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[maxdimario]

Sahib wrote on Thu, 03 May 2007 01:40

Hi Bruno, I was just reading Audio Power Amplifier Design Handbook by Douglas Self. He states that he carried out serious tests (at 0.0006% level)which includes spectrum analysis of the residual and found no evidence of thermal distortion in discerete power amplifiers (page 153-156). Are you able to mention your test data to make comparisons? Regards, Cemal

SIGNAL DEPENDENT.

[Quince]

It would make sense that opamps would have the worst thermal memory distortion because of the tiny semiconductor sizes and having the output stages in the same thermal package as the input an VAS.
In another forum I brought up the issue, and someone claimed that it wouldn't matter where the opamps are used in unity gain.  However I'm not sure that is so, since the open loop gain remains high.  What do you guys think?

I'm working on implementing thermal memory models for transistors in SPICE, but I don't know how I could do that for opamps unless simulating a discrete opamp.

[bruno putzeys]

Any error committed by the input stage gets multiplied by noise gain, which equals gain for noninverting ckts and 1+gain for inverting circuits.

Op amp designers have become extremely adept at keeping thermal gradients (or parameter gradients due to processing) across the chip from producing offsets. Tons of papers have been written on how to construct differential amplifiers and current mirrors using multiple parallel transistors laid-out such that the errors cancel out. The result is that it's quite rare to see the input offset wobble with instantaneous power dissipation in the output stage. If we can ignore the effect, that's owing to extremely diligent work from the chip designers.

[Quince]

The measurement in the Perrot paper is not trivial, so I've been working on SPICE modeling for thermal memory on individual transistors.  I'm not sure how to translate that to opamps, unless I simulate a discrete opamp based on internal schematics as are available in some datasheets.  But how on earth to guess thermal capacitance and conductance values for transistors on a chip...