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

Genome wrote:

"daceo" <hbc@xxxxxxxxxxxx> wrote in message news:1172758238.506798.143980@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Hi,

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...

I have built a circuit (highly simplified description), which uses an
N channel mosfet IRF730 with a capacitor between gate and source, and
a 4 meg resistor to my bias circuit. The drain is connected to 250
volts, the bias can be between 0 and 200 volts. When measuring the
source impedance of this circuit when under load (a resistor) by
injecting an AC current and measuring the voltage change, it appears
that the impedance gets lower as the current increases.

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.

Does anybody have any bright ideas if I am doing something daft? Or
got the wrong end of the stick?

daceo



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.

DNA


Actually, MOSFETs are good to the (tens of) nanoamp region; just beware of the temp sensitivity (at all currents).
.

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