[SOLVED] Op-amp summing voltages of opposite polarity?

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David_

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Hello.

Can you sum two voltages with opposite polarity with a op-amp summer circuit?

I have a 0-3,3V signal that is going to be amplified through a op-amp to 0-33V, but!
When the signal is 0V the op-amp should output ~ -3V, could I use a op-amp summer with the 0-3,3V signal and a -0,3V to end up with -3V to +30V?

Giving that the op-amp is being supplied with say +35V/-5V.
 

This is one way to solve it.

You have specified 0~3.3V in and -3 to +30V out.
This requires a gain of 33/3.3 = +10 and an offset of -3V.

Using an inverting gain of (-9) and non-inverting gain of +(1+9)=+10, the (-ve) input would need a reference input of +0.333V with a gain of -9 = output offset of -3V.

The input signal goes direct to (+ve) input.

You can look after the rest of the OA design parameters..
 
That seems like a much easier way to do it, although i don't really understand how to adjust both inverting and non-inverting gain but i managed to come up with this result. The scenario i asked about is one of two nearly identical circuits only the output polarity is changed, in the second version(bottom of picture) i had to decrease the reference input to 0,27V. But that is not a problem, but I'm curious regarding if there is another solution.

Thanks for your advice.
 

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