Cascode

The cascode is a two-stage amplifier that consists of 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 (reduces 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.

The use of a cascode (sometimes verbified to cascoding) is a common technique for improving analog circuit performance, applicable to both vacuum tubes and transistors. The name "cascode" was coined in an article written by Frederick Vinton Hunt and Roger Wayne Hickman in 1939, in a discussion on the application of voltage stabilizers. They proposed a cascode of two triodes (the first one with a common cathode setup, the second one with a common grid) as a replacement for a pentode, and so the name may be assumed to be a contraction of "cascaded triodes having similar characteristics to a pentode".

...
Wikipedia