# What will be the output signal of op-amp?

Discussion in 'Electronics Homework Help' started by za_slovom, Jun 20, 2017.

1. ### za_slovom

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Apr 20, 2017
Hi everyone. I am stuck on op-amp. There are some problems that I can deal with, and some thay I can't. I know that if our signal is applied into inverting input then there will be phase shift of 180 degrees in the output. For the example below, this rule doesn't work. The signal applied to the non-invering input doesn't have to change in the output. What am I doing wrong?

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2. ### davennModerator

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Sep 5, 2009
double ended output ???
what is the op-amp ?

what I see in that diag. doesn't even make sense

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

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Jan 21, 2010
The input signal is the difference between the two input voltages.

4. ### Audioguru

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Sep 24, 2016
Your opamp has no power supply.
Its inputs have no biasing.
It has no negative feedback.
It has an extra output (opamps have only one output). Some audio power amplifiers have a differential outputs (two outputs) and some have built-in biasing and negative feedback.

Here are two opamp circuits that amplify AC signals. One has a single polarity power supply and the other has a dual polarity power supply.
They are biased at half the supply voltage so their outputs can swing up and down.
They have negative feedback.
They have only one output.

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

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Jan 21, 2010
Perhaps you should Google for op amps with differential output.

The other points are not relevant either.

There is no suggestion that this is a practical circuit, and it is not uncommon to omit the power supply connections.

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6. ### Audioguru

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Sep 24, 2016
I explained why his circuit did not work and I showed all the details on my schematic that works perfectly.

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

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Jan 21, 2010
It's not a working circuit. It's a demonstration of how the device works.

You've probably never seen the equation Vo = a(V+ - V-)

8. ### Laplace

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Apr 4, 2010
Texas Instruments has an application note (sboa092b.pdf) that covers operational amplifiers with balanced output.

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9. ### davennModerator

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Sep 5, 2009
cool

I have never had need to play with those

10. ### Harald KappModeratorModerator

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Nov 17, 2011
An opamp with differential output needs feedback, too, which is missing in the circuit from post #1.
Assuming the amplifier shown in post #1 has finite gain (e.g. internal feedback or feedback simply not shown). one would still expect the differential output voltage
Vd = Vo1-Vo2 to be in phase with the input voltage Vi.
The graphical representation in the figure suggests that the definition of Vd is Vd = Vo2-Vo1 which explains the 180 ° phase shift. In fact, this is the only exlanation I can think of lacking more information from the op. The app note shown by @Laplace in post #8 supports this.

11. ### Audioguru

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Sep 24, 2016
The OP says that his input is applied to the inverting input but it is shown applied to the non-inverting input!
Another problem is that his inverting output is not indicated.

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12. ### Harald KappModeratorModerator

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Nov 17, 2011
does he?

Right, which leads to my questioning the definition of Vd.