5 - Logarithmic amplifiers and related circuits

Abstract

This chapter discusses the use of nonlinear components to provide nonlinear relationships. The techniques used to achieve nonlinear amplification generally fall into two categories. In one method, the desired nonlinear response is synthesized by a network that uses a number of semilinear elements (piecewise linear). The other method uses the inherent nonlinearity of semiconductors. The nonlinear effects of diodes and transistors are often used to obtain logarithmic amplification. The logarithmic performance obtained by using an op-amp with nonlinear components is influenced by the characteristics of both the amplifier and the nonlinear component. The lower limit to the range of a logarithmic converter is, in many DC applications, determined by the op-amp offsets rather than by the logarithmic range of the transistor. Initial op-amp offsets may be balanced. Errors then may occur owing to offset drifts. Separate biasing of voltage and current offsets reduces errors and gives maximum logarithmic range. Voltage bias should be applied to the noninverting input of the op-amp. Current bias should be applied to the inverting input of the op-amp. Log and antilog converters can also be combined to generate a variety of both linear and nonlinear functions. The circuits are interconnected in manner that they perform operations that are normally involved in logarithmic computations.