Discrete Opamp Open Design

[benb]

Originally Posted by Wavebourn

Open loop?
Because as soon as you close the loop it isn't H2 anymore.

That brings up a thought/question. Would it help this to take the feedback from the output of the VAS instead of THE output? It would avoid the problem of spreading out low-harmonic distortion into higher harmonics (is there a name for that?) in the feedback loop.

Since the VAS output is a rather high impedance point, a voltage follower (emitter/source follower and CCS) could be added, and the feedback taken from that. For those on a "low transistor count budget" this could blow it, but as any engineer knows you can't have everything...

[Wavebourn]

I proposed long time ago in this thread to have nested feedbacks. And feed-froward by a resistor from input to output of the output follower, balancing such a way power amplification between stages.

[scott wurcer]

Originally Posted by bear

I was just looking at the pdf for the Fairchild devices with a hopeful eye. From what I can see they do not seem to be particularly complementary.

_-_-bear

I'm afraid you will never find very good complimentarity with JFET's anywhere. They are majority carrier devices one holes one electrons, the mobility difference will always cause something to be amiss. At the very least my observation is that the output characteristics on the P's are always softer, also PFET's do not have impact ionization a direct result of the mobility difference.

[grhughes]

Scott: If p jefts exhibit this non-complementarity, I might be displaying my ignorance but what about an all n jfet output stage (totem pole)? But that would require a balanced push push input signal to the output stage? Thanks Ray

[grhughes]

Correction: Push-pull.

[Wavebourn]

Originally Posted by grhughes

Scott: If p jefts exhibit this non-complementarity, I might be displaying my ignorance but what about an all n jfet output stage (totem pole)? But that would require a balanced push push input signal to the output stage? Thanks Ray

Like, if after washing a baby in a water it is still relatively dirty, why do not throw it out together with the dirty water?

JC already showed balanced input to the output stage. It looks smooth on the paper, but is bumpy as usual in the reality. What we discussed here, many pages, how to find more optimal design. However, before doing that we did not stop to decide about optimization criteria, so everyone have own in mind.

[kgrlee]

Originally Posted by Wavebourn

However, before doing that we did not stop to decide about optimization criteria, so everyone have own in mind.

It's still not too late to put forward your criteria for consideration.

I think Scott & I share very clear common criteria; eg low THD, sensible stability adjustment for different loads & gains. no funnies on overload or startup, no Unobtainium, robust to component variations so flexible with voltages, devices etc, optimised for audio rather than RF, good sound.

We might also admit to some 'show off' criteria like zillon V/us slew so we outperform Golden Pinnae stuff on the important Hirata & Quan tests.

I'd like to do as well as SW-OPA on the above with less current, less devices & less real estate.

You'll excuse me for not adopting 'new' topologies' unless they show improvements on these criteria over tried & tested.

[qusp]

i'd also love to see a circlotron type OPA with BF862, that would be 'so fun'

but first this show should run to its conclusion

[bcarso]

Originally Posted by qusp

i'd also love to see a circlotron type OPA with BF862, that would be 'so fun'

but first this show should run to its conclusion

I suspect things are close to at least the curtain on the first act.

[grhughes]

No, I'm not advocating throwing the baby out with the bath water. I'll build it the way Scott designs it according to his reccomendations. Ray