Other IRF-MOSFETS

[ljozsef]

Hallo experts, please Mr. Pass,

What about the range of the yet not mentioned IRF-FETs?
There are some web offers with IRFP 460, 3710, IRFZ 44 etc. What would decide if they were champions or audio garbage?
Why do we see the 04x, 14x and 24x only? - please forget their price this time.
Are there some limits regarding Ciss, Rdson or other parameters? In my particular case, the A-X would be the basic topology, with single output FETs.

I think your help would be of general interest.
Looking forward to your quick kind answers,

Laci

[uli]

Hi Laci,

Normally the IR Mosfets are switches. When you "abuse" them

for linear purposes not all of them are well suited. From long

year experience the bigger the 2nd number, the less suited

is the device. EG 044 is much better (sonically) than 064.

There are many reasons for that e.g. gate-source capacity which

depends heavily on the number of parallel devices on the dye,

If you look at old NP designs (A75) he used 230. As they are

out of production you better use the devices with the lowest

number possible (mostly 140, 240, 9140, 9240).

There are even "rumours" that higher voltage devices such as

240 work better than 140 or even 044, but I personnally have

no proof of that.

Uli

[ljozsef]

That would mean, under my items IRFZ44 could be considered, while 460 were worse and 3710 the worst (?).
Regarding Cgs, its absolut value cannot be so critical considering the original Pass projects with a palmful of paralleled devices (4-6 x Cgs), more its nonlinearity.
BTW, in that case is the bandwidth the same because of the also paralleled series gate resistors or having the same driver will be altered?

Laci

[uli]

Originally posted by ljozsef
That would mean, under my items IRFZ44 could be considered, while 460 were worse and 3710 the worst (?).
Regarding Cgs, its absolut value cannot be so critical considering the original Pass projects with a palmful of paralleled devices (4-6 x Cgs), more its nonlinearity.
BTW, in that case is the bandwidth the same because of the also paralleled series gate resistors or having the same driver will be altered?

Laci

As every fet forms its own lopass pole (Rg +Cgs) it depends

on Cgs too. Driving a huge capacity through Rgs needs a lot

of "overshoot" in the driver and a lot of current too. This in

turn leads to the need of more bias in the driver and more

nonlinearity between driver and output which has to be

corrected by global feedback and can cause distortion through

overload of the driver (SIM).

Uli

[Upupa Epops]

Modern vertical fets have " internal Rg " only a several ohms, which is insignificant against typical resistor connected in series with gate electrode ( 100 - 500 ohms ) .

[ljozsef]

Yes, I was referring to those external resistors; I think so was Uli's answer, too.
Now, apparently all being paralleled, the resulting pole will be at the first look the same. Is this the reality? I am disposed to believe that the bandwidth will be compressed.

But --- ?
Laci

[uli]

The higher Cgs the lower Rg should be to achieve the same

pole frequency. As impedance level drops the amount of

transient current to drive that pole rises. Having such poles

in the NFB loop leads to SIM simply because those transients

saturate the driver stage without contributing anything to

open loop gain. Try it, you will hear the difference!

Uli



Edit: not to forget that a low Rg can lead to severe oszillation

as the source impedance of the driverstage drops too low!

[Upupa Epops]

I was build first mosfet amp twenty years ago. Do you mean, that my experiences are not sufficient ?

[ljozsef]

I would be happy to listen to differences, but also very pleased to be able to choose a proved set of MOSFETs...

Regarding the serial gate resistor, I did not mention dropping its own value, only the resulting parallel resistance in case of a "battery" of devices.

All told,
1. searching AUDIO at the irf.com were not the best solution (interesting results...),
more important
2. to prefere datasheets with low Ciss (IR-notation) and low voltages (?) and Rds (?). Or these letter not so cardinal?

Laci

[ljozsef]

What do you mean?
I feel nobady was withsaying your post; I definitely not...

Laci