# [SOLVED] simple op amps problem - summing signals from buffers

#### kooar

Joined Apr 3, 2020
6
Hi, I have some trouble getting this circuit to work, the circuit on my breadboard does not behave like the simulated one.
The circuit is shown below (I know it does not make a lot of sense, it is a dumbed down version of an actual circuit I need but the problem I came across is present here).

Voltage source is 3.3 V, then I am just summing 2x (3.3/2) V, in simulation I get 3.3V, great, that is what I need. But results on my breadboard are weird for me.

I use lm358n op amps and AVT3072 power supply - picture below. Op amps are powered with ~12 V voltage (11.35 V).
At the output of buffers I have the correct (~1.65 V) voltage, but at the output of the summing circuit I get 9.83 V. Why?

#### kooar

Joined Apr 3, 2020
6
oh and sure, I forgot to ground the + of the summing circuit, after fixing that I still get a result I do not understand - 0,68V

#### ericgibbs

Joined Jan 29, 2010
13,652
hi,
What are the OPA power supplies.?
E

#### kooar

Joined Apr 3, 2020
6
Hi If I understand your question correctly: op amps are powered with AVT3072 power supply, 12 V (mentioned in the post).

#### ericgibbs

Joined Jan 29, 2010
13,652
hi kooar,
If it is a single positive supply and you input a positive signal into the Inverting input of an OPA, what will the output try to do.?
E

#### kooar

Joined Apr 3, 2020
6
So voltage at the output is a difference between input voltages (times gain): V_noninv - V_inv, V_noninv is grounded so is 0, V_inv is positive so we get a negative value, and while powered by a single positive supply, OPA cannot generate a negative voltage?

#### kooar

Joined Apr 3, 2020
6
so I ground the inverting input, and my buffers outputs go to the noninverting input and I am golden

#### ericgibbs

Joined Jan 29, 2010
13,652
hi.
If you can modify your simulation circuit to only a single 12V for the plus supply to the OPA's and make 0V for the negative supply to the OPA, you will see that the OPA output will be at close to 0V.
Have you considered summing the two signals as a positive Non inverting input.?
E

#### kooar

Joined Apr 3, 2020
6
Yes, I came to this conclusion in a message I managed to squeeze in right before yours, it works for me. Thanks a ton, Eric. Have a great day.

#### ericgibbs

Joined Jan 29, 2010
13,652
hi k,
Look at this LTSpice sim.
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#### Audioguru again

Joined Oct 21, 2019
3,544
Why do no many people draw a schematic using opamps that are not powered and have no part number??"

#### Wolframore

Joined Jan 21, 2019
2,342
Hi, I have some trouble getting this circuit to work, the circuit on my breadboard does not behave like the simulated one.

View attachment 222283
I’ve broken this type of simulator in the past, it has many issues, it flips phase on outputs on its own, it won’t start oscillations in circuits (Missing noise), I’ve seen it “work” when real circuits and mathematics show it cannot and vice versa. Get LT Spice or multisim or similar.

Why do no many people draw a schematic using opamps that are not powered and have no part number??"
Because they have simple simulators that run on tablets but they’re only good for simple current dividers and should never be used for more than complex voltage dividers. The power is “hidden” in a menu. The way it calculates MOSFETs and transistors and even diodes make me cringe. While it can offer some use for a quick check the results will never be close to reality.