# Is a MOSFET really a good current source???

Discussion in 'Electronic Design' started by daceo, Mar 1, 2007.

1. ### daceoGuest

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

2. ### JonGuest

Depletion mode FETS (MOSFET or JFET) make good current sources. Just
tie the gate to the source. Since virtually all JFETs are depletion
mode, this is probably your best bet. Most MOSFETS are enhancement
mode.

3. ### Winfield HillGuest

Your data, IIUC, is wrong. MOSFETs in fact have a highly
constant current vs drain voltage, for a fixed Vgs. If
you at first measure otherwise, recheck your setup until
you get it right. For example, be sure you that don't
have Vds = 0, or under say 1 to 2 volts. Natch.

4. ### GenomeGuest

You are well down at the bottom end of the scale for such a device. Have a
look at the dirty sheet 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

5. ### GenomeGuest

Ah, perhaps I'll retract that. Mr Win is on the case and I seem to have got
it wrong..... :-(

DNA

6. ### Jim ThompsonGuest

Channel-length modulation is the culprit.

It's also not clear to me that your definition of impedance is
deltaVDS/deltaID... DC numbers are meaningless.

...Jim Thompson

7. ### Fred BartoliGuest

Genome a écrit :
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.

To the OP, if you want to increase the dyn impedance you'll have to add
a source resistor and set the bias such that Vbias = Vgs + Rs*Id
This would be necessary for good thermal stability and some
reproducibility too.

And since it's for toobs a few volts don't matter much.

You'll have to select low capacitance mosfets too if you want to get
past a few kHz BW.

WRT this, PNPs might be a better choice.

8. ### Fred BartoliGuest

Jim Thompson a écrit :
He said he was using AC current for the measurement.

9. ### Phil HobbsGuest

How are you measuring Zd? You have to measure partial Vds/partial Id
for *fixed* Vgs. From a quick squint at the numbers, it looks as though
you're calculating Id/Vs as you vary Vgs, using about a 150V supply that
is sagging as you pull current out of it. That's a misunderstanding of
what you're trying to measuring. You're basically measuring the
large-signal *source* impedance, which is 1/transconductance, and should
show the behaviour you're seeing.

A simple way to measure it properly with one DVM would be to put a
resistor between the source and ground, apply a floating Vgs using a pot
and a battery, and compare the 120 Hz ripple current you measure in the
resistor to the ripple voltage on the drain. (From the sag, I'm
assuming the supply is unregulated, which means it probably has enough
hum for the measurement.)

Cheers,

Phil Hobbs

10. ### Tim WescottGuest

everyone else:

Didn't you look at the numbers, and compare them to some data sheets?

daceo:

10-50K is about right for a small-signal MOSFET, so these numbers look
good to me. If anything they're better than I would expect since you're
using (I think) a power MOSFET. Considering that 15K at 7mA would
require 105 volts from a resistor and much less from your MOSFET I think

You'll get a higher impedance if you use some source degeneration, with
the drain impedance increasing by the proportion that the apparent
source impedance increases (i.e. if you have a 20 ohm source impedance
then a 20 ohm degeneration resistor will double the drain impedance).
If you keep the degeneration resistance significantly higher than the
source impedance it'll also make the drain a more linear current source.

o
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(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

11. ### Robert BaerGuest

One-point values are not useful.
Apply Vgs of some amount, say 3.1V and sweep the drain voltage from
zero to near breakdown; repeat for 3.2V, 3.3V, etc to say 4.5V.
The idea is that one will see a flat Id VS Vds above some voltage;
that "flatline" being the hi-Z area that you are looking for.
As the Vgs increases, the current "flatline" will increase, but will
still be flat.
Granted, the IV curve will look like a low value resistor (value
decreases as Vgs increases) until it curves to the "flatline", but that
is the area you do not want to use.
A curve tracer is rather useful here, as the drain current during
"flatline" will change as a function of temperature (it is sensitive,
like Vbs in a bipolar) - so that at higher powers (meaning over 10mW in
a TO-220 package) point measurements become useless.
Handling the device will mess up the measurements (hot hands, cold
heart?).

