Transistor Amplifiers and the Miller Effect

The bipolar junction transistor (BJT) is a three-terminal semiconductor device whose collector current is controlled by a small base current. This current control is the basis of amplification: a weak signal applied to the base produces a much larger, scaled copy in the collector circuit. To work as an amplifier the transistor must be biased into its active region, where the relationship between input and output stays roughly linear and the output faithfully follows the input waveform.

How the transistor is connected determines its behaviour. In the common-emitter configuration the input is at the base and the output at the collector, giving substantial voltage and current gain along with signal inversion; it is the most common amplifying arrangement. The common-collector or emitter-follower offers near-unity voltage gain but high current gain and a low output impedance, making it a useful buffer. The common-base arrangement has low input impedance and is favoured at high frequencies. Each is chosen according to the required gain and impedance levels.

At higher frequencies the gain of an amplifier falls, and the Miller effect is a key reason. The small capacitance between the input and output of an inverting stage appears at the input multiplied by the stage's voltage gain, so its effective value can be many times larger than the physical capacitance. This enlarged input capacitance forms a low-pass filter with the source resistance and rolls off the high-frequency response. Designers reduce the Miller effect with techniques such as the cascode configuration, which lowers the voltage swing across the critical capacitance.

Frequently asked questions

What is the Miller effect?

It is the apparent multiplication of the capacitance between an inverting amplifier's input and output, so that this capacitance loads the input by a factor proportional to the stage's voltage gain.

Why does the Miller effect limit high-frequency response?

The enlarged effective input capacitance combines with the source resistance to form a low-pass filter, which reduces gain as the frequency rises.

Which transistor configuration gives the most voltage gain?

The common-emitter configuration provides high voltage and current gain with signal inversion, which is why it is the usual choice for general amplification.