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Is a MOSFET really a good current source???

T

Tony Williams

Jan 1, 1970
0
[massive, massive snip]
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!

I haven't seen a decent description yet of how the OP
configured an N-channel MOSFET into a pullup cc source.
His results had the sniff of a quasi-source-follower.
 
D

daceo

Jan 1, 1970
0
[massive, massive snip]
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!

I haven't seen a decent description yet of how the OP
configured an N-channel MOSFET into a pullup cc source.
His results had the sniff of a quasi-source-follower.

I have improvised myself a curve tracer and plotted a few devices....

Over a reasonable voltage swing (about 150v) and at 10mA nominal
(minimum) current, I got the following results for drain impedance (dv/
di).

IRF730 (ST!!) 12k
IRF620 (IR) 70k
2SK2350 (TOSH) 200k

yes it is an ST IRF730.... Unfortunately my mosfet draw is a bit
lacking in choice at the moment. Looks like I need to order in a few
samples, and test some more. I guess as Winfield says, maybe the
quality has got worse...

Incidentally, when you say "low-leakage performance for many power
MOSFET types", is that the leakage of the body diode?

Anyway, I see some light at the end of my tunnel, now off to see if I
can track down some Toshiba 400v devices!!

"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!"

I checked another one of the same device, just to check if the first
sample was dodgy, and it was largely the same.

Vgs 0 10 20 50 100
2.6 0 0.0034 0.0037 0.0042 0.0053
2.7 0 0.0078 0.0086 0.0101 0.0129
2.8 0 0.0194 0.0219 0.026 0.0337
2.9 0 0.0506 0.0565 0.0691 0.0909
3 0 0.1343 0.1511 0.1862 0.248
3.1 0 0.3584 0.4139 0.5035 0.6791
3.2 0 0.9528 1.085 1.44 1.99
3.3 0 2.58 3 3.7 5.14
3.4 0 6.49 7.59 9.71 14.16
3.5 0 15.86 18.49 25.06 43
3.6 0 36.05 44.3 65


Hi Tony,

With regards to my original circuit it does look very much like the
below circuit in essence, with a 4M resistor to a variable voltage
source driving the gate. My "load" is connected to the bottom of the
resistor.

o
|
|
|
||-+
||<-
.---||-+
| |
| |
--- .-.
--- | |
| | |
| '-'
| |
'------o
|
o

However, all other measurements since then have been performed on a
bare mosfet by itself, either with lab PSUs and amp meter etc, or the
newly improvised curve tracer, which is a function generator driving
an amplifier and a step up transformer, then bridge rectifier, and A
100R source resistor for current measuring, and a floating psu to
connected directly between the gate and source for biasing. Hope this
is satisfying enough!!

Cheers
Daceo
 
G

Genome

Jan 1, 1970
0
daceo said:
[massive, massive snip] [me too]

Cheers
Daceo

Are you measuring AC performance with AC stuff and ignoring how the device
behaves with AC and its capacitative stuff?

I am ask becoz you mentioned a four meg resistor.

DNA
 
D

daceo

Jan 1, 1970
0
[massive, massive snip]
[me too]
Cheers
Daceo

Are you measuring AC performance with AC stuff and ignoring how the device
behaves with AC and its capacitative stuff?

I am ask becoz you mentioned a four meg resistor.

DNA

Er not sure, but whatever the 4 M is not the limiting factor.

Incidentally, I have been having this problem for a couple of years,
with this circuit, with probably approaching 30 devices of this type
all, behaving in the same way. I had convinced my self that the
problem was something else. Sods law / fickle finger of fate dictated
that I chose this device (cheap and easily available) and at last I am
getting a fix on where the problem is.... Grrr.

Daceo
 
F

Fred Bartoli

Jan 1, 1970
0
Winfield Hill a écrit :
Fred said:
Winfield Hill a écrit :
Winfield Hill wrote:
Fred Bartoli wrote:
Genome wrote
Genome 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 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.

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.

I first was thinking of parasitics (the OP made his measurements with an
AC source), but it can't be on such a wide impedance span.

Probably self heating if the frequency is low enough.

Measuring mosfets other than pulsed at those currents levels is at least
difficult because you fight with self heating on one frequency end and
parasitics on the other end.
 
