base capacitance, or by using a circuit that isolates the output signal from the input signal.
The other options are incorrect because:
- Option B: The presence of external and internal capacitance can cause the amplifier gain to vary with frequency, but this is not the main cause of frequency distortion.
- Option C: The logarithmic increase in the output signal is a characteristic of all amplifiers, and it does not cause frequency distortion.
- Option D: An inter stage transformer can be used to reduce the effect of the Miller effect, but it is not the main cause of frequency distortion.