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Stacking OPA's (like stacking buffers)?

post #1 of 11
Thread Starter 
Hi,

since i read that it is possible to stack buffers in the output stage of an amp, i asked myself if this would be a useful thing to do with the OPA in amps with no buffers (Cmoy, for example); especially, since the limited output ressources should be the weak spot for this kind of amps.
Should'nt stacking OPA's improve the output capabilities of the amp (possible output current, impedance,...)?
Am i missing something?
post #2 of 11
I'm going to say no you cannot. If it could be done people would do it.
post #3 of 11
Thread Starter 
Quote:
Originally Posted by heatmizer View Post
I'm going to say no you cannot. If it could be done people would do it.
My thought as well. Otherwise i could just try, as i've got an unemployed OPA 2132 lying round here.

But why should'nt it work? I don't wanna fry the poor bug for nothing.
post #4 of 11
I'm looking up the pinout right now, but I'm pretty sure something will get really screwy in the feedback loop.
post #5 of 11
Each opamp needs its own feedback resistors (loop). Stacking them without will not work.
post #6 of 11
Thread Starter 
OK then.

So putting a Feedbackresistor between the -Input and Output should do the trick?

Damn, this stuff is complicated.
post #7 of 11
No. Look at the resistors on both the + and - inputs of the opamp. Usually, the + input has something like 100K to ground (R2??). The - input has 1K to ground (R3 IIRC stock cmoy value for gain of 11) and 10K from the - input to the output (R4 IIRC stock gain of 11). You need to duplicate them all. You can feed the + input from the same cap if you are using input caps.
post #8 of 11
Also, most opamps won't like to have its output tired directly to the output of another opamp, even if both opamps are swinging the "same" signal. You will need current-sharing resistors in series with each opamp to provide a bit of isolation (usually >22ohms or more). Of course, such resistors cause current-limiting, and sort of counteracts the purpose of parallaling opamps in the first place. The net gain is output current capability is therefore not as much as you might think.
post #9 of 11
Thread Starter 
So, in short: Better build a buffered amp.

Thanks a lot, guys.
post #10 of 11
That has been the consensus, otherwise you would probably see more of that. The Apheared A47 is somewhat down this path, although the second opamp is configured as a buffer, and the outputs (v amp and buffer amp) each go thru a 47 ohm resistor.
post #11 of 11
Analog actually recommend connecting AD8615/16/18 in parallel to reduce noise. They recommend a 100 Ohm output resistor on each amp. I can't see why you shouldn't try this if you're up to some experimenting. If you do, post your impressions please.
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