4 – RF amplifiers

Publisher Summary This chapter discusses small signal radio frequency amplifiers and preamplifiers. These circuits are used to amplify radio signals from antennas prior to input to the mixer, in order to improve signal-to-noise ratio and front-end selectivity. A preamplifier is simply a radio frequency (RF) amplifier which is external to the receiver, rather than being built in. The performance of some radio receivers can be improved by the use of either a preselector or preamplifier between the antenna and the receiver. Most low priced receivers suffer from performance problems that are a direct result of the trade-offs the manufacturers have to make in order to produce a low cost model. In addition, older receivers often suffer the same problems, as do many homebrew radio receiver designs. The weakest radio signal that one can detect on a receiver is determined mainly by the noise level in the receiver. Some noise arrives from outside sources, while other noise is generated inside the receiver. Most RF amplifiers use bipolar junction transistors (BJT) or field effect transistors (FET). These may be discrete, or part of an integrated circuit.