Cascode Amplifiers
by Dennis L. Feucht
Innovatia Laboratories
Two-transistor combinations, such as the Darlington configuration, provide advantages over single-transistor amplifier stages. Another two-transistor combination in the analog designer's circuit library combines a common-emitter (CE) input configuration with a common-base (CB) output. This article presents the design equations for the basic cascode amplifier and then offers other useful variations. (FETs instead of BJTs can also be used to form cascode amplifiers.) Together, the two transistors overcome some of the performance limitations of either the CE or CB configurations.
Basic Cascode Stage
The basic cascode amplifier consists of an input common-emitter (CE) configuration driving an output common-base (CB), as shown.
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