True Current Feedback N-channel Mosfet Amp - diyAudio
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Old 16th January 2004, 07:06 AM   #1
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Arrow True Current Feedback N-channel Mosfet Amp

##DEDICATED TO "CIRCLOTRON THE DIYER FRIEND"##



This is the true current feedback amp using only N-channel Mosfets.
I am still Prototyping it while posting!
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Old 16th January 2004, 07:51 AM   #2
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ACTUAL PROTOTYPE COMPLETED!
Supply Voltage = +-50VDC
Slew rate = 250V/uS
Power = 200W TRUE RMS@4ohms@100Khz
Zero degree phase shift @50KHz
THD = 0.0012%
Frequency Response = DC TO 300KHz


ANY SUGGESTIONS ARE MOST WELCOMED!

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Old 16th January 2004, 08:40 AM   #3
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It's nonsens : IRF 640 have Id max. 12 A and in your connection it will be 12,5 A ! Also I don't see any thermal compensation of biasing, what is for these devices necessary !
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Old 16th January 2004, 08:45 AM   #4
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Mistake : Id max. is 18 A, but Pd of one device is too high and without compensation of biasin will be " life " of this device very short.
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Old 16th January 2004, 09:08 AM   #5
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Arrow CARIFICATIONISATIONS

SOME CLARIFYING COMMENTS FOR UR [Upupa Epops] COMMENTS



I have 5 YEARS of EXPERIENCE with IRF MOSFETS .
I have never used any thermal biasing scheme and CURRENT Sharing Source resistors ever on my MOSFET amps DUE to their
POSITIVE TEMPERATURE COEFFICIENT which is self regulating in itself.
AND never RUN into THERMAL RUNAWAY in 5 years!
I matched the output MOSFETS to 10% tolerances when Paralleling is required.
My amps are tested through out using 20Hz squarewave for 24Hours continously, without the sign of any so called thermal runaway[which is most dearest to BJTs].

MY ADVICE IS TAKE SOME REFERENCE ELECTRONIC BOOKS AND BRUSHUP UR KNOWLEDGE!
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Old 16th January 2004, 09:16 AM   #6
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My experiences with mosfets are much older . IRF 640 have possitive TC over app. 15 A ( ! ). Your words are true for lateral mosfets, which have it poss. TC at app. 100 mA.
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Old 16th January 2004, 10:09 AM   #7
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MR Upupa Epops(OR WHAT SO EVER IT IS)

Regarding ur mosfet experience!
Which i dont know??
ALL I KNOW FROM WHAT I HAVE EXPERIENCED AND GAINED THROUGHOUT MY MOSFET AMPLIFIER DESIGNING
1. I NEVER MET A GUY NAMED "THERMAL RUNAWAY" YET in 6 Years(maybe he is ur friend)
2. I HAVE SEEN THE EFFECT OF POSITIVE TEMP. COEFF.
IN CURRENT SHARING , STABILISING etc Right from the
1 A current flow.
3. AS FAR as LATERAL MOSFETS are concerened , They are NOTHING in terms of Performance as compared to N-CHANNEL VERTICAL MOSFETS from IRF.

REGARDS!!

THE amp_man_1

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Old 16th January 2004, 10:19 AM   #8
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Hi amp_man_1. Mmmm.... that's interesting. I have a bag full of IRF540's that I could probably use in something like that.

As far as temperature effects are concerned, there can be two different situations - In a switching type cct like an SMPS or suchlike, or in fact any time two or more fets are paralleled and turned *fully* on, they might start out sharing the current fairly closely but the one that decides to get a bit hotter will increase it's Rdson and as a result back off a little. Current sharing is still more or less achieved.

The other situation is gate threshold voltage variation with temperature. Put two fets in parallel but on separate heatsinks and bias them both *partway* on to several amps but keep the drain voltage at say 20-30 volts. As they get hotter the gate voltage threshold decreases by about 5.5 mV per deg C (equivalent to raising the gate bias voltage by this amount per deg C) so the hotter fet will really dig in. Big unbalance!

So... saturated switching and linear mode have opposite temperature effects. I think this is where the confusion comes from.
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Old 16th January 2004, 10:47 AM   #9
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Your amp are probably set up to the pure class B . What about crossover distortion ? How is setting I bias in this amp ? You can't to learn me about fets - I was learning by good teacher in 80's - Borbelly, Cordell and several others. I was build many fet amp and my amp with fets is in our country in DIY community " sound reference "
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Old 16th January 2004, 10:49 AM   #10
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Hi FACTOR, STALLION, amp_man_1... are you back in town?
220.224.29.xxx.

/Hugo
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