Diode Function Generators

published 2026-01-06

by Christopher Howard

I've been interested recently in building a classic diode-based variable function generator. The basic idea behind it is that a diode can act like a switch, not conducting when reverse biased, and otherwise conducting with low resistance. This is not exactly true, but it is close enough to be practical for this application. So, using a diode in conjunction with an amplifier — in the negative feed back configuration — you can cause the output voltage to rise in a linear fashion, for a range of input voltages. Also, potentiometers are used to bias the diode toward the reference voltage — e.g., toward negative 10 volts — which alters the specific point — i.e., input voltage — in which the output voltage starts to rise. Another potentiometer can be used to change the slope of the rising voltage. When a number of these diode modules are combined together, and fed into the amplifier like standard inputs, the effects can be summed together to create a straight-line approximation of any arbitrary function.

MODELS 701 AND 709 VARIABLE DIODE FUNCTION GENERATORS

There is a nice section on these in an old book I checked out from the library, "analog and hybrid computing" by D.E. Hyndman, 1970. Section 2.3 Nonlinear Computing Units, pp. 26-35. Some information is available on the Internet also, but unfortunately searches will give a lot of noise, due to the use of the term "function generator" to refer to any equipment that can output sine, square, and sawtooth waves. In contrast, the diode-based variable function generator can approximate any function via arbitrary straight line segments.

I have made some progress toward building my own, experimenting with the basic construction of a diode module on protoboard. It has been slow though, as there are a some details like appropriate resistor values, and exactly how to handle the biasing, that are still fuzzy to me.

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