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Old 17th December 2008, 01:51 PM   #1
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Default Triple emmiter follower

Hi, I am exploring bjt buffer output stage using MicroCap. There is a problem (at least in simulation) when output is cap-loaded. Input of the driver is loaded with +/-30V 10KHz rise and fall time 100nS.
I attached the schematic and output signal when loaded with 100nF and 2uF. (I used MJL model by Andy C)

Is this simulation problem real and could this be fixed without using output coil ,zobel and feedback?

Thanks.
Borko.
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Old 17th December 2008, 01:52 PM   #2
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schematic:
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Old 17th December 2008, 01:55 PM   #3
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100nF-8-Ohm-load:
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Old 17th December 2008, 01:57 PM   #4
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2uF-8-Ohm-load:
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Old 17th December 2008, 02:02 PM   #5
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Quote:
Originally posted by bogdan_borko
2uF-8-Ohm-load:
hi Bogdan,
send me your .cir and I will play with it as well. I use MC9. My e-mail is nico_ras@telkomsa.net

Nico
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Old 17th December 2008, 02:07 PM   #6
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Quote:
Originally posted by Nico Ras


hi Bogdan,
send me your .cir and I will play with it as well. I use MC9. My e-mail is nico_ras@telkomsa.net

Nico
The file i sent.
Cheers!
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Old 17th December 2008, 02:14 PM   #7
Bonsai is offline Bonsai  Taiwan
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I have not seen this in practice on a square wave.

However, on a number of amps I've designed and built, on a10-15Khz sine wave output, I got 'ringing' at 100Khz and higher on the final 90 degree quadrant (so as the sine wave comes off the negative pk voltage and heads towards 0V.

In these cases the cause is almost always parasitic oscillation in the output stage. Additionally, if your amp has a 'sticky' problem, this can make the situation worse on output voltages that swing close to the rails, but don't quite clip. ('sticky is when the amp sticks to the rails when overdriven - i.e does not recover cleanly)

A few things to fix it:-

1. Use a Zobel directly on the output before the output inductor (10Ohms + 0.1uF)
2. Use a 100 Ohm in series with the pre-driver base transistors from the VAS
3. In really stubborn cases, I use a 27 ohm in series with the the positive driver base and the negative driver base. From each driver base, I put 1nF 1kV capacitor to 0V.

Parasitic oscillation in triples is a common problem - in fact, Microcap even published an ap note on this issue and how to cure it (sorry, I do not have it anymore) - this is where I got the 27 ohm and 1nF cap solution from (they used different values - I got mine from trying it practically).

Good luck
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Old 17th December 2008, 02:40 PM   #8
lineup is offline lineup  Sweden
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Bonsai

Why settle for Tripple

When you can try the brand new

Lineup Quattro Output Stage
.. just invented

high gain super plus!!!!
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Old 17th December 2008, 02:56 PM   #9
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Quote:
Originally posted by lineup
Bonsai

Why settle for Tripple

When you can try the brand new

Lineup Quattro Output Stage
.. just invented

high gain super plus!!!!
Hi Lineup.

Very interesting "invention", I would like to try this. Have time to kill.

Nico
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Old 17th December 2008, 03:21 PM   #10
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Quote:
Originally posted by lineup
Bonsai

Why settle for Tripple

When you can try the brand new

Lineup Quattro Output Stage
.. just invented

high gain super plus!!!!

NOPE got to think this one through a little.
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