Cascode

The circuit on the right is the usual configuration, and the circuit on the left is the cascode configuration. At low frequencies both behave nearly identically, but at very high frequencies the cascode performs much better. This is because the upper transistor keeps the lower transistor’s collector voltage nearly constant which eliminates the miller effect. The 1nF capacitors represent the parasitic capacitances of the transistors from their collectors to their bases and are responsible for the miller effect. Note that the miller effect doesn’t actually reduce gain directly, but rather decreases the input resistance at high frequencies. Because of the resistance of the input sources (100Ω here) this means a reduced input signal, and therefore reduced output signal. The cascode configuration is better in this regard because it prevents the input resistance from dropping as quickly with frequency.

published 3 years ago