What is a cascode amplifier and what are its advantages?

A cascode amplifier stacks a common-emitter (or common-source) stage with a common-base (or common-gate) stage. Advantages include: very high output impedance (product of individual output impedances), improved high-frequency response by reducing Miller effect, higher voltage gain, and better isolation between input and output. It is widely used in RF amplifiers, current sources, and high-gain stages where bandwidth and output impedance are critical.