The Miller Effect

In a gain stage (common emitter) there is a limit to the achievable bandwidth at some set gain: i.e., the higher the gain, the lower the bandwidth; conversely, the lower the gain, the wider the bandwidth. This is the now famous, Gain Bandwidth Product.

The dominant mechanism for this is found in the intrinsic feedback capacitance, C_cb, between the collector and the base. The effect -- as frequency increases -- is to increase feedback via C_cb's capacitive reactance, XC_cb, thus reducing the overall gain.

To compound this problem: XC_cb is dependent on the intrinsic capacitance, C_cb, multiplied by the gain, i.e., as the gain is reduced, the bandwidth is increased.

There are ways of reducing this effect, such as peaking coils in the collector (X_l cancels X_c); pre-emphasis of the signal's higher frequencies at the input; frequency selective feedback, etc...