# How to connect op amps in parallel

Discussion in 'General Electronics Discussion' started by Looee, Jan 7, 2017.

1. ### Looee

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Nov 8, 2016
We are trying to put 5 different signals on a 120 volt power line. We are running each signal through an op amp to increase the amplitude. We connected the output of all the op amps together. However, this causes the amplitude of the signals to attenuate. Is there a way that we can connect all the signals together without any attenuation?

2. ### AnalogKid

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Jun 10, 2015
Yes.

ak

Arouse1973 likes this.
3. ### BobK

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Jan 5, 2010
To be a little more helpful, lookup summing amplifier.

Bob

4. ### duke37

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Jan 9, 2011
You can sum with resistors to a common point. If you make this point at the -input of the op-amp, you will have a virtual earth amplifier. The current going into this common point will carry on through the feedback resistor and so will generate a polarity reversed voltage. One input will not affect any other input.

5. ### AnalogKid

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Jun 10, 2015
Actually, I think my response was very helpful. It was carefully crafted to encapsulate all of the technical requirement in post #1.

ak

6. ### Alec_t

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Jul 7, 2015
What opamp are you using to drive a 120V power line?

7. ### tedstruk

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Jan 7, 2012
I thought all op amps were connected in parallel...

8. ### AnalogKid

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Jun 10, 2015
Is that stereotyping?

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Sep 5, 2009

10. ### Audioguru

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Sep 24, 2016
In the United States your circuit is feeding the entire country's electricity grid. You need the power from millions of paralleled opamps to drive it. If you add a series inductor to isolate your electricity line at each end then you will need the power only from hundreds of opamps. Is your receiver sensitive?

How do you prevent the 120VAC from destroying the low voltage outputs of the opamps?

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

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Jan 21, 2010
This is typically and commonly achieved by capacitively coupling your signal to the mains. The inductance of the mains is quite high at higher frequencies due to the transformers from which it is fed.

I'm not sure if the exact requirements for driving these signals, but a quick Google should find them for you.

Edit: search for "power line communication" (PLC), "Broadband over power line" (BPL), "Ethernet over power", and "carrier current signalling".

Last edited: Jan 8, 2017
12. ### BobK

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Jan 5, 2010
I have seen capacitors and transformers used for coupling a signal to mains.

Bob

13. ### AnalogKid

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Jun 10, 2015
The classic National Semiconductor app note circuit used a strange semi-resonant transformer that was available through distribution for a while. Both AM and FM "wireless" intercoms used some variation of it.

ak