F

Fred Bartoli

Jan 1, 1970
0
daceo a écrit :
[massive, massive snip]
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!
I haven't seen a decent description yet of how the OP
configured an N-channel MOSFET into a pullup cc source.
His results had the sniff of a quasi-source-follower.

I have improvised myself a curve tracer and plotted a few devices....

Over a reasonable voltage swing (about 150v) and at 10mA nominal
(minimum) current, I got the following results for drain impedance (dv/
di).

IRF730 (ST!!) 12k
IRF620 (IR) 70k
2SK2350 (TOSH) 200k

yes it is an ST IRF730.... Unfortunately my mosfet draw is a bit
lacking in choice at the moment. Looks like I need to order in a few
samples, and test some more. I guess as Winfield says, maybe the
quality has got worse...

Incidentally, when you say "low-leakage performance for many power
MOSFET types", is that the leakage of the body diode?

Anyway, I see some light at the end of my tunnel, now off to see if I
can track down some Toshiba 400v devices!!

"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!"

I checked another one of the same device, just to check if the first
sample was dodgy, and it was largely the same.

Vgs 0 10 20 50 100
2.6 0 0.0034 0.0037 0.0042 0.0053
2.7 0 0.0078 0.0086 0.0101 0.0129
2.8 0 0.0194 0.0219 0.026 0.0337
2.9 0 0.0506 0.0565 0.0691 0.0909
3 0 0.1343 0.1511 0.1862 0.248
3.1 0 0.3584 0.4139 0.5035 0.6791
3.2 0 0.9528 1.085 1.44 1.99
3.3 0 2.58 3 3.7 5.14
3.4 0 6.49 7.59 9.71 14.16
3.5 0 15.86 18.49 25.06 43
3.6 0 36.05 44.3 65


Hi Tony,

With regards to my original circuit it does look very much like the
below circuit in essence, with a 4M resistor to a variable voltage
source driving the gate. My "load" is connected to the bottom of the
resistor.

o
|
|
|
||-+
||<-
.---||-+
| |
| |
--- .-.
--- | |
| | |
| '-'
| |
'------o
|
o

However, all other measurements since then have been performed on a
bare mosfet by itself, either with lab PSUs and amp meter etc, or the
newly improvised curve tracer, which is a function generator driving
an amplifier and a step up transformer, then bridge rectifier, and A
100R source resistor for current measuring, and a floating psu to
connected directly between the gate and source for biasing. Hope this
is satisfying enough!!

Cheers
Daceo

Just occurred to me: what's the value of your gate-source bypass cap?
 
D

daceo

Jan 1, 1970
0
Just occurred to me: what's the value of your gate-source bypass cap?

Hi,

22uF, and I am using a 100hz squarewave, the corners are slightly
rolled off, indicating a comfortable bandwidth. I just dropped one of
those 2sk2350s in to my circuit.... it is good. Too sleepy to
calculate source Z, but lets say it is prabably good enough!!!

cheers

Daceo
 
W

Winfield Hill

Jan 1, 1970
0
daceo said:
Fred Bartoli wrote,

22uF, and I am using a 100hz squarewave, the corners are slightly
rolled off, indicating a comfortable bandwidth. I just dropped
one of those 2sk2350s in to my circuit. it is good. Too sleepy
to calculate source Z, but lets say it is prabably good enough!!!

Actually, that's a classic, rather dangerous way to operate a
high-voltage MOSFET in the linear region, with Vds above 25V,
encouraging RF oscillation. In fact, the moderate gate-source
capacitance of a common gate-protection zener diode is enough
to set off many MOSFETs. With such capacitance, it's strongly
recommended to place a 100 or 220-ohm resistor in series with
the gate, and add a few other passive elements as well. I know
from personal experience how important such workaround schemes
are. The MOSFET's internal 10 to 20nH bond lead inductances,
plus a high Ciss, Coss and Cgs, etc., makes an excellent set
of LC-oscillator components.

|
+---Lwiring--+---- Cstray
| |
Ld |
| |
,------+----, _|_Cload
Crss _|_ | | ---
--- ||-+ _|_ |
| ||<- --- Coss |
,--Lg--+---||-+ | |
| | +----' |
Cgs_|_ '--||--+ |
--- Ciss | |
| Ls |
| | |
'-------------o--Lwiring---+--- Cstray
|
o

The problem with this RF MOSFET model is a lack of lossy elements.
15nH and 2000pF resonates at 30MHz. Have you checked for this?
 
