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Old 26th June 2004, 10:55 AM   #1
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Question PWR stage in miller loop

I red again (I do that time to time) old articles about amplifiers.
Ed Cherry writes EW July 1997 "Ironing out distortion" about the miller compensation. He recommended to include the power stage into miller loop. This should lower the cross over distortion.
However D. Self wrote in EW Feb 1994 that this kind of solution will lead easily in to parasitic oscillation and he also wrote that the benefit is minimal if the bias in the power stage is set right.

No doubt that both gentlemen have their point. However the idea was so interesting that I started to figure how it could work with Fet power stage. The Fets generate more distortion, and the cheap "hexfets" or similar has big differences (p vs. n) in conductance over the cross over region.
The Fets are much faster than bipolar which could make them better in the millering what Ed Cherry recommends.

Further... what about if the miller compensation is cut in to parts. One including the power stage and one part as the traditional including only Vas. It could solve the oscillation problem but still probably lower the Fet cross over region distortion and to make open loop distortion lower on higher freqs.

Anyone tested or any opinions ?
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Old 26th June 2004, 12:56 PM   #2
sreten is offline sreten  United Kingdom
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You need some voltage gain to miller the output stage,
its not possible to seperately miller a standard unity gain
output stage.

Adding some gain and local feedback :

You then get into the subject of nested feedback loops,
search on this.

sreten.
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Old 26th June 2004, 01:41 PM   #3
Bricolo is offline Bricolo  France
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can someone explain me what a miller loop is?
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Old 26th June 2004, 01:59 PM   #4
sreten is offline sreten  United Kingdom
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In a standard amplifier all the voltage gain comes from the VAS.

Cdom is usually connected between the VAS output and input.

The Miller effect makes this capacitor appear to be much larger
and it forms the dominant pole compensation of the amplifier.

Note that the Miller effect only takes place when the loop
has voltage gain, the more gain, more Miller effect.

Cdom also forms a local feedback loop around the VAS, as
it reduces gain it also reduces the output impedance of the
VAS improving its capability to drive the output stage.

However a possibility is to connect Cdom between the VAS
input and the output of the amplifier, including the output
stage within the "Miller Loop", the loop that is subject to
the feedback through Cdom.

sreten.
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Old 26th June 2004, 02:10 PM   #5
Bricolo is offline Bricolo  France
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Quote:
Originally posted by sreten
In a standard amplifier all the voltage gain comes from the VAS.

Cdom is usually connected between the VAS output and input.

The Miller effect makes this capacitor appear to be much larger
and it forms the dominant pole compensation of the amplifier.

Note that the Miller effect only takes place when the loop
has voltage gain, the more gain, more Miller effect.

Cdom also forms a local feedback loop around the VAS, as
it reduces gain it also reduces the output impedance of the
VAS improving its capability to drive the output stage.
Yes, that's Miller's theorem.
I know this one. But that's the 1st time I read about a Miller Loop


Quote:
Originally posted by sreten

However a possibility is to connect Cdom between the VAS
input and the output of the amplifier, including the output
stage within the "Miller Loop", the loop that is subject to
the feedback through Cdom.

sreten.

how?
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Old 26th June 2004, 02:15 PM   #6
sreten is offline sreten  United Kingdom
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How ?

You just move the connection of Cdom from the VAS output
to the amplifiers output. It may or may not go up in smoke
at this point depending on oscillations in the output stage.

sreten.
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Old 26th June 2004, 02:20 PM   #7
Bricolo is offline Bricolo  France
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I'm not sure I got it

Is Cdom a "real" capacitor, or an intrinsec one?
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Old 26th June 2004, 02:32 PM   #8
sreten is offline sreten  United Kingdom
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In a real amplifier Cdom is a real capacitor that takes Cbc of
the VAS transistor out of the equation, usually 47pF to 200pF.

You have a point, if Cdom is connected to the output, then Cbc
of the VAS transistor would have some effect, and not particularly
predictable either.

Perhaps its this that causes " intractable HF oscillation with
Inclusive Miller-compensation " according to D.Self.

Perhaps you'd need to cascode the VAS (to prevent local Miller
effect) for it to have a chance of working without oscillation.

sreten.
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Old 26th June 2004, 02:42 PM   #9
Bricolo is offline Bricolo  France
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This topic becomes more and more interesting

I'd like to begin in designing my own amplifiers, and I'm currently looking for informations about this.

Tell me if I'm off topic, so I'll maybe start a dedicated topic.



-what's the benefict of adding a Miller compensation (no matter which one (local or loop)) ?

-And what's the advantage of a Miller compensation, compared to a "global" compensation?
By global, I mean placing a cap across the feedback resistor (from output to input). In this scheme, the total speed of the amp is reduced, but all stages are running locally at "full speed"

-"In a standard amplifier all the voltage gain comes from the VAS."
Nothing from the differential?
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Old 26th June 2004, 02:54 PM   #10
sreten is offline sreten  United Kingdom
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The issues are standard design issues with power amplifiers
and operational amplifiers, a power amplifier is an overgrown
op-amp.

Its all textbook stuff, especially compensation for closed loop stability.

sreten.
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