7 - RF small-signal circuitry

Abstract

This chapter focuses on the radio-frequency (RF) small-signal circuitry design. A fairly exact circuit design can be effected using an RF oriented computer aided design (CAD) package, which would probably have a model of the transistor to be used in its component library. The results of the simulation give a fair idea of the performance to be expected from the hardware as built, provided great care be taken in the practical layout to avoid introducing parasitic capacitive and inductive elements, which do not appear in the circuit as modeled. With the continued heavy usage of the HF band, RF stages with their front-end selectivity provided are making their come back. However, an RF amplifier with both input and output circuits tuned needs very careful design to ensure stability, especially when using the common emitter configuration. The potential source of trouble is the collector/base capacitance, which provides a path by which energy from the output tuned circuit can be fed back to the base input circuit. A common technique for increasing the stability margin of an RF amplifier is mismatching. This simply means accepting a stage gain less than the maximum that could be achieved in the absence of feedback. In particular, if the collector (or drain) load is reduced, the stage have a lower voltage gain. However, the emitter follower has a reputation for instability unless care is taken in the layout and decoupling of the stage. In particular, if an emitter follower drives a mainly capacitive load, it exhibits an input impedance having a negative resistance component.