how many ways to stablise?

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Yes, I think we agree. The quote about the integrator was from the earlier discussion when the question came up of whether individual components of the loop had to be stable for the overall loop to be stable. I had brought up the ideal integrator in that context, since it's the most common case where an individual block is not stable by itself but the overall loop can be (PLL's and other servos etc.). But I think we all agreed later in the thread that stability of individual blocks isn't strictly necessary for stability of the overall loop.

And you also came up with the example where it's not always possible in a practical sense to stabilize the overall loop in the presence of an arbitrary local instability. This was Peufeu's example of the VAS with a local 50 MHz instability if I remember correctly.

So I think we're all on the same page :).
 
andy_c said:

So I think we're all on the same page :).

I agree... :)

You know, peufeu's discussion and hearing tests provide an interesting insight into how misleading subjective beliefs can be...

Further down this page on the 'Whoa' section:

http://peufeu.free.fr/audio/memory/memory-8-complete.html

Peufeu finds that, (contrary to Otala and others who believe in wide foward-path bandwidth), reducing second-stage gain to obtain increased bandwidth actually 'sounds' bad....

..and that very large foward-path gain is in fact a 'good' thing...

So much for consistency among subjectivists..... :rolleyes:
 
jcx said:
Hi mikeks,

the TDA7294 block diagram shows the Cherry stlye output stage inclusive compensation (I would call the output devices “power transistors” in this chip amp):

http://us.st.com/stonline/books/pdf/docs/1057.pdf

“A significant aid in keeping the distortion contributed
by the final stage as low as possible is provided
by the compensation scheme, which exploits
the direct connection of the Miller capacitor
at the amplifier’s output to introduce a local AC
feedback path enclosing the output stage itself.

Perfectly feasible (and so it should be done) with a MOSFET output stage, (as is the case with the TDA7294 monolithic device to which you've refered), whose dominant poles are located in the 100s of MHz.

This is in contrast to power BJTs whose dominant singularities reside between 200KHz and 2MHz...if your lucky...
 
From Bob Widler...

...the designer of the LM3875's direct antecedent...the LM12:

Compensating directly from the output requires well-controlled response in the output inverter out to nearly 100 MHz if spurious oscillations in the integrator loop are to be avoided.

This is not easily accomplished over the full range of dc and ac loading conditions that a power op amp can encounter (including capacitors resonating with lead inductance).

See pg 2 here:


http://www.national.com/an/AN/AN-446B.pdf
 
jcx said:
Hi mikeks,

How about some substantive discussion instead of uninformative derision

In nested feedback you have to apply Bode’s multiloop criteria, individual loops are permitted to be “unstable” without the overall amplifier being unstable – if you are referring to conditional Nyqusit stability, Cherry brings this up himself, the existence of physical prototypes with scope probe traces in his articles strongly suggest some range of stable operation

There are only few hundred million counter examples to your claims that output inclusive and nested compensation can’t work

NE5532/4 compensation is described in these terms in Eschausier’s 1995 book “Frequency Compensation Techniques for Low-Power Operational Amplifiers”

The IEEE has more than a few papers in the “nested miller” genra

Other op amps use output stage inclusive compensation

LT1028/1128/1115 has a unity gain buffer type output stage inside the Ccomp loop

all common emitter/source types have to use output connected compensation

the TDA7294 block diagram shows the Cherry stlye output stage inclusive compensation (I would call the output devices “power transistors” in this chip amp):

http://us.st.com/stonline/books/pdf/docs/1057.pdf

“A significant aid in keeping the distortion contributed
by the final stage as low as possible is provided
by the compensation scheme, which exploits
the direct connection of the Miller capacitor
at the amplifier’s output to introduce a local AC
feedback path enclosing the output stage itself.


http://www.diyaudio.com/forums/showthread.php?postid=475131#post475131
 
Indeed my current design incorporates positive feedback within an output stage nested feedback loop!


It's stable AND clean clipping!

But I'm keeping it under wraps as my last design (Eidetic) has apparently been patented by locals some 8 years after I published it.

Cunning as sh*thouse rats they are!
Oh well imitation is the highest praise.

Greg
 
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