Summing Op Amp circuit doesn't sum

Thread Starter

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
2,587
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
8,695
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
11,249
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.
 

Thread Starter

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
11,249
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.
 
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