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Voltage followers with outputs connected

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What happens if I connect the outputs of four opamps configured as voltage followers?

I am not sure if I'm applying the right analysis, but if I use superposition and short out the output of the other voltage followers, I get 0V, but if I run it on SPICE, it gives me the value of the lowest output voltage (I am modeling the input voltage for each follower as different voltages that are up to 100mV different from 3V).

Also, will output currents from the other opamps be a concern with this setup?

Thanks!

Reply to
MRW
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You can't connect them together like this! Op-amp outputs have very low impedance and directly connecting two or more together is equivalent to a short circuit. Each voltage follower will "compete" with the others, trying to bring its output to the same level at its (+) input, resulting in high currents flowing from one output to the other. Don't do it.

In your SPICE simulation, try to measure the output currents of the op-amps, and note their magnitude and direction.

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Regards,
Costas
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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Reply to
Costas Vlachos

That's provided the opamps are different! Spice likes to make them all the same. Absolutely identical opamps will share the load politely, in some simulated universe.

You can connect multiple followers to a common output node if you put a small resistor in series with each one. If the max opamp offsets were +-1 mV, and you used 2 ohm resistors, the maximum possible opamp-fighting-opamp current would be 1 ma, which might be tolerable.

John

Reply to
John Larkin

If the followers share the same input then yes you can do that. But the OP mentions about 100mV max. input difference between followers... What use would that circuit have in practice? If he wants to increase output current capability, why the 100mV input difference? Just use a push-pull or AoE's op-amp-only circuit idea (AoE, 2nd Ed., p253).

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Regards,
Costas
_________________________________________________
Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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Reply to
Costas Vlachos

They'll have an argument with each other.

See 'offset voltage'. You'll need to use combining resistors. Is your intention to obtain a greater output current ?

Graham

Reply to
Eeyore

I've done that: four opamps, each with a gain of about +4, using 1% resistors to set the gains so the outputs match pretty well. Each then has a 200 ohm resistor to the output. The result is a nice, fast 50 ohm drive.

John

Reply to
John Larkin

Was there a specific reason you didn't use one amp as a x4 stage and use the others as followers ? Concerned about the HF characteristics presumably ?

Graham

Reply to
Eeyore

I don't recall exactly, but I think it was an input common-mode-range issue. I was trying to get a clean, very fast +-10 volt drive with just +-12 volt supplies, and couldn't find a single opamp that would work. They were THS3062's, maybe.

Saves one opamp, too!

John

Reply to
John Larkin

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