# Summing Op Amp circuit doesn't sum

#### James George

Joined Jul 26, 2019
3
I'm attempting to add an additional 0.5V to a signal using an op amp TL072CP. For whatever reason I get an output of 4.2V. I was hoppng for an output of 1.5V with this example circuit. Have I wired it correctly?

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#### BobTPH

Joined Jun 5, 2013
8,649
You show no connection to the V- pin.

That cinfiguration is inverting, so the correct output would be - 1.5V, but you need a negative supply to get negative output.

Bob

#### AnalogKid

Joined Aug 1, 2013
10,943
Note that the classic voltage summing circuit is inverting. That is, with correct power supplies connected and a circuit configured for unity gain, if you sum a +2.0 V signal with a +3.0 V signal, the output will be -5.0 V.

ak

#### Audioguru

Joined Dec 20, 2007
11,248
The minimum total power supply voltage for a TL072 is 10V on some datasheets when its inputs have a narrow range of useable voltages.
But I have powered many TL072 opamps with a 9V battery that drops to 6V and my circuits all worked fine because I biased the inputs at half the supply voltage and used AC signals. The inputs always had plenty of voltage.

Your circuit has no power supply voltages shown. The output of a TL072 goes as high a voltage as it can if an input voltage becomes within a few volts from its negative supply voltage which might be ground (0V) in your circuit.

#### James George

Joined Jul 26, 2019
3
I have changed it to go into V+ it works now.

With a 5V input I get a range of 0.5V to - 4.2V. Ideally I'd like a 5V output. Do I need a different Op-amp a 'rail to rail one? Can anyone recommend me one?

Thanks

#### Audioguru

Joined Dec 20, 2007
11,248
We still do not know the voltage or voltages of the opamp's power supply or the load resistance and current!
The datasheet of a TL072 shows an output voltage loss of from 1.25V with a low output current to a voltage loss of 3V with a higher output current.

The datasheet of a rail to rail opamp also shows how much is its output voltage loss is when loaded.

#### Dodgydave

Joined Jun 22, 2012
11,234
I would use a TLC272 / 277 op amp..