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
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
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...
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...
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.
a+b) two single transistor versions.
c+d) two same sex versions.
e+f) two versions with complementary transistors.
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
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
A VAS using CFP's :
http://pagesperso-orange.fr/francis.audio2/Rds236_palouda.gif
Simulation shows it is very linear :
http://francis.audio.monsite.wanadoo.fr/page5.html
(chapter "SIMULATION DU DEUXIEME ETAGE DE L'AMPLIFICATEUR")
http://pagesperso-orange.fr/francis.audio2/Rds236_palouda.gif
Simulation shows it is very linear :
http://francis.audio.monsite.wanadoo.fr/page5.html
(chapter "SIMULATION DU DEUXIEME ETAGE DE L'AMPLIFICATEUR")
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
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
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.
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.
AKSA said:
Wow! Never heard that kind of expression before. I thought he was joking or something... Well, another day I learned something new...
Thanks Hugh.
Self is obsessed with single ended LTP and his CDOM, apart from this nothing exist in amplifier designs according to his views.
Kanwar,
Nice to see you here. Perhaps you are right......
But then, he did pick the simplest of all configurations to explain, so this is it, I guess.
Hugh
Nice to see you here. Perhaps you are right......
But then, he did pick the simplest of all configurations to explain, so this is it, I guess.
Hugh
He did admit they existed , but "scared" readers away from
more advanced designs.
Saying "too hard to compensate" or "tends to be unstable"
attracted me to other topologies. 😀
OS
AKSA said:
Hello Hugh,
Nice to see you too🙂
Yes, but other topologies are worth consideration as well and complementary thing is not that complex.
Cheers
Hi forr
Now double it up to make a differential VAS, and don't skimp on the parts. Go all out for quality matched arrays.
Now double it up to make a differential VAS, and don't skimp on the parts. Go all out for quality matched arrays.
CBS240 said: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.
CBS240 said:Hi forr
Now double it up to make a differential VAS, and don't skimp on the parts. Go all out for quality matched arrays.
Hi CBS
Well, I was going to ask you to show your best VAS...
I am very interested by constant power schemes.
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