Cascode

A cascode is a two-transistor amplifier arrangement in which the input transistor is configured as a common-emitter (BJT) or common-source (MOSFET) stage, and its output drives a second transistor arranged as a common-base (BJT) or common-gate (MOSFET). The second transistor provides a buffering action that keeps the voltage at the first transistor’s output relatively constant, which improves isolation between input and output and enhances the frequency response.

A central advantage of the cascode is the suppression of the Miller effect. By fixing the voltage at the input transistor’s collector (or drain) with the cascode transistor, the effective input capacitance is reduced, allowing higher bandwidth. The configuration also yields a higher output impedance and can result in higher gain for a given load. In simple terms, the overall gain is approximately the product of the transconductances and impedances of the two devices, while the dominant bandwidth-limiting pole moves to a higher frequency.

Common applications include radio frequency RF amplifiers, mixers, and various analog integrated circuits. Cascodes are valued for their wide bandwidth, improved input-output isolation, stability, and linearity. They are especially prevalent in integrated circuit design because the two devices can be implemented on the same chip with favorable biasing and isolation characteristics.

Variants of the cascode exist for different technologies: a BJT cascode uses a common-emitter input stage with a common-base cascode, while a MOS cascode uses a common-source input with a common-gate cascode. More stages can be stacked to achieve higher gain, though at the cost of increased biasing complexity.