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Old 6th December 2009, 05:58 PM   #1
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Default Why high voltage Fets when low voltage types gives less distortion?

I can see many people using IRFP240 and other similar fets in simple amps. Reading Nelson Pass article "Zen Amplifier Revisited" i could see that IRFP040 gives half the distortion compared to IRFP240

Quote:
First, the choice of Mosfet will have a significant influence on the
distortion figures. I have built the circuit with IRF240, IRF140, and
IRF040. These devices are very similar, but have been doped somewhat
differently for the purpose of voltage ratings. The higher the voltage, the
lower the transconductance, and the higher the distortion in the circuit.
The original curves reflected the 040, and subsequent use of a 140 gave
harmonic distortion figures about 50% higher, and the 240 about 100%
higher. For example, at 1 watt at 1 KHz, the 040 gave about .6%, the 140
gave about .9%, and the 240 about 1.2% harmonic distortion.
This is from that article. http://www.passdiy.com/pdf/retofzen.pdf

I bought a couple IRF3706 that should give even lower distortion.
They can only take 20 volts so i will have to cascode them with a higher voltage type on top. That should lower the distortion even more?!

Why does anybody use IRFP240 in a circuit with only 20 volts across them?
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Last edited by Circlomanen; 6th December 2009 at 06:17 PM.
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Old 6th December 2009, 06:55 PM   #2
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Probably for the power dissipation and bigger package (to247).
Hence better reliability.
If you want to use these then you going to have to parrallel quite a few of them.

However it is hard to give advice when we have no idea what you are trying to build with them.

Last edited by Melon Head; 6th December 2009 at 06:57 PM.
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Old 6th December 2009, 08:54 PM   #3
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Im sorry if my question is a bit unclear. I find myself having a hard time writing exactly what i want to in english. I dont find the right sentenses and words.

Anyway. Thanks for the answer. I will try to explain.

Quote:
Probably for the power dissipation and bigger package (to247).
There is the IRFP3703. A 230 watt, 30 volt TO247 package - device, and ofcourse IRFP040, IRFP054 and some others. I never see people using them, even though the amp never uses more than 40 volts rails.

Quote:
If you want to use these then you going to have to parrallel quite a few of them.
I hope a TO220 device can handle 2 amperes and 5 volts. That should be what it will see in a cascode, in something like the Zen v9 but with a low voltage Hexfet instead of a J-Fet. (and ofcourse some bias-circuitry).

Quote:
However it is hard to give advice when we have no idea what you are trying to build with them.
Any one of Nelson Pass different Zen/Aleph/First-Watt-amps. This is in the Pass Labs forum....

The question is more a general one about the use of high voltage fets in low voltage applications, when Papa himself has written about lower distortion from low voltage devices. It seems this point is not considered ( or forgotten) when trying to squeeze maximum performance out of simple circuits. If Nelson Pass could reduce distortion to 1/2 just by using IRFP040 instead of IRFP240, everyone should want to use IRFP040?!?! But i never see them used by Pass or anyone else....????
is there some other reason that i dont know about? It doesnt make sense?!
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Old 9th December 2009, 01:22 AM   #4
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Default Why not use the IRFP140?

Quote:
Originally Posted by Circlomanen View Post
Why does anybody use IRFP240 in a circuit with only 20 volts across them?
That does seem like a good question. The IRFP040 does not seem to be available anymore, although I see a IRFP044 available from Digi-Key. The IRFP044 is a 55 Volt device and its use would be marginal at best in an amplifier with +/-25 Volt rails. The IRFP140 is a 100 Volt device and does seem (to me, anyway) like it would be a good candidate for use in various amplifiers which use the IRFP240.
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Old 9th December 2009, 02:37 AM   #5
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obviously Nelson has tons of IRFP240 lying around. Using them up seems logical.
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Old 9th December 2009, 02:42 AM   #6
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Hola Y'all

I do not presume to know what Papa thinks, However I think he is using these higher voltage fet's at lower voltage and higher current to get a more linear response. Ala Klasse A...

From the sidelines in the Deep South,
Elwood
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Old 9th December 2009, 03:57 PM   #7
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Quote:
a more linear response. Ala Klasse A...
Yes ofcourse, but according to Pass own words you get a more linear respons from a lower voltage part, due to more conductans and more possible feedback, local or global. The class A is device-independent.

Quote:
obviously Nelson has tons of IRFP240 lying around. Using them up seems logical.
Yes, i have read something along these lines somewere....

Quote:
The IRFP044 is a 55 Volt device and its use would be marginal at best in an amplifier with +/-25 Volt rails.
That shouldnt be a problem since the steadystate voltage across the device will be 20 - 25 volts. A mosfet rated 55 volts will take 55 volts without a problem. Probably it could take 60 volts too, if not to hot or running to much current at the same time.

I will buy some IRFP054 to my next project.

Johannes.
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Old 9th December 2009, 04:18 PM   #8
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Quote:
Originally Posted by Circlomanen View Post
That shouldnt be a problem since the steadystate voltage across the device will be 20 - 25 volts. A mosfet rated 55 volts will take 55 volts without a problem. Probably it could take 60 volts too, if not to hot or running to much current at the same time.
Johannes.
It isn't the steady state voltage that is the problem, but the peak voltage. If you play at full volume, and the amp has a +/- 25V supply you will have at least 45V across the FETs and perhaps as much as 48V - let's call it 50 V-peak. If the line voltage is 10% high, then you could be at 55V-peak.

If you are comfortable with those safety margins, then everything is fine. It's DIY and mostly for ourselves, so skimping on the device safety margins can be part of the fun. After all, who hasn't considered building a SOZ amp with way undersized heatsinks just to see how it sounds, knowing it won't last very long.

Jeremy
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Old 9th December 2009, 04:20 PM   #9
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Quote:
Originally Posted by Circlomanen View Post
That shouldn't be a problem since the steady state voltage across the device will be 20 - 25 volts. A mosfet rated 55 volts will take 55 volts without a problem. Probably it could take 60 volts too, if not to hot or running to much current at the same time.
Johannes.
Yes, but at full output Vds will swing from near 0 to near 50V. That does not provide much of a safety factor, especially when varying lines voltages are considered.

FWIW, the IRFP054 has considerably higher gate charge and input capacitance (2900 pF) compared with the IRFP044 (1500 pF) so I would expect about half the bandwith from an amp built with them. IRFP140 and IRFP240 have similar input capacitance to the 044 part, in the range of 1500 pF.

Cheers,
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Old 9th December 2009, 04:55 PM   #10
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Quote:
Originally Posted by Jacques Merde View Post
...although I see a IRFP044 available from Digi-Key. The IRFP044 is a 55 Volt device and its use would be marginal at best in an amplifier with +/-25 Volt rails. ...
These are original to the Zen v4 as published by NP...which I have on mine.
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