3 - FETs and BJTs: Comparison of Parameters and Circuitry

Abstract

Solid-state RF power transistors consist of two basic types of devices, namely bipolar junction and field effect. In certain applications, the bipolar junction transistor (BJT) will yield superior performance without any doubt, whereas in other areas, a field effect transistor (FET) will do a better job. Many types of FETs are commercially available for RF power use. These include static induction transistor (SIT), which is a version of a depletion mode junction FET, and the metal gate Schottlky FET (MESFET). The vertical channel silicon MOSFET is the most common RF power FET. One main difference between a BJT and a MOSFET in RF amplifier use is the need for base/gate bias voltage. Regarding impedance matching, the largest difference can be noticed at the base-emitter and gate-source impedances. Common emitter and common base circuit configurations are most widely used in RF amplifiers. The common emitter and common base circuits have very different gain characteristics. They exhibit good stability, good linearity, and a high power gain up to UHF. A common collector (emitter follower) circuit is widely used where high input and low output impedance levels are desired. As in a common base configuration, there is no phase reversal between the input and the output. Unlike the emitter follower, variations in the load impedance in a source follower are not reflected in the input. This makes the source follower suitable for RF power amplifier applications at least up to VHF. High power linear amplifiers are probably the most suitable application for this mode of operation.