Re: Is a MOSFET really a good current source???

Fred Bartoli wrote:
Winfield Hill a écrit :
Winfield Hill wrote:
Fred Bartoli wrote:
Genome wrote
Genome wrote
"daceo" wrote
I am under the impression that a mosfet is considered to behave
like a good current source, and is quite often recommended as
an anode load for valve amplifier instead of a resistor...
[ snip ]
Puzzled by this I have today bench tested my IRF730 and recorded
results of Ids against Vds for a set of Vgs values, starting
just above the threshold. From this I have calculated what I
believe to be the drain impedance with respect to Vgs.

The results are quite interesting
Vgs Zd aprox Id in mA
2.6, 50M, 0.0039
2.6, 19M, 0.0093
2.7, 7.30M, 0.0235
2.8, 2.66M, 0.0606
3.0, 976K, 0.159
3.1, 379K, 0.426
3.2, 103K, 1.095
3.3, 46K, 2.91
3.4, 15K, 7.37

Vds 75
gm 4
When plotted on the graph with a logarithmic ohms scale it is
interesting to see that impedance curves are equally spaced for
each step of the Vgs.

This is interesting but my point is that at a drain current of
7 mA 15 Kohms does not seem like a good current source to me.
You are well down at the bottom end of the scale for such a
device. Have a look at the dirty *** for the set of ID vs VDS
plotted WRT Vgs and you will see that you are operating the
device in it's linear (resistive) region rather than its
saturation region...... where the curvy bits are bending down
to zero rather than being flat.

I think that's right.

Down there it behaves as some sort of square law resistance
depending on something to do with Vth, Vg and a possible K.

Ah, perhaps I'll retract that. Mr Win is on the case and I
seem to have got it wrong..... :-(
For this time it seems Win did read a bit too fast. (believe
he took the 'M' for milli).
The figures seems pretty reasonable to me and 15K at 7mA Id
and 75V Vds is to me a good figure.
No, I stand by my remarks, based on the theory for ideal MOSFETs
and on many hopefully-precise measurements I've taken. But Fred,
you've thrown down the gauntlet, so I'll drag out my data, and
perhaps add some new measurements on that old part, the IRF730.

One caution when using high-voltage MOSFETs in the linear
region: they love to oscillate at high RF frequencies!
This can certainly cause changes in their observed low-
frequency characteristics. A small series gate resistor
to isolate the recommended gate-source protection zener,
and other precautions are in order, along with checking
their activity in the 20 to 80MHz region with a scope.

I have rechecked my data, and stand by my assertions. However,
let me remark, my data was for high-voltage MOSFETs operating
at 50% or less of the maximum voltage rating. My measurements
above 50% show continued textbook low-leakage performance for
many power MOSFET types, continuing nearly to their avalanche
voltage, and abysmal leakage performance for some others. I
had poor 1000V mosfets (the manufacturer shall remain unnamed)
that had 50x higher leakage at 500V than a 600V mosfet. But
we can't blame such things on "poor current-source behavior".

I did not have time to test any IRF730 parts. But anyway,
given all the different manufacturers, and the different fab
facilities over the years, as they strove mightily to probe
the bottom floor of production costs, I imagine it'd be a
real crapshoot to see what leakage results one came up with.

Let see the figures presented by the OP and compare them to a
simple resistor giving the same current:

Vds is 75V.

Vgs Zd ap. Id(mA) Req=Vds/Id Zd/R
2.6, 50M, 0.0039 19M 2.6
2.6, 19M, 0.0093 8M 2.35
2.7, 7.30M, 0.0235 3.2M 2.29
2.8, 2.66M, 0.0606 1.24M 2.15
3.0, 976K, 0.159 472K 2.07
3.1, 379K, 0.426 176K 2.15
3.2, 103K, 1.095 68.5K 1.50
3.3, 46K, 2.91 25.8K 1.78
3.4, 15K, 7.37 10.2K 1.47

That's an already awful current source and there's little room to
improve that awfulness (except under current multiplication-avalanche
the dynamic resistance can't be worse than a true resistive behavior).

Interesting addition to the data.

I agree the OP's data looks horrible. I'm saying that's either his
particular "quasi-defective" MOSFET, or a measurement error of some
kind. Even the poor 1000V mosfets I mentioned weren't that bad!

Sheesh, I can't have been supremely lucky in the measurements I've
taken over the years? Actually, if the parts I've used routinely
to make my high-voltage amplifiers, five or six different types
from four or five manufacturers, several thousand MOSFETs in total,
were that bad, surely I would have noticed. But I'll double check.

.

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