Class-A-Burn: an amp with capacitive feedback divider around MOSFET - Page 3 - diyAudio
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Old 24th August 2009, 04:49 AM   #21
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Lumba Ogir View Post
Wavebourn,
tell me very confidentially, why extend the already huge and extremely harmful internal capacitances? What are they supposed to divide?
I don't get it either... so I'll try to guess. Looking at the simplified schematic in post 15

C3 samples the output to the top device, like Nelson's 'modulated Cascode' for better linearity ???

C1 bootstraps the gate capacitance on the output device so that it's no longer signal-dependent ???


are we seeing here the 'Wavebourn Tower' perhaps...
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Old 24th August 2009, 05:02 AM   #22
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No, it's not a 'Wavebourn Tower'. It's a crazy idea about vacuumless triode, made of a MOSFET with capacitive voltage divider in feedback.

Lumba, I don't extend already huge and extremely harmful internal capacitances. I just shunt them by external capacitances that are useful.
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Old 24th August 2009, 06:30 AM   #23
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Here is a better version, with idle current trimpot. Still, needs some gate stoppers.

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Old 25th August 2009, 08:54 PM   #24
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This version of PIM-free amp can be driven by a line out of a player.

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Old 26th August 2009, 04:42 AM   #25
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Finally, I've found the way to fit in gate stoppers so they won't screw up everything.

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Old 27th August 2009, 02:42 AM   #26
Bigun is offline Bigun  Canada
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You have to consider me electronics illiterate - I've just started to read the Zen articles and am getting a little bit more comfortable with MOSFETs etc. I amy even build a Zen amp next. So, since your circuit looks similar to Zen I'd like to understand the benefits of your capacitive divider - can you explain a bit more about it using the Zen amp as a comparison ???

While I have you here - I remember reading your comments on a hybrid output device where you have a BJT with heavy emitter degeneration in parallel with a MOSFET and you called it 'the perfect device' - what are it's benefits, perhaps it's a candidate for a Zen amp design too ???
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Old 27th August 2009, 04:43 AM   #27
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The problem is, I still can't find neither perfect device, nor perfect usage for non-perfect devices.

What is the difference between this amp ant the rest of them, in usage of capacitors for feedback.


MOSFET has big capacitances. If to use resistors in feedback there will be always phase shift between currents flowing through this resistors to MOSFET's gate and a voltage that is created on the gate, that controls the MOSFET. That means, feedback will be always shifted by phase. Since that capacitances are non-linear there is non-linear modulation of the complex transfer function, and it is frequency dependent.

If to use capacitors for a feedback divider, it would be frequency independent, and there will be no phase shifts between feedback current and voltage.

Now we have a stage with capacitive input impedance, it is a new problem we created solving the previous one.

The world is not perfect!
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Old 27th August 2009, 01:34 PM   #28
Bigun is offline Bigun  Canada
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thanks - it makes sense now. I see what you are hoping to achieve. Would it be correct to say that the feedback phase shifts are going to be more of a problem at high frequencies, so your proposal benefits these frequencies the most ? And, isn't the input impedance capacitive anyway with a MOSFETs ?
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Old 27th August 2009, 05:13 PM   #29
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Quote:
Originally Posted by Bigun View Post
thanks - it makes sense now. I see what you are hoping to achieve. Would it be correct to say that the feedback phase shifts are going to be more of a problem at high frequencies, so your proposal benefits these frequencies the most ? And, isn't the input impedance capacitive anyway with a MOSFETs ?
Exactly.
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Old 27th August 2009, 07:49 PM   #30
Bigun is offline Bigun  Canada
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Still learning....


The latest schematic shows no d.c. path to the gate of the lower MOSFET. Yet it has to be biassed. Is it self-biassing ?
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