Re: Voltage followers with outputs connected

John Larkin wrote:
On 4 Jan 2007 10:37:41 -0800, "MRW" <mr.whatever@xxxxxxxxx> wrote:

Thanks Costas!

Costas Vlachos wrote:
MRW wrote:
Happy New Year!

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!
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.

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

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).

--
Regards,
Costas
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