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Old 31st January 2004, 03:55 PM   #11
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Hi Sam9!

Have you a RC Zobel at the output??
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Old 31st January 2004, 03:59 PM   #12
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Quote:
Originally posted by sam9
After this project, I think I've had enough CFB for a while the next amp is going to be an EF output.

my experience has been that CFBs are tough to get to work stablely, this is particularly true with mosfets.

It can work but it takes a lot of tweaking.

I have resigned from using CFB in my amps. just emitter followers,
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Old 31st January 2004, 04:28 PM   #13
sam9 is offline sam9  United States
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"my experience has been that CFBs are tough to get to work stablely, this is particularly true with mosfets."

At least two authors list the benefits of CFB, and then as a kind of afterthought mention that they are subject to oscillations for which a sound understanding and remedy remains elusive. It's easy to miss this caveat after reading how wonderful they otherwise are. I suppose one additional benefit is that if I ever get this thing to run the way I want, it will be a big ego boost.
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Old 31st January 2004, 05:12 PM   #14
rickpt is offline rickpt  Portugal
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The problem is that your output stage is just too slow, that why you decreased compensation and the oscillation problem got better....

I can see four solutions:

1-use only one output device...
2-increase driver current...
3-put a folower between the driver and the output device...(this creates other problem because of the big increase in the local open loop gain)
4-scrap that crapy output stage and build a folower that gives a much better results wen conected to a reactive load...

millwood

if you realy understand the problem you will see why mosfets(gate capacitence is multtiplied by the gm of the mosfet in commun source mode) are a big no no in a conpound folower

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Old 31st January 2004, 07:20 PM   #15
sam9 is offline sam9  United States
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"The problem is that your output stage is just too slow, that why you decreased compensation and the oscillation problem got better...."
Pehaps, but at least as far as the actual devices go 2sa1943/2sc5200 anf 2sd669/2sb649 are about as "fast" as one is going to find from the usual sources. But see "2-", below.

"I can see four solutions:
1-use only one output device..."
Not an option given tne intended application. Also future applications, I anticipate would need even more.

"2-increase driver current..."
I'm trying out some mods today to see if this this can be done. Plus another idea or two.

"3-put a folower between the driver and the output device...(this creates other problem because of the big increase in the local open loop gain)"
Most likely I would be substituting one agravation for another -- not progress

"4-scrap that crapy output stage and build a folower that gives a much better results wen conected to a reactive load..."
This is the most likely resolution. However, I don't agree with the adjective, "crapy". More like "tempermental", such as when young I had an experience with a French girl friend and a very fast Italian car. Frustrating, expensive and ultimately a waste of time - but educational.
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Old 31st January 2004, 09:13 PM   #16
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Default The tale of a French girl....

Quote:
Originally posted by sam9
Frustrating, expensive and ultimately a waste of time - but educational.
When something is educational...is never a waste of time...

Cheers
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Old 31st January 2004, 10:02 PM   #17
AKSA is offline AKSA  Australia
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Sam,

I suspect you have oscillation of the Complementary Feedback Pair.

Place a 100pF (for starters, might need more) between the base and collector of each driver.

This will reduce gain of the driver at high frequencies, improving Nyquist stability.

Furthermore, you can sometimes stop oscillation without sonically destructive base/collector caps on the driver by introducing 0R22 to 0R47 emitter resistors into the output devices, between rail and emitter. This reduces gain within the CFP also, but does not introduce sonic penalty. It does however cost rail efficiency.

I would tactfully agree with the comments about 'crappy CFPs'. I'd strongly suggest a couple of emitter followers in cascade.

Cheers,

Hugh
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Old 31st January 2004, 10:25 PM   #18
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Originally posted by sam9
I suppose one additional benefit is that if I ever get this thing to run the way I want, it will be a big ego boost.

Sam, you only get the ego boost after your ego has been busted working out a solution in vein, .

I think many of the problems associated with CFB is in wiring / layout. I can count the number of my working breadboarded CFB amps I have done in one hand (a couple at best). One such beast took a couple years and I still failed, . I guess the stray capacitance / inductance of the wires on a breadboard does wonders in getting the circuitry to oscillate.

Your chance of getting it to work on a pcb is likely better but for small time hobbist, we really cannot afford to revise PCBs endlessly. so it may be one of those areas where it pays to get a proven layout for a CFB amp - I think P3A uses CFB.
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