# Linear DC voltage inverter

Discussion in 'General Electronics Discussion' started by pyrohaz, Jan 4, 2013.

1. ### pyrohaz

33
0
Oct 28, 2012
Hey guys, i'm designing a passive diode based VCA and I was wondering what I can use as a linear DC voltage inverter. As in, if the input goes up by 1v, the output goes down by 1v. In AC terms, this would just be an amplifier with a gain of -1. How can I achieve this?

The power supply will be at 5v and it will need to be able to source/sink a minimum of 500uA.

Thanks,

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
The classic way to do this is with an op-amp.

With a 5V supply rail, the issue then becomes, how close to the supply rails do you need to get? This will be perhaps the major issue in selecting an op-amp.

I'm presuming that the frequency is low, and that you don't want the output to go outside of 0-5V.

3. ### pyrohaz

33
0
Oct 28, 2012
Ok, generally as close as possible to rails, complementary outputs would be useful but if I have a buffer before the inverter, I can just tap off each.

What kind of circuit am I looking at here? Can I use a standard single supply inverting amplifier without a capacitor on the input?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Assuming you get something capable of rail to rail inputs and outputs, you bias the non-inverting input to Vcc/2 using a pair of resistors (R3 and R4). Then you make a standard inverting amplifier with Feedback resistor R1 and input resistor R2.

R1 = R2, and R3 = R4 = 2*R2

The values of R3 and R4 are not hugely important, but keeping them at twice the value of R2 is optimal (I think).

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