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Old 14th December 2016, 08:51 PM   #21
suzyj is offline suzyj  Australia
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Bear in mind this is all simply simulation. I had to play with the compensation values a little to keep it from oscillating. This is one of the reasons I favour smd for these complex amps - fewer parasitics, so the reality is closer to the simulations and it makes for less frustration when it comes time to build it.
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Old 14th December 2016, 09:24 PM   #22
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That's what I though you meant but wasn't sure. How do you set and stabilize the vas quiescent current with the mirrors in place? I suspect that the followers will reduce the hf distortion of the amp. Why not add some more output pairs - that will certainly reduce the distortion further.
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Old 14th December 2016, 09:32 PM   #23
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Ah, thought you'd actually built it. So in real life you might find the vas quiescent current difficult to calculate and keep stable. A trick I've used in the past in similar cases is to place matched two terminal constant current loads between the collectors of Q15/Q17 and Q16/Q18.
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Old 15th December 2016, 03:06 AM   #24
suzyj is offline suzyj  Australia
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Just so we're on the same page, here's my sim:

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Fourier components of V(out)
DC component:0.000620999

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.530e+01 1.000e+00 -0.11 0.00
2 2.000e+03 1.892e-05 7.478e-07 75.30 75.42
3 3.000e+03 2.918e-05 1.153e-06 -103.85 -103.73
4 4.000e+03 2.716e-06 1.074e-07 164.72 164.84
5 5.000e+03 1.718e-05 6.793e-07 -95.39 -95.27
6 6.000e+03 2.293e-06 9.063e-08 175.07 175.19
7 7.000e+03 9.412e-06 3.721e-07 -90.77 -90.66
8 8.000e+03 2.108e-06 8.333e-08 -177.53 -177.41
9 9.000e+03 4.871e-06 1.925e-07 -87.39 -87.28
Total Harmonic Distortion: 0.000160%(0.000000%)

I added emitter degeneration to the mirrors. Your questions regarding drive current stability are very good - I think it's a case of "suck it and see", but I'm wondering if there's not a reasonable tradeoff to be made between gain and stability through degeneration.

Last edited by suzyj; 15th December 2016 at 03:08 AM.
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Old 15th December 2016, 08:47 AM   #25
chalky is offline chalky
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The voltage at the Q10/Q16 collector junction isn't rigidly fixed, so the current through the Q14/Q15 vas transistors isn't well defined, which in turn means in real life its value would be a bit pot luck. You can solve that problem by inserting a two terminal constant current source ( two transistors, two leds, two resistors ) between the collector of either Q14 or Q15 and R36. I've used this trick with a two differential pair version of this topology ( ie the original non-symmetric Hitachi topology ) and it works well with no stability problems. The problem with applying it to your circuit is the further large increase in open loop gain, which might lead to stability problems, and necessitate some work with compensation networks. I'd be intrigued to see if it works however.
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Old 15th December 2016, 10:29 AM   #26
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Hmm another thought. You might have trouble simulating the simple two terminal current source I mentioned. With ideal devices it should never self start, but in practice its 100% self starting and reliable. We might need to find a two terminal device that works in theory as well as practice.
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Old 15th December 2016, 10:56 AM   #27
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See also this thread "HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles" and the other threads it refers to.
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Old 15th December 2016, 11:33 AM   #28
suzyj is offline suzyj  Australia
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I see what you mean now. Looks like I've fallen into a rather classic trap...

I'll go read threads and see if anything jumps out.
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Old 15th December 2016, 12:18 PM   #29
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But as you can see there are lots of solutions other than the one I used. I like stocchino's solution where he makes the vas one half of a current mirror to fix its operating point. Bob Cordells solution is neat too - uses a common mode signal to fix the operating point. I use the two terminal current source because its easy to undestand and you can fix the quiescent current precisely. This can be got to work and I'd like to see an updated/improved AEM6000. Not sure it'll sound any better but I'm dedicated to the pursuit of elegant circuits and the last ppm.
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Old 15th December 2016, 11:24 PM   #30
suzyj is offline suzyj  Australia
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How's this look. I really wanted to preserve the symmetry. I figured I could add another current source/sink to the VAS to stabilise, separating out the two VAS transistors and tying the whole lot back to the current source/sink for the previous stage...

Fourier components of V(out)
DC component:0.000917462

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.828e+01 1.000e+00 -0.04 0.00
2 2.000e+03 2.333e-05 8.249e-07 79.25 79.28
3 3.000e+03 2.341e-05 8.276e-07 -108.35 -108.31
4 4.000e+03 2.028e-06 7.169e-08 144.80 144.84
5 5.000e+03 1.475e-05 5.214e-07 -95.81 -95.78
6 6.000e+03 1.319e-06 4.663e-08 162.22 162.25
7 7.000e+03 8.930e-06 3.157e-07 -87.34 -87.30
8 8.000e+03 1.163e-06 4.112e-08 178.86 178.90
9 9.000e+03 5.607e-06 1.982e-07 -81.01 -80.98
Total Harmonic Distortion: 0.000134%(0.000000%)

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