Can I ask a naive question?
I think I remember reading that the bias voltage had to come on before, in this case, the 60V. What makes that happen here?
Thanks
Bob
I think I remember reading that the bias voltage had to come on before, in this case, the 60V. What makes that happen here?
Thanks
Bob
Zero bias voltage means zero current thru transistor ?
When bias voltage increases current also increases, or ?
When bias voltage increases current also increases, or ?
Bob you may be thinking about those SIT transistors like the Sony 2sj60 . They
do require the bias voltage be on before the drain voltage is turned on. But in the
case of that big IXFN14020n we were talking about it's like Thornilsson just said no
bias voltage no current.
do require the bias voltage be on before the drain voltage is turned on. But in the
case of that big IXFN14020n we were talking about it's like Thornilsson just said no
bias voltage no current.
Big Bang moment
cap across zenner (or pot wiper to gnd ) is empty - zero voltage
it goes slowly up
mosfet is enhanced device , meaning - it need some positive voltage to be cranked
so ........ bigger the cap across zenner (wiper to gnd) , soft start is longer
cap across zenner need is having no more volts across than zenner is maintaining
simple , isn't it ?
cap across zenner (or pot wiper to gnd ) is empty - zero voltage
it goes slowly up
mosfet is enhanced device , meaning - it need some positive voltage to be cranked
so ........ bigger the cap across zenner (wiper to gnd) , soft start is longer
cap across zenner need is having no more volts across than zenner is maintaining
simple , isn't it ?
I don't entirely understand your question. Without power from the supply you cannot produce a bias voltage
Provided Zener doesn't fail, the cap will never see more than 5V (max adjustment on triimpot).
25V cap is more than adequate for this application. Use what you already have.
Thanks to all that answered my question
My reference was to the 2sK82/2sj28 s as per the Nelson's VFET1 article
"The parts aren't very critical, but the important thing is to get a consistent and quiet set of voltages for both the bias and the main supplies. The other key item is that the bias voltages need to be in place much more quickly than the main power supply to prevent too much current from flowing through the output stage on turn-on. You will see that the time constants for these circuits have the bias regulators coming up much faster than the mains."
Sorry I wasn't more clear when I asked but you guys got me there. 😀
My reference was to the 2sK82/2sj28 s as per the Nelson's VFET1 article
"The parts aren't very critical, but the important thing is to get a consistent and quiet set of voltages for both the bias and the main supplies. The other key item is that the bias voltages need to be in place much more quickly than the main power supply to prevent too much current from flowing through the output stage on turn-on. You will see that the time constants for these circuits have the bias regulators coming up much faster than the mains."
Sorry I wasn't more clear when I asked but you guys got me there. 😀
make difference between enhanced and depletion mode parts , also normally-on and normally-off parts
so , in case of big mosfet puck - enhanced, normally-off
being auto biased in upper half , cap connected to outer world , so no biggie regarding entire power-up procedure
so , in case of big mosfet puck - enhanced, normally-off
being auto biased in upper half , cap connected to outer world , so no biggie regarding entire power-up procedure
How about doubling up on the Zener and add one more in parallel - more stability plus redundancy to avoid the possibility of a runaway bias.
A reasonably large cap will make a kind of soft start for this circuit ?
(not for the powersupply but for the amplifier cicuit)
(not for the powersupply but for the amplifier cicuit)
As you can see that is already taken care of for bias.
Making a quiet supply is up to you, looks like anything with 10mV ripple will be ok. I can test at higher ripple amplitudes.
I suppose you could do that, but no one has done that in any of Nelson's other amps. I'm not too worried.
Well you will need to put parrallel zeners in F6 too
Nelson never seemed worried about it.
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