Class-A Opamps

[mikeks]

Originally posted by nuvistor
I wonder, what is the purpose of the 47p capacitors? the time constant with the 150R is 7ns.

Improve minor-loop stability margins...

[darkfenriz]

Mike
Is this a machanism of introducing a zero for 'miller-type' pole?

[mikeks]

Originally posted by darkfenriz
Mike
Is this a machanism of introducing a zero for 'miller-type' pole?

No...Miller compensation rolls off foward path gain by increasing minor-loop transmission about the second stage....This is denoted by a pole in the foward path gain-frequency response..

It follows that the reduction in foward path gain must simultaneously be accompanied by an increase in minor loop transmission about the second stage...denoted by a zero....

Bypassing the second stage's emitter resistor merely increases minor loop transmission at ultrasonic frequencies...near the local loop's unity gain frequency....ergo...introduces yet another arbitrary zero....which enhances stability in the local loop.....

[darkfenriz]

Do you mean local miller integrator might need a zero for better stability?
I remember Horowitz saying like "one must be extrmely talented to make an integrator instable".
How about integrator around one device?
Is this really an issue?

Thanks
best regards

[mikeks]

Originally posted by darkfenriz
Do you mean local miller integrator might need a zero for better stability?

Yes...and No...you do not have to be 'extremely talented' to make the second stage of an op amp. unstable...Just enclose three or more transistor stages within the loop...

[lineup]

Originally posted by lineup

Generally, why is almost no Chip OP-amplifier in Class A
I think is
when you go for an IC ( and not discrete ) Operational amplifier
- you want it simple
- you want it as low power supply as possible
- you might want to build a great number of identical devices

If you want to find TRUE Class A OP-amps, you have to go for discrete.
Like this one by Graham Maynard:
http://www.mitedu.freeserve.co.uk/Ci...dio/preamp.gif

Hi-Fi Preamplifier by Graham Maynard
.

Originally posted by amplifierguru
Hi lineup,

There are many modern op amps
that operate enough standing output stage current
that they will operate Class A into av high Z load
- so they can be lightly loaded with a discrete buffer stage.

Class A may be a goal in itself in low power and preamps but in power amps it's generally a wasteful fallback position.

Cheers, Greg

Hi, Greg Guru.
I am still not convinced enough to begin to replace
my Class-A discrete pre amplifiers and power amplifiers.
And if I will build myself a new pre/power amp ( in case I have ANY USE for another amp )
I certainly will go 100% Class-A, push-pull complementary or quasi-complementary.


Like for example Graham Maynard Class-A (based on work of JLH, John L. Hood ).
See my ATTACHED SCHEMATIC
This is in my Lineup Audio book,
how a good Audio Amplifier should be!



Audio Regards to Australia, all kangaroos and amplifierguru
lineup
-------------------------------


More info, background and description at Graham Maynard
website
http://www.gmweb2.net/
Latest published version of GM25W Class-A is 2007-01-26
Cheers & have some bears. Russian Bear is great & big!

[jcx]

B.VDBOS skectch for biasing monolithic op amps class A wasn't too far off

I've acutally built a monolithic op amp headphone amp that is biased Class A for up to 400+ mA ~ 5W into 60 Ohms


http://www.head-fi.org/forums/showth...12#post2276512

scroll around that thread for my (jcx) posts showing a rather extreme implementaiton

basic principle:

[lineup]

thanks
for your interest jcx

now, it is with amplifiers
as is with eat ice-cream

... to each his own ... by the taste of his tongue

.... what is TASTELESS to one, is something ELSE for one other


lineup with pen ... not with his tongue

[lineup]

Class A IC-Chip op-amps do not exist.
At least not manufactured in any quantity and so commonly found for us.

To single end bias the output stage of one Class AB op-amp
will not give you one Class A op-amp.
Because the op-amp was designed & calculated for a specific output stage operation
= Symmetrical Class B / Class AB, in 90% of all known cases.

If you want Class A op-amp, you must go discrete.
There are plenty of discrete transistor operational amplifiers in Class A.
In fact, it is more The Rule than the exception.
all sorts of, JFET or BJT input, complementary or one LTP differential pair inputs.

Because when a good designer is left with the choice,
he will go Class A all the time, if there is no special thing against it.
And most often is not. Except for Power Op-Amps applications.

[EWorkshop1708]

Originally posted by EWorkshop1708


Too bad I found your post AFTER I built a small preamp using a 324

But thank your for that valuable information.

Wow, old posts!

I now use an MC33079 opamp with 0.002% THD and am VERY pleased. No other changes to the preamp other than changing the IC chip.