Finding inverse of a signal using analog circuits

How do I use an op amp to find inverse of a voltage signal? In other words, my output signal has an amplitude inverse of amplitude of input signal.

My solution was to convert the signal to digital form, find inverse digitally and then convert back to analog form.

Is there another solution using just analog techniques?

• Can you give an example of exactly what you mean by "inverse of a voltage signal" ? Feb 17 '15 at 17:53
• If you mean 1/x when you say inverse, see my answer here Feb 17 '15 at 17:59

If your variable is constrained to be > 0 you can use a simple log-antilog circuit which would likely cost a lot less than a purpose-built multiplier chip. For example, this one in an older TI app note.

For the transistors you can use matched duals which cost less than 20 cents each, and the amplifier could be a quad op-amp.

In this case E2 would be your input voltage. and E1 and E3 can be connected to a reference voltage.

• Love this circuit. Learned about it a few months back. The transistors should be matched for accuracy because this circuit really uses "diode" currents, and its important to have Is and Vt matched because matched transistors can cancel out terms. Feb 18 '15 at 23:36