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Chip Amps Amplifiers based on integrated circuits

Composite amplifier: LM3886 + LME49710
Composite amplifier: LM3886 + LME49710
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Old 16th April 2019, 04:13 PM   #81
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by asbjbo View Post
After a discussion here some time back, I read up on control theory, worked out the transfer function for a composite LME49710 + LM3886 chipamp and simulated it in Scilab (Matlab clone). Basically, each opamp adds 90 degrees of phase to the loop gain, so the composite is guaranteed to oscillate unless the phase rotation is compensated somehow. Playing with the transfer function was quite useful in figuring out possible compensation schemes.
Yep. I used to do that in MathCAD some 20+ years ago. That works very well for gaining understanding of how the circuit works and how to compensate it. It doesn't work so well in cases where the transfer function changes dynamically. For example, the LM3886 has different gain as its output approaches the negative rail (and the VAS starts getting into saturation). That tends to be the reason reality and simulation don't always match. That's not modelled in the SPICE/TINA model either, by the way.

Quote:
Originally Posted by asbjbo View Post
We now have a simulated version with sixth order compensation to deal with the kinked loop gain from the OPA1611 in addition to the decompensated LM3886.
Nice!! Yeah. That loop gain/phase kink in the OPA1611 is annoying.

Quote:
Originally Posted by asbjbo View Post
If and when it is working as intended, it probably deserves a thread of its own here. Until then, a big thank you for sharing your knowledge both here and on the Neurochrome site. Very inspiring, enough info to start looking in the right direction, but not so much that all the challenge is gone either.
You're quite welcome. I'm glad you found it useful.

Tom
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