What is "cascode"

The cascode is a two-stage amplifier comprising a common emitter stage feeding into a common base stage.

Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance, higher gain or higher bandwidth.

In modern circuits, the cascode is often constructed from two transistors ( BJTs or FETs), with one operating as a common emitter or common source and the other as a common base or common gate. The cascode improves input-output isolation (or reverse transmission) as there is no direct coupling from the output to input. This eliminates the Miller effect and thus contributes to a much higher bandwidth.