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Old 11th November 2004, 10:03 AM   #21
Bricolo is offline Bricolo  France
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I want to know something about this Baxandall super pair

The "control transistor" seems saturated, since it's Vce is approx 0.7V (quite normal; since the control transistor's Vce is equal to the output transistor's Vbe)

Isn't this a problem?
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Old 11th November 2004, 04:18 PM   #22
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not saturated, just not strongly biased

many "current-mode" signal processing circuits bias the cb juntion at ~ 0 V

usually we want more cb bias to reduce Ccb magnitude and modulation with Vcb but in the super pair Vcb is "bootstrapped" by the output transistor to nearly constant voltage so little ac current flows in it , obviously a few V bias wouldn't hurt if it didn't further complicate the circuit
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Old 11th November 2004, 04:52 PM   #23
Bricolo is offline Bricolo  France
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OK thank you for the details.

I have an oscillation problem with this super pair, does someone have an idea on how to solve it?

"Super-Pair" I/V for TDA1543
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Old 30th August 2005, 04:23 PM   #24
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Hi, JCX,

Is this Super Pair not suitable for audio power amp's VAS because it always oscilate?

What do you think about CFP in place of VAS, why it seldom used in VAS?
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Old 30th August 2005, 04:54 PM   #25
jcx is offline jcx  United States
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somewhere here I showed some R-C damping connections that seem to tame the sims - I've protoed a few super pair circuits but can't claim they will always work without problems - you certainly can't expecct them to be less twitchy than regular cascodes - which can have probelms too


for more vas options:

Quote:
Originally posted by jcx
see "Gain Stage Investigations" Electronics World, july '98 by Hood

he shows a final config of the VAS as a mirrored diff stage driving a small mosfet common source (no degeneration) that is cascoded with a bipolar to get the max gain from high input impedance at the input to the VAS stage as he assumes a constant load on the VAS from the output stage

the high output Z available from the elaborated cascodes and super-pair enable higher gain if output stage drive impedance can be increased with further buffering - a point that i think can be seen in http://www.ne.jp/asahi/evo/amp/J200K1529/report.htm going from fig 7 to fig 13 (lots of mods but i think the performance couldn't get near these levels without the buffering of the output fet gate impedance)
I believe eva recently reiterated the point that cfp can have complicated clipping behavior/poor recovery - they look really good as long as they're working class A
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Old 30th August 2005, 05:20 PM   #26
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Quote:
Originally posted by Pedja
Hi Pierre,

Not really 100MHz per MicroCap and neglecting parasitic properties, but it seems like it doesn’t happen accidentally. Try the compensation capacitor (from the base of the first to the collector of the second transistor), say 68pF.

Pedja

Roll off the gain after a couple of MHz - a small cap across I2 should do it.

Jan Didden
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Old 30th August 2005, 05:21 PM   #27
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Quote:
Originally posted by Bricolo
OK thank you for the details.

I have an oscillation problem with this super pair, does someone have an idea on how to solve it?

"Super-Pair" I/V for TDA1543

See my previous post.

Jan Didden
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Old 3rd March 2007, 02:52 AM   #28
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This schematic uses superpair for VAS and VAS load. Will this oscilate?
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File Type: jpg superpair.jpg (67.5 KB, 1712 views)
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Old 4th March 2007, 04:59 PM   #29
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Hi peufeu

going back to the start of this thread ...

the Baxandall pair is an inverter ... so perhaps you just need a small resistance in series with the input with a small Miller capacitor connected from the collector of the output device to the base of the input device.

I'd suggest 1kohms + 10 pF ...

cheers
John
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Old 4th March 2007, 09:11 PM   #30
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Quote:
Originally posted by lumanauw
This schematic uses superpair for VAS and VAS load. Will this oscilate?
What does the OP stage look like?

Terry
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