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Old 28th August 2005, 06:32 PM   #1
1001 is offline 1001  Italy
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Question Feedback & audio amp

Is it possible to build an audio amp whit a feedback factor of 250-300dB?
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Old 28th August 2005, 06:44 PM   #2
MikeB is offline MikeB  Germany
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Are you aware that 250db are a feedbackfaktor of 3.162.277.660.168 ?

So, the answer to your question is: NO !

Mike
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Old 28th August 2005, 06:49 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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If you cascade enough stages you can have any feedback factor you want up to infinity, but making it stable would be correspondingly close to impossible.
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Old 28th August 2005, 07:11 PM   #4
tlf9999 is offline tlf9999  United States
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Sure you can: just make sure that you have enough open loop gain.

Whethere it is stable is another question.
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Old 28th August 2005, 07:34 PM   #5
MikeB is offline MikeB  Germany
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I thought he was asking for an audioamp, not a radiotransmitter...
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Old 28th August 2005, 08:18 PM   #6
jcx is online now jcx  United States
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will that be global or local?

Feedback is limited by stability requirements, at some high frequency all physical systems have excess phase shift which turn the desired negative feedback into oscillation causing positive feedback

For audio power amps with conventional output devices/packaging the unity loop gain intercept is usually limited to (optimistically) <20MHz for mosfets or <5MHz for bjts, the choice of how you roll off loop gain to reach the unity gain intercept then determines your allowable audio frequency loop gain

Single pole dominant compensation is most popular and the most conservative, limiting loop gain roll off to 20bB/decade which limits loop gain to 40-60 dB at high audio frequencies (work backwards from 20 MHz; 20KHz to 20 MHz is 3 decades of room to roll off the audio frequency loop gain)

Bode’s maximum feedback can be approximated with multiple pole-zero compensation and increases gain slope to 30dB/decade; 80-90 dB loop gain would be possible – but you never see this type of compensation

2-pole compensation abandons Bode’s unconditional stability and may require inspired clamping/limiting of internal nodes to avoid bad clipping/current limiting behavior but allows over 100 dB loop gain at audio frequencies

Additional local feedback loop gain is possible, nested feedback loops have been used around the output stage so that the output stage may be wrapped in 100 + dB of local gain which is not visible to the outer loop – your 200 dB number might be approached in this way by a Halcro amp's output stage
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Old 28th August 2005, 11:22 PM   #7
tlf9999 is offline tlf9999  United States
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Quote:
Originally posted by jcx
[B]Feedback is limited by stability requirements, at some high frequency all physical systems have excess phase shift which turn the desired negative feedback into oscillation causing positive feedback

I disagree with that: as long as you have small enough of a gain (less than unity) when the feedback turns positive, you will have a stable system.

Instability isn't inherent to negative feedback. It is the combination of gain and phase shift that cause feedback to be unstable.
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Old 28th August 2005, 11:59 PM   #8
1001 is offline 1001  Italy
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Ok, so I could build this amp but it's impossible to stabilize it, is it true?
Why is not possible to stabilize this amp and how much high could be the open loop gain before it is impossible to stabilize it?
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Old 28th August 2005, 11:59 PM   #9
1001 is offline 1001  Italy
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Quote:
Originally posted by jcx
will that be global or local?

Feedback is limited by stability requirements, at some high frequency all physical systems have excess phase shift which turn the desired negative feedback into oscillation causing positive feedback

For audio power amps with conventional output devices/packaging the unity loop gain intercept is usually limited to (optimistically) <20MHz for mosfets or <5MHz for bjts, the choice of how you roll off loop gain to reach the unity gain intercept then determines your allowable audio frequency loop gain

Single pole dominant compensation is most popular and the most conservative, limiting loop gain roll off to 20bB/decade which limits loop gain to 40-60 dB at high audio frequencies (work backwards from 20 MHz; 20KHz to 20 MHz is 3 decades of room to roll off the audio frequency loop gain)

Bode’s maximum feedback can be approximated with multiple pole-zero compensation and increases gain slope to 30dB/decade; 80-90 dB loop gain would be possible – but you never see this type of compensation

2-pole compensation abandons Bode’s unconditional stability and may require inspired clamping/limiting of internal nodes to avoid bad clipping/current limiting behavior but allows over 100 dB loop gain at audio frequencies

Additional local feedback loop gain is possible, nested feedback loops have been used around the output stage so that the output stage may be wrapped in 100 + dB of local gain which is not visible to the outer loop – your 200 dB number might be approached in this way by a Halcro amp's output stage
I refer to global, overall, whole feedback factor, of course

Why do I never see multiple pole-zero compensation? What do you mean? Doesn't exist commercial amp (pre amp, headphone amp, opamp, etc.) with this type of compensation?

2-pole compensation allows over 100 dB loop gain at audio frequencies would mean as well 200-300 dB?

very interesting... so an Halcro amp's has 100 dB of FB in the output stage and 100dB in the global loop, is it true?
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Old 29th August 2005, 01:54 AM   #10
jcx is online now jcx  United States
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tlf9999;

feel free to be more precise in a short quick post, be sure to mention minimum phase/nonminimum phase, device carrier transist time, Bode's integral of feedback...


1001;

I'm merely speculating based on what little is public about Halcro's amp - he claims to use "video op amps" in local loops around the output devices - probably only 60-80 dB local V gain - what the final number is is only known by Halcro himself - but if you want to spend the $25K it is perfectly legal to reverse engineer one!

Prof Ed Cherry wrote about nested feedback loop audio amplifiers in the 70's

Bode's maximum feedback has a s^1.5 slope - it requires many R-C stages to approximate this slope - see "pink noise" filters for an idea of fractional roll off approximation with discrete parts

high feedback is a tool with limitations, no one has demonstrated that 200 + dB feedback is necessary for audio - few audio playback signal sources can deliver 120 dB S/N

as a precision instrumentation designer I know how many layers of implementation detail have to be addressed to get to even 16-bit resolution - high feedback is only a minor part - uOhm amplifier out Z at the feedback terminal is meaningless with 100s of mOhm cable and 1-10 mOhm connectors to mention only one limitation - measuring Halcro's ppm distortion is an exercise in defining where the amplifier’s output is - to avoid nonlinear load currents in interconnect z from swamping the measurement

if you really want to pursue high feedback then b j Lurie's "Feedback Maximization" is the book for you - after completing a undergrad controls curriculum –the pricey “Classical Feedback Control” by Lurie is more readable but also full of publishing errors – not the book to learn the material from cold
http://www.luriecontrol.com/Classica...ackControl.htm
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