Which node(s) to worry about in CFB amplifiers

[Pjotr]

Originally posted by pjacobi

Summary:
Your standard CFB layout (as in EL2020 or QRV06) has (approximately) only one voltage gain stage, as the input stage is working into a low Z node.
An archetypical "standard" audio amplifier with LTP inputs (as seen in the Self, Slone and numerous other books) has two voltage gain stages, as in the input stage is working into a high Z node.
But you can also have it the other way around, as shown above.

Regards,
Peter Jacobi

Hi Peter,

Sorry, but I am missing the point what you actually want to tell us.

Regards

[pjacobi]

Originally posted by Pjotr
[...]
http://www-s.ti.com/sc/psheets/slva051/slva051.pdf
[...]

When seeing the schematic for the THS4001 in this paper, I realized that I've forgotten another important topology: folded cascode.

This is another possibility to let a LTP input work into a low Z node and consequently resulting in a single voltage gain stage.

See these schematics:

Symmetrical version:
PDF: http://www.linearaudio.de/scratch/fo...ascode-sym.pdf
LTSpice: http://www.linearaudio.de/scratch/fo...ascode-sym.asc

Asymmetrical version:
PDF: http://www.linearaudio.de/scratch/fo...scode-asym.pdf
LTSpice: http://www.linearaudio.de/scratch/fo...scode-asym.asc

Some observations:
- It's not straightforward to mix this with a diamond input stage, as you must add 180° somewhere for the right polarity.
- Especially the symmetric version behaves in simulation very similiar to a diamond+current mirror topology. It would be most interesting to compare measured and perceived performance
- Further cascoding with JFETs will give those low THD numbers in simulation, which are often laughed about (but not near -300dB as Bernhard reqularely achieves...)
- Curiously, in the asym version K2 and K3 are of same (low) height wheras in the sym version K3 is 10dB lower. Go figure!

Regards,
Peter Jacobi