Bob Cordell Interview: Negative Feedback - Page 341 - diyAudio
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Old 3rd October 2012, 12:26 AM   #3401
davada is offline davada  Canada
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Hi Bob,

I'm still not totally clear as to how cascoding takes care of the early voltage.
Can you explain this in more detail?
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Old 3rd October 2012, 07:06 AM   #3402
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Originally Posted by lumanauw View Post
This is very likely if this high global feedback poweramp is made with bad output stage. The problematic section is output stage, but the troubleshooting is made in differential pair+VAS, building huge OL gain in these front stage. It's like a car having problem in the tyres, but you keep fixing the steering wheel. Not shooting the real problem.
Better to have good output stage first (like using classA or using EC) and then using suitable gain front stage than building bad output stage with enormous front end gain.
I agree very much on this; most often the work of speeding up the vas is nulled again by applying a relatively high Cdom from output to input, just to slow the whole input down again to match the slower output stage.
Negative Feedback: The Need for Speed
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Old 3rd October 2012, 11:14 AM   #3403
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Originally Posted by Bob Cordell View Post
Hi keantoken,

I came up with the snubber idea in my MOSFET power amplifier with error correction. I don't recall having used it in BJT output stages, although it might be useful there as well. I refer to them as gate zobel networks on page 226 of my book in the MOSFET chapter, but unfortunately do not show a schematic there with them. I cite as a reference my MOSFET amplifier paper. That paper is available on my website at - Home.

The basic idea is to damp out resonances that can be part of forming an oscillator topology, such as a Hartley or Colpitts. The technique may very well work with BJT arrangements as well. Don't rule out the possibility it is the drivers oscillating.

It was a little unclear whether the problem you saw was in simulation or prototype, but it sounds like the latter. In that case, Triples of any type can be susceptible to power supply wiring inductances and HF feedback through the power rails. When using ordinary thriple EFs, I often recommend some small-resistance R-C filtering in the rail as it travels back to the driver and thence the pre-driver (i.e., two LPF stages, often with resistances on the order of 1 ohm and 10 ohms, respectively). If the shunt capacitors are of a moderate value, the R-C combination not only acts as an LPF, but also acts as a Zobel on the rails. Cheers, Bob
I don't understand this term:
ordinary thriplet EFs (= triplett emitter followers)
There is a very large difference between the true complementary triplet follower (Harman's "T-Circuit") or quasi complementary triplet (Quad triplet) emitter followers.

In case of so called "true complementary" triplet (super ) emitter follower (one stage more than ordinary darlington) - in the kind like mentioned by the paper
"An Ultra-Low Distortion Direct-Current Amplifier"
or about the attachments by post #15 about
Collection of Class B topologies <100mA Idle and Sound closest by Class A
you are right in your explanation. Until now I was always successful with this steps

But the situation at quasi complementary triplets is completly different because the right steps to remove unwanted oscillations must be also "Quasi Complementary", this means not a mirror image between the positive and the negative half.
read post #20 about
Quad 303 triple cascade
and have a look to post #21, third file (QuadTriple) about
Quad 303 triple cascade
so as pdf file attachment post #22 (EQUIN super emitter follower from German's magazine "ELEKTOR") about
Edwin 20 watt (Elektor 1970 - May)

I am looking for an appropriate design rule to remove any unwanted oscillations for the various quasi complementary triplett topologies. This I haven't find until know.

Perhaps you know basic articles about the compensation of such three stage super quasi complementary stages.
I know too few English keywords therefore. I recall, that I have read some years ago a JAES article regarded that. But I don't know the headline.

Last edited by tiefbassuebertr; 3rd October 2012 at 11:29 AM.
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Old 3rd October 2012, 12:25 PM   #3404
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The best I can come up ith is that if you reduce the oscillation to a negative resistance in parallel ith a resonator, hich describes most oscillators, and then selectively cancel the negative resistance ith a positive resistance such as an RC snubber, then you've succeeded. There is also the concept of altering parasitics so that the resonance is out of range of the negative impedance. Every technique used to quell local output stage oscillation must be one of these. Phase shift oscillators don't necessarily yield to this analysis, but I am not really a math guru so hat do I kno.

I may have already done this, if I had found a good description of the parasitics around a modern poer BJT folloer that oscillates and that I can simulate successfully. So far I'm having difficulty getting that 50MHz resonance... Perhaps it is a driver that as oscillating.
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Old 8th October 2012, 12:52 PM   #3405
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Originally Posted by keantoken View Post
Yes, this is a real prototype. I simulated it and then built it. I have a 40MHz signal generator so I have some indication hen simulations and reality don't converge.

Thanks for the info. I onder if I coul find the source of the oscillation by probing the air around the traces. I suppose this ould be a job better suited for an FET probe, unfortunately I don't have one.
Hi Keantoken,

I'm sorry for being late to getting back to you. I wouldn't think probing the iar around the traces would be very effective. Tracking these things down can be difficult. If you haven't tried it, tray base stoppers fir the driver transistors. If you have base stoppers for the output transistors, try probing on both sides of them to see the amplitude of the oscillation. BTW, sometimes just probing with a 10pF 10:1 probe on a sensitive node can stop the oscillation.

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Old 8th October 2012, 02:48 PM   #3406
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Yes, I'm aware of the difficulties. My scope has about 180pF probe capacitance counting the probe and scope, which makes it totally useless for probing anything but the power supply and output.

I think that a low-Cob active FET probe, with a special insulated frilly tip to maximize surface area, may be able to probe traces through the air. This is because the capacitive input impedance, in series with the capacitance through the air, would form a voltage divider just like resistors would. It would only be an approximate probe at LF because of the lopass needed and the varying probe gain with trace proximity. Nonlinear capacitance would need to be cascoded away to increase the accuracy and sensitivity.

The 150p/47R RC stopped oscillation at 50MHz. Then I discovered oscillation at 150MHz! But, this oscillation remains even when I turn off my amp... How confusing. I wonder who is transmitting!?

Yesterday I shorted the output of my amp into the positive rail, and blew my bench fuse. At 63V variac, 2A gives 120W, which through the trafo becomes 3.4A drawn from the supply. My amp's protection limited it to 5 or 6A, so max instantaneous dissipation of 210W. I am glad to say the protection worked even in this extreme case. I replaced the fuse and turned it on again and it works.

After I get the oscillation sorted out my next order of business will be the thermal compensation. There appears to be thermal positive feedback. If I turn the bias up too fast, it will go up and up and up over time, and if I set it slowly, it will eventually drift down to 30mA or so and then stay there.
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