Patrick Turner
Guest
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 Posted:
Wed Dec 08, 2004 2:13 pm Post subject:
Re: BJT for plate CCS load |
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Ian Iveson wrote:
| Quote: | "Patrick Turner" <info@turneraudio.com.au> wrote
When you use a pnp device, and the turn on voltage goes
way above the rating, all 3 electrodes, emitter, base, and
collector all
are at the high B+
until the triode starts conducting.
Not quite true, Patrick. The valve anode will initially be at 0V. If
500V is supplied instantly to the other end of the current source,
then it has 500V across it.
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But no current flows, and the anode voltage whizzes up to whatever
voltage the
CCS collector wants it to be.
| Quote: |
The anode capacitance must be charged.
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Its only a few pF.
| Quote: | The current source may charge
this at its designed current, which won't take very long, so you
will get an initial short pulse of voltage across the source. Or the
source may not be fast enough and you will get a current pulse
instead.
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It depends on how the CCS is set up.
I have an R and C between the base and B+, then an R from base to 0V,
so the CCS B+ to base voltage takes some time to establish, the anode
capacitance is low,
no problems so far....
| Quote: |
Then there is the capacitance into the following stage which is
probably fed from the anode. That will take much longer to charge at
a much lower current, being a larger capacitance and a much higher
resistance.
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Still only pF quantities....
| Quote: |
Also stray inductance, etc, that will tend to resist current pulses.
Also the capacitance of the source device electrodes, and the rest
of the source circuit.
That is what I fear, too. BJTs seem to be a bit more forgiving.
Not as much as tubes, but probably more than FETs, I suppose.
Perhaps the mosfets you smoke are more vulnerable to all these
start-up transients?
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I'm not the one to smoke mosfets, because I don't use then for CCS
when there is a cheaper easier locally available bjt.
| Quote: |
Plenty mosfet mu-stages seem to work. It is commonly argued that
they make better current sources than BJT (similar to pentode/triode
difference?). Problem however with electrode capacitance.
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I am not sure.
Patrick Turner.
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