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Reduced Emitter Resistance Error in Translinear Circuits

IP.com Disclosure Number: IPCOM000061484D
Original Publication Date: 1986-Aug-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 51K

IBM

Related People

Bond, PW: AUTHOR [+2]

Abstract

This article describes a circuit which reduces the emitter resistance error of translinear circuits through the use of different emitter area ratios for the bipolar semiconductor devices used in the core of the circuit. Translinear circuits are an extremely useful classification of circuits that perform algebraic functions on input currents [*]. For example, a translinear principle can be used to provide a square- law function to control the linearity and focus of a CRT electron beam. Simply stated, translinear circuits make use of the logarithmic diode law characteristic of bipolar devices. Simple circuit configurations essentially take the logarithm of various input currents and then take the anti-logarithm of a developed voltage to perform various mathematical functions on the input currents. Fig.

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Reduced Emitter Resistance Error in Translinear Circuits

This article describes a circuit which reduces the emitter resistance error of translinear circuits through the use of different emitter area ratios for the bipolar semiconductor devices used in the core of the circuit. Translinear circuits are an extremely useful classification of circuits that perform algebraic functions on input currents [*]. For example, a translinear principle can be used to provide a square- law function to control the linearity and focus of a CRT electron beam. Simply stated, translinear circuits make use of the logarithmic diode law characteristic of bipolar devices. Simple circuit configurations essentially take the logarithm of various input currents and then take the anti-logarithm of a developed voltage to perform various mathematical functions on the input currents. Fig. 1 is a simplified schematic of a square-law cell, and is an illustration of the translinear principle. R represents the extrinsic emitter resistance of the devices, and may be assumed to be zero for the moment. All devices are of equal area. Summing base emitter voltage drops around the loop, we obtain Io = (Iin)2/IR where Iin is the input current, IR is a reference current, and Io is the output current of the cell. There is a significant error in translinear circuits caused by the emitter resistance of the devices. The emitter resistance contributes to the non-logarithmic behavior of the devices which causes errors in the desired mathematical function. Generally speaking, the error gets worse at higher currents because the voltage drop across the emitter resistance gets larger. The emitter resistance is modeled as "R" in Fig. 1. A straightforward technique to reduce the error would be to increase the emitter area of all the involved devices which would reduce the emitter resistance. However, this consumes silicon area and is not as effective as the...