Current feedback - Voltage feedback, how do I see the difference? - diyAudio
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Old 21st June 2004, 08:51 AM   #1
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The vast majority of power-amp designs that I have seen have used a single dominant pole for compensation.

My own home-made PA uses a current-feedback architecture so doesn't have too many problems with slew-rate limiting (transient or otherwise)
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Old 21st June 2004, 08:52 AM   #2
dimitri is offline dimitri  United States
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> This is verifiably untrue......Slew induced distortion (before the onset of slew limiting) only occurs with any significance, in single-pole Miller compensated amplifiers..

This is only gross TIM, soft TIM is generated as soon as the input signal of the given stage is large enough to stress any nonlinearity mechanism. Sorry mike, you are really mixed everything, apples and carburettors.
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Old 21st June 2004, 08:58 AM   #3
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Old 21st June 2004, 09:01 AM   #4
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Ouroboros
The vast majority of power-amp designs that I have seen have used a single dominant pole for compensation.

My own home-made PA uses a current-feedback architecture so doesn't have too many problems with slew-rate limiting (transient or otherwise)
Indeed the vast majority of commercial designs use single-pole minor loop compensation...this is because it is easy to implement, linearises the TIS...and is often entirely trouble-free with regard to instability.....

..However it increases the current swing demanded of the input stage with increasing frequency, provoking a propotionate increase in input stage distortion,.....

.... while simultaneously leaving the output stage exposed due to the fall in feedback, as the compensation capacitor rolls off loop transmission...



Quote:
Originally posted by dimitri
> This is verifiably untrue......Slew induced distortion (before the onset of slew limiting) only occurs with any significance, in single-pole Miller compensated amplifiers..

This is only gross TIM, soft TIM is generated as soon as the input signal of the given stage is large enough to stress any nonlinearity mechanism. Sorry mike, you are really mixed everything, apples and carburettors.

No Dimitri....

'gross TIM'=Outright slew limiting

'soft TIM'=Distortion provoked in input stage by increased loading of minor loop capacitor before it goes into outright current clip...
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Old 21st June 2004, 09:07 AM   #5
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Ouroboros
My own home-made PA uses a current-feedback architecture so doesn't have too many problems with slew-rate limiting (transient or otherwise)

Would avoid 'current' feedback in audio applications, as the input stage operates in class-AB at the feedback node.....

It's bad enough that one has to endure class-AB operation in the output stage....

...extending it to the input stage in audio frequency applications is near perverse....
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Old 21st June 2004, 10:04 AM   #6
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Quote:
Originally posted by mikeks



Would avoid 'current' feedback in audio applications, as the input stage operates in class-AB at the feedback node.....

It's bad enough that one has to endure class-AB operation in the output stage....

...extending it to the input stage in audio frequency applications is near perverse....

Mikek,

This is interesting. Do you mean that the feedback current exceeds the standing current in the low-impedance input pair?

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Old 21st June 2004, 10:21 AM   #7
AKSA is offline AKSA  Australia
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C'mon, I know what a carburettor is, I was being ironic.......

Quote:
Would avoid 'current' feedback in audio applications, as the input stage operates in class-AB at the feedback node.....
I'm not sure I get you here; can you explain more? I would have thought the input stage operates in pure class A; as I see it, at no stage would either diff pair device switch off completely until clip, or on those rare occasions when the feedback loop loses control.

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Old 21st June 2004, 10:40 AM   #8
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We have the word "karburator" in swedish,
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past 50 years or so.
we in the states have those beauties until late 1980s: my 1987 chevy nova has one.

Quote:
Originally posted by mikeks
...extending it to the input stage in audio frequency applications is near perverse....

mikeks, you aren't suggesting that current feedback applies to class-ab at the input stage, are you?

the jlh1969 for example has a current feedback input stage which operates in class a (the input stage).
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Old 21st June 2004, 11:39 AM   #9
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You can run a current-feedback input stage in class-A, but it does mean that the dissipation in the current-mirror stages is rather high.

Normally the input stage will run in class AB, to ensure there's plenty of current available to charge and discharge the compensation cap between the output of the current mirrors and ground.
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Old 21st June 2004, 11:59 AM   #10
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by janneman



Mikek,

This is interesting. Do you mean that the feedback current exceeds the standing current in the low-impedance input pair?

Jan Didden
This is particularly true at higher freqs., and for typical feedback resistor values....

Note the particularly low values commonly used......

Avoid current feedback in audio apps. in general, unless this is part of a composite arrangement in which the current feedback circuitry acts as a compound output stage....particularly in 'small signal' applications..

Otherwise stick to voltage feedback, where ultra high slew rate if desired, can be readily obtained by other means.....as i have shown elsewhere.....
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