A useful pseudo‐logarithmic circuit

Abstract

Purpose

This paper proposes a useful pseudo‐logarithmic circuit as a basic building block in the construction of a logarithmic amplifier made from piecewise approximations.

Design/methodology/approach

The circuit employs two operational and a handful of resistors, and mimics the logarithmic function over a predefined range. Control of the pseudo‐logarithmic function is achieved by a ratio of resistances defined as x, one of which may be digitally switched, or implemented by tunable transconductors. When controlled by digitally switched resistors, the circuit is particularly attractive because of the commercial availability of such resistors, and with the aid of simple control logic, individual blocks can be algebraically summed to extend the dynamic range of the basic pseudo‐logarithmic block which is 28 dB

Findings

Experimental results using off the shelf operational amplifiers (opamps) and a handful of resistors show that the circuit yields a maximum log error of 0.6915 dB for x in (0.22, 4.65).

Originality/value

Proposes a novel circuit capable of realizing the pseudo‐logarithmic function (x−1)/(1+x). The circuit is simple and easily implemented using readily available opamps.

Citation

Maundy, B., Westwick, D. and Gift, S. (2007), "A useful pseudo‐logarithmic circuit", Microelectronics International, Vol. 24 No. 2, pp. 35-45. https://doi.org/10.1108/13565360710745584