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Old 21st February 2008, 03:38 PM   #1
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Default CFP output emitter resistors

Which of these circuits is the more correct way to construct a CFP output with parallel pairs? Is there a better topology than either version?

Slone used the circuit on the left, but I've seen versions like that shown on the right. The only difference being the position of the emitter resistors.

Has anyone had any good experiences with ferrite beads as gate stoppers to prevent parasitics?
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Old 21st February 2008, 03:55 PM   #2
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R3x in the circuit on the right will have some say in stabilising the output current and also provides local nfb . The one on the left will not affect the quiesent current of the output stage. The MOSFETs on the left should be well matched so that the bias current is equal in both devices .
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Old 21st February 2008, 04:07 PM   #3
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The circuit at right is the correct one.
The one at left is wrong.
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Old 21st February 2008, 04:07 PM   #4
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Neither are correct. Look at the ESP site P27 amp for how it should be done.
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Old 21st February 2008, 04:23 PM   #5
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Thanks for the tips, I'll get on with some more research!

I've had a quick glance at the ESP circuit, and it does seem to make a lot more sense. The circuit on the left is from one of Slones designs, but it just doesn't seem right to me.

Any suggestions for more reading on CFP? Self and Slone mention it, but dont go into great detail, particularly regarding parallel pairs. Info in the art of electronics is pretty scant too.
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Old 21st February 2008, 04:46 PM   #6
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Quote:
Neither are correct.
Whats wrong with the circuit at right?
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Old 21st February 2008, 09:46 PM   #7
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hi,

both are incorrect:
mosfets have no overdrive protection.
a zener should be there to limit the gate voltage & a diode in series w/ it to reduce the capacitive effect introduced.
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Old 21st February 2008, 10:05 PM   #8
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Quote:
Originally posted by longthrow
hi,

both are incorrect:
mosfets have no overdrive protection.
a zener should be there to limit the gate voltage & a diode in series w/ it to reduce the capacitive effect introduced.
The question was that which method is correct related to paralelling mosfets in an audio amplifier's output stage.
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Old 21st February 2008, 10:24 PM   #9
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Quote:
Originally posted by longthrow
hi,

both are incorrect:
mosfets have no overdrive protection.
a zener should be there to limit the gate voltage & a diode in series w/ it to reduce the capacitive effect introduced.
Thanks for your comments. I have omitted pretection circuitry for simplicity though. Could you clarify that you believe other defects exist apart from this, or not?
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Old 21st February 2008, 11:11 PM   #10
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Richie, I stumbled across an old thread of yours:

here

I'd be interested to hear how you eventually got on? Is there any chance you could post the schematic for the OPS for reference please?
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