How to get y=x/(a*(x-1)+1) with opamps?

Thread Starter

freshtapescent

Joined Dec 27, 2015
4
Ultimately trying to design an AC amplifier stage that has y=x/(a*(x-1)+1) for values between 0<x<1. A is a constant where 0<a<1 Order of operations should be as follows:
1. x-1 = add -1V DC offset to x with noninverting amplifier
2. a*(x-1) = use a voltage divider and buffer
3. a*(x-1)+1 = add +1V DC offset to x with noninverting amplifier
4. x-(a*(x-1)+1= no idea

This website's section on analog computing is great (http://www.allaboutcircuits.com/textbook/semiconductors/chpt-9/computational-circuits/) and I should be able to figure out steps 1-3, but as both x and (a*(x-1)+1) are different voltages and neither constant, I'm lost. Anyone point me in the right direction?

Attached images are from the online Desmos graphing calculator and show what happens when variable a is adjusted. Higher values of A create more nonlinearity from y=x.
 

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crutschow

Joined Mar 14, 2008
24,360
You can't multiply or divide two variable voltages together with just op amps.
Op amps only add, subtract, or multiply/divide by a constant.
You need a non-linear device such as an analog multiplier to multiply/divide two variable voltages.
 

kubeek

Joined Sep 20, 2005
5,682
I think you can do it with an logarithmic amplifier, that uses just bjts and opamps. Or you could build a Gilbert Cell which allows multiplication - see
 

ErnieM

Joined Apr 24, 2011
8,019
The order of operations should be to first reduce the expression you are trying to synthesize.

Then you can do this with just a few op amps.
 

Thread Starter

freshtapescent

Joined Dec 27, 2015
4
The order of operations should be to first reduce the expression you are trying to synthesize.

Then you can do this with just a few op amps.
Steps 1-3 in the original post outline how I plan on dealing with the order of operations. I can get that to work. Its the fourth step which is the division of two variables. How can I do that with just a few opamps?
 
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