# Old Man looking for guidence on a linear inversion of an analog DC control voltage

#### stagekraft

Joined Jan 18, 2022
12
Hey Folks,

Old man here scratchin' his head...

Was involved in analog audio design back in the 70's/80's, but life took me into industrial electronics in the late 80's.

Now retired, I got time to follow up on ideas I had in my youth, but the brain has become fuzzy...

Looking for a simple means to do a linear inversion of an analog DC control voltage...
that is, a variable 0-1.25VDC control voltage with a linear inversion to 1.25-0VDC..

In the u-controller/plc world I was involved in for the last 30 some years, this was a simple task with a couple lines of code.

Of course, all things digital evolved from real world analog of the past...

My first thought was a simple voltage divider, from 15VDC to get my 1.25VDC, with a BJT driven from my 0-1.25VDC control voltage.
I get my 1.25VDC with 0VDC to the base of the BJT, and (close to) 0VDC when the base of the BJT is driven 1.25VDC..
But it is not linear in between.. I get about the same result with a FET in place of the BJT..
Of course, this is all simulation in LTSpice...

I keep thinking something like this should be do-able with an op-amp, but my feeble old brain is not clicking...

Any ideas out there??

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#### Papabravo

Joined Feb 24, 2006
18,403
If I understand what you are asking for you want the linear transfer function:

$$V_o\;=\;1.25\;-\;V_{in}$$

Is that correct?

It took me a minute to whip up the answer. You'll have to recompute R5 & R6 if you use a different supply voltage. The voltage at the non-inverting input should be 1.25V/2 = 0.625V

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#### crutschow

Joined Mar 14, 2008
29,779
I keep thinking something like this should be do-able with an op-amp
Yes, that's the easiest way.
Example circuit below:
The op amp is configured as a gain of -1 (inverter) with a positive 0.625V offset applied to the (+) input to give a positive 1.25V output offset (since the non-inverting gain is [1 + R2/R1] = 2).
Note that the op amp needs to be a single-supply or rail-rail type to operate from a single-supply.

#### sparky 1

Joined Nov 3, 2018
716
I can't beat the op amp circuits above. If jogging memory might bring out one better than old analog scope's ramp generators, that might be a significant recollection. The diode switch control, a comparator, taking advantage of the symmetry of an AC signal to charge and discharge a capacitor using constant current. If you actually build this circuit you will have a better chance of recalling an arrangement having less parts if the simulations don't help.

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