U-input absolute value circuit

What is the third amplifier for? http://everycircuit.com/circuit/4654595195535360

BillyT your circuit is not working. You have remove the retifier stage. You can remove the first OpAmp if your source impedance is low enough. But cant remove the core functionality.

http://everycircuit.com/circuit/5229480594636800

It seems to work for me, if I vary the input voltage the output voltage follows.

@BillyT ... The absolute value of a number is obtained by squaring it and then square rooting it. It is a handy way of just getting rid of the minus sign. If we do that with a set of values where we are trying to obtain the average (ie the mean), then we obtain a good reading instead of ending up with zero, which would occur if we didn't do that....because the negatives would have cancelled out the positives to leave us with zero. Hence the usage of rms..... root-mean-squared.

@BillyT ... Your two op amp schematic follows that which you put in, so if you turn your input anti-clockwise until it reads say -2v (care not to look at the op-amp input next to it which has a reversed sign due to the source here having its + connected to earth) then you will see a -2v final output, ie your schematic does not find the absolute value. The Hurz schematic two-opamp does however. For maths lovers:  |x| = (x^²)½  http://everycircuit.com/circuit/5283024345759744

For this example.... if x is our input of -2v, then -2 squared = +4 ... (a minus times a minus gives a plus). Then the square root of +4 is +2 .... That is our answer ...just the same 2 that we started with, but without the minus sign in front of it. Likewise, the absolute value of say -93.47 is just that without the minus sign in front of it: 93.47 Note also that the absolute value of +93.47 is also 93.47 ... The absolute value is always the positive answer of the two possibilities, but we don't usually bother putting the + sign there.... if we don't have a sign in front, then it is taken as read that it is + ... Sorry if you knew all that, hope it can be of use to someone.