Driving a MOSFET power follower

[gromanswe]

Originally posted by HarryHaller
HarryHaller: 'Transconductance.'

IRF530 IRF540 are still my favourites
of various other reasons
you did not have me your way

[dshortt9]

Did I miss something? Show me a JFET that runs at 50 ma.

[gromanswe]

I haven't any knowledge about JFETs, that can handle more
than max 24 mA.
A good working point could be some 10 mA.

I have made stages mostly with 2-6 mA working current.
Also is highest voltage limited.
most wouldn't handle more than 40, usually 30
Drain to Source

Anyone used BF245B for audio?
It is a HF-JFET. High Frequencies, radio equipement etc

[yves]

Try RF power mosfets. They exhibit extremely low
capacitances, which is why they are great for RF.
However, you will definitely need an RF spectrum
analyser to check for oscillations. These things oscillate
at the slightest provocation and with the least
parasitics. But once you get them to work
stably, they are stable. And I *believe* they make
the best audio mosfets too.
See www.advancedpower.com. I would like to
purchase small quantities, but I can't find a source
that would sell them.

One other alternative with audio freq mosfets
is to operate them such that their capacitances
are minimised, or the change in capacitances
are reduced. If you look at the C-V curves
for any audio mosfet (or any mosfet for that
matter) they are anything but linear vs voltage.

A cascode works well to reduce the Miller effect,
but some people find that the sound is not
the best, YMMV.

yv

[gromanswe]

MUST be good
what else would price them so high
I am no stranger for to use
"HF" "RF" devices for Audio.
On the contrary. Think it is way narrow
not to test that option ....

Speed can be good

[jcarr]

dshortt9:

>Did I miss something? Show me a JFET that runs at 50 ma.<

Among the devices that I have actually tried, I'd suggest the following.

N-ch: 2N4856. Minimum idss, 50mA. Maximum idss, 175mA.

The closest complementary that I have been able to find is the 2N5114. Minimum idss, 30mA. Maximum idss, 90mA.

regards, jonathan carr

[dshortt9]

Thanks, I didn't know these critters existed.

[RichardJones]

ok,

I hooked up my MOSFET power follower last night.. I have zener protection diodes on all the mosfets.

I connected my batteries and the lower 0.2ohm resistor in the CCS turned bright red!!!

It got so hot that some solder joints melted..

well this isn't right.

So I checked some resistances... the resistance across the mosfets drain/source is 220 ohms.... hmmm...

So I think I fried both MOSFETS on the left channel.

And I have no clue how I did it.

So I will replace the mosfets and the MPSA18 sense transistor on my CCS.

I think the 0.2 ohm is still ok.

very strange.

any ideas / clues?

[mrfeedback]

Originally posted by RichardJones
..any ideas / clues?

Hi Richard,
Yup use a variac with a lamp in series with the output, or wire a lampholder across the amp fuseholder, and have a selection of lamps.
If you have a fault the lamp will do the dissipating.
This saves a lot of grief whilst doing the initial running up.

Regards, Eric.

[RichardJones]

Well the point is that the amp was working fine before. 0.7v across the 0.2ohm resistor... about 3.25A bias..

My heatsinks get nice and toasty... then I can trim output offset down to <10mV per channel... adjusting the upper mosfets bias.

well I will check everything tonight... very odd..

Unless I blew my MOSFETS before I turned it on.... very possible.

oh well.

I guess thats whats great about tubes... really cannot blow them up.