ABSTRACT
This chapter describes whether power FETs or bipolar junction transistors (BJTs) are superior in power amplifier output stages. The chapter suggests the well-established power FETs, when used in conventional Class-B output stages, are a good deal less linear than BJTs. Gain deviations around the crossover region are far more severe for FETs than the relatively modest wobbles of correctly biased BJTs, and the shape of the FET gain-plot is inherently jagged, due to the way in which two square-law devices overlap. The incremental gain range of a simple FET output stage is 0.84 to 0.79, range 0.05, and this is actually much greater than for the bipolar stages; the emitter-follower stage gives 0.965 to 0.972 into 8 Ω, with a range of 0.007, and the complementary feedback pair gives 0.967 to 0.970 with a range of 0.003. FETs are more linear was based not on Class-B power-amplifier applications, but on the behaviour of a single device in Class-A.
