CFP For The VAS

[selim]

Hi All,

Most of the circuits I have seen here in this forum and elsewhere
utilize a single transistor for the VAS. Mr.Self, describing his
'Blameless Amplifier', maintains that it is best to use a darlington
connected pair for the VAS.I would like to hear your opinions
on swapping this darlington configuration with a Sziklai pair.

One downside I can think of is, in a darlington connection as in
Mr. Self's amps, the input transistor of the darlington pair
operates in an almost constant Vce because the collector is
tied to the ground.In a CFP configuration, both transistors
making up the VAS will see the rail-to-rail excursion, forcing
the first transistor to be a high voltage type as well.

Selim

[peufeu]

Sziklai has advantages for voltage followers but the VAS is just a current amplifier, so using a darlington or a CFP is about the same.

View the input stage as a current source (it converts the error voltage into a current) if you have problems understanding this.

If you don't use a current mirror in the input stage, it's a bit less clear, though...

[mikeks]

Originally posted by peufeu
......the VAS is just a current amplifier.......

Nyet. Transimpedance amplifier.

[Workhorse]

I think there is an advantage of using CFP in VAS and that is more linearity .

[AndrewT]

Self shows 6 variations of the VAS, all with his preferred Cdom.
a+b) two single transistor versions.
c+d) two same sex versions.
e+f) two versions with complementary transistors.

[blueskynis]

Originally posted by AndrewT
c+d) two same sex versions.

I just can't get it what are you saying. Is this a miss-type?


Cheers

[AKSA]

No, it is not a misquote. Andrew is very precise in his contributions.

It is a reference to both transistors being identical types, that is, both npn (fixed at the negative rail) or both pnp (fixed at the positive rail).

Cheers,

Hugh

[forr]

A VAS using CFP's :

http://pagesperso-orange.fr/francis....36_palouda.gif

Simulation shows it is very linear :

http://francis.audio.monsite.wanadoo.fr/page5.html

(chapter "SIMULATION DU DEUXIEME ETAGE DE L'AMPLIFICATEUR")

[AKSA]

Thanks Sebastien. I have used this precise VAS configuration in one of my amp designs.

Make each stage as linear as possible, then apply feedback, not much at all in this case..... there is a LOT of degeneration in there.

Output stage, another CFP, shows signs of very great design care. I assume it's Class AB, however, with some issues at switching.

There is nothing new under the sun.

Thank you for sharing this circuit, Sebastien,

Cheers,

Hugh

[CBS240]

Hi

It depends on how you perceive the circuit. The best VAS I have ever built consists of a bipolar, constant power, differential bridge. If you look at the constant power part, it resembles a CFP that happens to be cascode. So I guess my vote goes towards the current feedback in that sense.