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op-amp theory: relation between gain and distortion?
op-amp theory: relation between gain and distortion?
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Old 3rd April 2006, 01:47 PM   #11
jcx is offline jcx  United States
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at any gain a OPA549 power "chip amp" will always have more distortion than any decent quality audio op amp used at gains up to +24 so the question is moot, other factors will determine what you can do

cascading separate feedback amplifiers is in any event a poor way to reduce distortion, distortion is reduced by high loop gain so using the same gain stages within a single feedback loop gives lower distortion at the same total gain than any partitioning of the gain between separate feedback amplifiers using the same op amps

the entertaining part comes from trying compensate a feedback loop with 2 op amps for stable operation and still get the distortion reduction advantages of their combined open loop gain

Walt Jung shows some multiloop feedback circuits but he uses fast current feedback output op amps and the slower input op amp supplies the dominant pole compensation

http://www.analog.com/library/analog..._handbook.html - section 6

an example with a fast output amp:


with the 900 KHz unity gain bandwidth of the OPA549 you have to really slow down the input op amp - and lose most of its added loop gain if you use these compensation schemes

an alternative is to let the OPA549 supply the dominant pole and add a very fast input op amp with local feedback that controls its added phase shift at 900 KHz to sufficiently low levels;

an option might be a LM6171 input op amp with local modified integrator feedback with a high frequency gain of Av +24 -- this gives ~ 3-4 MHz pole from the input gain stage which adds ~ 10-15 degrees excess phase at 900 KHz with the global closed loop gain set to +24 (a lead capacitor in the global feedback network can cancel this added phase and improve the phase margin even further)

the added gain of +24 improves the composite amplifierís distortion at all frequencies by a similar factor vs the OPA549 supplying all of the gain, even more of the LM6171ís gain can be made available to reduce audio frequency distortion by the extra pole of the modified integrator local feedback adding gain below ~ 100 KHz so it doesnít add too much more excess phase at 900 KHz
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Old 3rd April 2006, 02:56 PM   #12
Bazukaz is offline Bazukaz  Lithuania
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op-amp theory: relation between gain and distortion?
Isn't it better to simply use a lower distortion power op amp IC ? OPA549 seems to have not so good distortion curve in datasheet, especially at higher freqs.

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Old 4th April 2006, 12:03 AM   #13
Dominique is offline Dominique  France
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thank you all fro participating! That's a very interesting subject to me!

The opa549 doesn't have to be used at higher gains, but you're right and I forgot: Most op-amps for the first stage won't have enough voltage swing!

that sounds very interesting! I only wish I'd understand all of you've said! I've read about those poles, but didn't understand that well to date.
But e.g. if I'd simply use a global feedback and an opa2132 first and the opa549 then, both with a gain of about 5, overall about 25, will the circuit be easily unstable?
From your description it sounds a bit difficult to get such a circuit stable.

And yes, from the THD graph, the opa549 doesn't look that fine, but I think it will perform a bit better with 8ohm speakers, or equally with two chips in parallel to drive 4ohm speakers.

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