Several different types of logarithmic amplifiers are used in receivers. The logarithmic circuits under consideration here are high-frequency types intended for operation at video frequencies up to 50 MHz and at receiver intermediate frequencies up to 1 GHz. At these frequencies, the logarithmic characteristic has to be approximated using multi-stage circuits. Three well-established approximation techniques are examined. Unfortunately logarithmic amplifiers of this type are not easily examined analytically because of the difficulties in dealing with both the non-linear aspects and the approximation techniques involved. For this reason, the techniques are discussed in general, simple terms. Complete logarithmic amplifiers are commercially available. They are commonly based on discrete-component hybrid construction and achieve high levels of performance. As systems become more complex, demands on space and weight increase, the ability to tailor construction of receivers to specific equipment requirements becomes more important. However, designing and building discrete-component logarithmic amplifiers is a complex and difficult task. Specialised integrated circuits (ICs) are available and these simplify the task of design and construction of amplifiers to meet individual requirements. The use of ICs provides the advantages of reproducibility, small size, greatly reduced component count and enhanced reliability. The general types of logarithmic ICs both current and future, are indicated.

Inspec keywords: approximation theory; video amplifiers; receivers; integrated circuits

Other keywords: integrated circuits; logarithmic receivers; receiver intermediate frequencies; discrete-component hybrid construction; video frequencies; logarithmic amplifiers; high-frequency logarithmic circuits; approximation techniques; multistage circuits

Subjects: Radio and television receivers