Ever made a MOSFET CFP/Sziklai that works? - Page 9 - diyAudio
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Old 7th January 2008, 03:47 PM   #81
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Mine is an opamp driven (LM4562) Sziklai pair biased with a pair of diodes loaded by a current source and a current sink.

The idea is to build a dorm-room amplifier that doesn't generate much heat and that gives its best distortion figure at arround 2 to 8 Wrms. Simulation results give less than 0.001% distortion for all the audio spectrum at this range. Simulation does not use any compensation part but a 100pF silver-mica between the two bases.

I'm happy to see that yours worked. I may build one in a month or so and post the results here since this thread really deters one from trying hibrid Sziklai arrangements.
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Old 9th January 2008, 12:56 PM   #82
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Click the image to open in full size.

Voltage reference diodes (or diode connected transistors) for CCS mounted to heatsink for outputs.
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Old 9th January 2008, 01:22 PM   #83
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by djk
...

Be aware this configuration suffers from serious crossover spikes due to the fact that the output device transitioning from cutoff to conduction must have its gate capacitance rapidly charged in order to reach threshold voltage, and this in turn implies a large current (gate capacitance is in the nF range) for a brief period that must be supplied by the drivers.
In practice what one sees is a feedback induced step in the driver's base and ringing until the loop stabilizes.

On simple way to overcome this is connecting a resistor from gate to ground for each output device so as to ensure it never reaches full cutoff, but the idle current is hard to stabilize and in practice results in a quasi class A mode with heavy disipation.

I posted here a more complex bias arrangement that works nice and have made amplifiers in the low 0.001% THD range that sound good.

Rodolfo
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Old 9th January 2008, 01:55 PM   #84
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Yes, the bias servo is a better idea.
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Old 9th January 2008, 03:38 PM   #85
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Hey that's really similar to my design (despite other current-source topology and other practical details). The innovative thing is the cap instead of a resistor to have a non-turn-off design you use.

Can I borrow that trick for my design if i plan to post them?
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Old 27th January 2013, 01:55 PM   #86
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eventhough i discovered this too late i also think its a nice idea to take a look at once more ..

perhaps after more than 5 years past since last post there might be something new that we have learned in between ..

Please look at it again

Kind regards
sakis
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Old 27th July 2014, 08:25 PM   #87
Bigun is online now Bigun  Canada
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anyone ? :-)
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Old 27th July 2014, 10:46 PM   #88
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I am working at the moment on the mosfet CFP output stage amplifier, the first prototype is working fine, sound is very delicate but I must spend some more time on it to learn a bit more about CFP output stages.
The schematic is simple. If someone can give me some more advices/ideas about improvements would be nice.
The config from post 82 is looking nice. The spice is throwing thd around 0.005%@10kHz@ 50Vp-p wchich looks ok for me but my board is throwing around 0,5% thd at the same conditions
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Old 27th July 2014, 11:43 PM   #89
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Looks nice !

... usually for me, for a CFP, the 'local' feedback from output drain to driver emitter should be connected directly and then isolated from the amplifier output by a resistor. You have a different arrangement, more like common emitter driver and common source power device. Well - just a thought anyhow.
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Old 28th July 2014, 12:18 AM   #90
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Quote:
Originally Posted by borys View Post
The spice is throwing thd around 0.005%@10kHz@ 50Vp-p wchich looks ok for me but my board is throwing around 0,5% thd at the same conditions
Since most THD comes from output stage try to make these changes:
- omit R22, R33
- put 50 Ohm parallel with 10 uF film cap between emitters of Q11, Q13
- connect R31, R32 to sources of M1, M2, not to rails
- lower degeneration resistors R20, R21 to 22 Ohm or less
- use low Z degeneration resistors to bases of Q11, Q13
- higher the voltage gain of VAS/mirrors by lowering emitter resistors
- lower R8 to 10 kOhm
- lower R1, R2
- connect 0,1 uf parallel to R16
- C13 to 1 uF
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