16 - RF amplifiers and frequency-changers

Abstract

This chapter discusses the modulator and amplifier circuits to be found in the transceiver circuits. The term Radio Frequency (RF) includes both the ranges: normal RF range that is from-100kHz to -30MHz, and the VHF (very high frequency) range from -30MHz to -1000MHz. For amplifiers and modulators of this class, the input and output circuits may be in the form of an LC resonator, a wideband transmission-line transformer, or a ceramic or quartz filter. The term “frequency changer” refers to the conversion of the output of the RF or VHF stages to the Intermediate Frequency (IF) and back. However, the conversion from the audio base-band to the first IF in the single-sideband transmitter, and the corresponding demodulation, are both a form of frequency conversion. In addition to the modulators, demodulators, frequency changers, and filter circuits also include: RF, IF, and VHF amplifiers. The chapter describes the low-noise RF and VHF amplifiers used at the input of the related receivers, and the RF and VHF power output stages for the corresponding transmitters. While the VHF circuits are resonator coupled, giving operation essentially at one frequency only, the RF circuits use the transmission-line transformer, giving coverage of most of the RF range for any one transformer.