Edited by Martin Rowe and Fran Granville -- EDN, 9/4/2008

An op amp in a negative-resistance topology subtracts the minimum programmable resistance from the total programmed resistance of a solid-state resistor.

A variety of solid-state, in-circuit-programmable replacements exist for the traditional electromechanical trimmer potentiometer. These replacements have many obvious advantages, such as automatic adjustability, miniaturization, and immunity to vibration. But these devices, unlike humble mechanical potentiometers, have relatively large minimum programmable resistance.

Although you can adjust a typical trimming potentiometer down to a fraction of 1Ω, solid-state-potentiometer substitutes usually bottom out at 10s, 100s, or even 1000s of ohms. This limitation can sometimes be problematic and frequently precludes use of the solid-state option in some design applications.

The Rejustor family of devices, which Microbridge recently introduced, provides an extreme example of this effect. You can program a typical Rejustor over only a narrow span of 30%. For example, you can program a 10-kΩ Rejustor to no lower than 7 kΩ, imposing a serious and obvious obstacle to general-purpose application of these devices.

Figure 1 suggests a generally applicable workaround that works not only with Rejustors, but also with all adjustable resistances. It uses an op amp in a negative-resistance topology that, in effect, subtracts R_MIN (minimum programmable resistance) from the total programmed resistance.