12. ### Robert BaerGuest

A Jfet does *not* have a "flatline" current VS voltage curve anywhere!
A depletion mode MOSFET is far superior!

13. ### Robert BaerGuest

And do not heat it up with hands or "large" IR drops.

14. ### Robert BaerGuest

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

15. ### Robert BaerGuest

Since a PNP is the compliment of an NPN, does that mean we look for
the Late effect?

16. ### daceoGuest

Hi and thanks for al the replies,

Yes JFETs are quite good as current sources, but in my set up the
mosfet sort of adjusts its current to whatever is required at the
voltage I want it to be at, and this works quite well.

Yes I am right down in the noise and off the scale with regards the
data sheet, and I feel that I am in uncharted territory for me.

Yes I suppose I am getting a benefit with the mosfet over a resistor
in increased impedance, I am sure I have seen people quoting Meg ohms
for similar circuits (frustratingly never kept the link) under similar
conditions..... and I did not expect the impedance to change with
current so much either, never mind, you live and learn!

I initially found the problem when looking in to the AC impedance (low
capacitance mosfet comments noted), so I set out as a sanity check to
do steady DC measurements, from which I calculated the results above,
which roughly reflect my AC measurements.

Anyway, I measured and plotted my own data using lab psus and dvms,
for increasing steps of Vgs of 0.1 volt and Vds of 10, 20, 50 and 100
volts (assuming no current is flowing at 0 volts), and these are the
results.

Vgs 0 10 20 50 100 Volts
2.61 0 0.0032 0.0034 0.0039 0.0049 mA
2.71 0 0.0072 0.0078 0.0093 0.0119
2.81 0 0.0175 0.0195 0.0235 0.0303
2.9 0 0.0444 0.0499 0.0606 0.0794
3 0 0.1144 0.1295 0.1589 0.2101
3.097 0 0.3025 0.338 0.4268 0.5584
3.193 0 0.7781 0.8782 1.0949 1.58
3.289 0 2.06 2.33 2.91 3.99
3.386 0 5.06 5.86 7.37 10.69
3.48 0 10.83 14.57 20 30.32
3.58 0 28.36 33.65 50.66 140

I think the channel length modulation looks like a good match to what
I am seeing, I will have to read up on it, a brief look at data sheet
however, and I can not see it quoted (lambda?). Is this the kind of
thing that may change with manufacturer even though the device is
nominally the same?

I am quite interested in the floating battery gate drive method of
measurement. Sounds fun!

Yes I am using a power mosfet, IRF730 and it is interesting that you
think I am doing quite well with this type of device, I do actually
have a bit of degeneration included in the way of a current sensing
resistor, 43 ohms and some other bits which probably help.

Yes, thermal effects are what I think I am seeing when I get the Vgs
above 3.4 volts, I took the precaution of screwing the device to the
biggest lump of heat sink I could find (about 1kg). I have had a run
in with the negative, ehem or is that positive temp co? in these
devices at low currents.... Another headache, from the past!!

Curve tracer, sounds nice, found a simple circuit for one that I was
going to build, but is on hold at the moment... (it started attractively
simple, and then I thought what if, and could I do that, and it
rapidly became less simple!!....)

With regards to a PNP it could work but it would make my circuit more
complicated, and its quite nice with no base current. May ultimately
be the way to go...

So my situation now is to do some more measurements to see what
happens at higher currents, just to see if it flattens off, build
(borrow or buy) a curve tracer, and try and find the channel length
modulation number for the devices I would like to use. Always
something new to learn!!

Cheers For all the input, lots to think about

Daceo

17. ### Winfield HillGuest

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 agree. As a longtime MOSFET fan, who loves to use them in
low-current linear applications, and who is happy with their
sub-threshold theory and measured properties, I'm constantly
struggling to find small enough parts (small die, that is),
and have to admit that BJTs often show superior performance.

18. ### Winfield HillGuest

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.

19. ### Fred BartoliGuest

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

Vds is 75V.

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).

20. ### Winfield HillGuest

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.