T

Terry Given

Jan 1, 1970
0
Winfield said:
Actually, that's a classic, rather dangerous way to operate a
high-voltage MOSFET in the linear region, with Vds above 25V,
encouraging RF oscillation. In fact, the moderate gate-source
capacitance of a common gate-protection zener diode is enough
to set off many MOSFETs. With such capacitance, it's strongly
recommended to place a 100 or 220-ohm resistor in series with
the gate, and add a few other passive elements as well. I know
from personal experience how important such workaround schemes
are. The MOSFET's internal 10 to 20nH bond lead inductances,
plus a high Ciss, Coss and Cgs, etc., makes an excellent set
of LC-oscillator components.

|
+---Lwiring--+---- Cstray
| |
Ld |
| |
,------+----, _|_Cload
Crss _|_ | | ---
--- ||-+ _|_ |
| ||<- --- Coss |
,--Lg--+---||-+ | |
| | +----' |
Cgs_|_ '--||--+ |
--- Ciss | |
| Ls |
| | |
'-------------o--Lwiring---+--- Cstray
|
o

The problem with this RF MOSFET model is a lack of lossy elements.
15nH and 2000pF resonates at 30MHz. Have you checked for this?

I presume you've read the paper in the Siliconix MOSPOWER handbook,
where a Routh-Hurwitz array is used to demonstrate the instability of
said FET. Until then, I'd never seen a practical example of R-H
stability analyses

Cheers
Terry
 
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

For reasons I'm not recalling at the moment (a mind is a terrible
thing to waste), lateral current flow devices are preferred for audio
use. They go under various names, lin mos, l-mos and the obvious
lateral mos. I googled these:
http://www.magnatec-uk.co.uk/mosdata.shtml
http://www.renesas.com/fmwk.jsp?cnt...ower_mosfets_for_amplifier/power_mos_gen_amp/
 
T

Tony Williams

Jan 1, 1970
0
daceo said:
With regards to my original circuit it does look very much like
the below circuit in essence, with a 4M resistor to a variable
voltage source driving the gate. My "load" is connected to the
bottom of the resistor.
o
|
|
|
||-+
||<-
.---||-+
| |
| |
---C .-.
--- | |Rs
| | |
| '-'
| |
'------o
|
o

Thank you. Interesting circuit. Never seen it
before. A source follower at DC makes Vout
reasonably predictable, but it is a cc load at AC.
If Rs >> 1/gm then it determines the sensitivity
to AC variations in the voltage across the C.
 
D

daceo

Jan 1, 1970
0
Thank you. Interesting circuit. Never seen it
before. A source follower at DC makes Vout
reasonably predictable, but it is a cc load at AC.
If Rs >> 1/gm then it determines the sensitivity
to AC variations in the voltage across the C.

I like to make things up.... It is similar ish to one of those valve
like pre P channel type circuits. Works quite well though. I can
switch the capacitor in and out and even at 4 Mohms it does a good
enough voltage source.

Comments noted on stability, it is something I am aware of... I have
used a 100R between the gate and the capacitor. It makes the current
source slightly less good, but gives peace of mind! I haven't seen RF
oscillations, (I use a 100MHz analogue scope).

I have calculated the AC impedance in my circuit from measurements I
made last night, using a 2SK2350, (which didn't blow up, even though
rated at 200v). Now that it is behaving more CCS like, it is more
difficult to measure, and the results are a bit noisy or not enough
digits on my DMM. Any way a very useful improvement...

Id mA Zout in kohms @ 100Hz
0 1525
0.5 439
1 357
2 366
3 452
4 587
5 660
6 789
7 405
8 405
9 320

Cheers

Daceo
 
W

Winfield Hill

Jan 1, 1970
0
For reasons I'm not recalling at the moment (a mind is a terrible
thing to waste), lateral current flow devices are preferred for
audio use. They go under various names, lin mos, l-mos and the
obvious lateral mos. I googled these:
http://www.magnatec-uk.co.uk/mosdata.shtml
http://www.renesas.com/fmwk.jsp?cnt...ower_mosfets_for_amplifier/power_mos_gen_amp/

The strong reason these are preferred for audio power amplifiers
is their negative Id temperature coefficient at modest currents
(e.g., above 0.1A, and low gate-threshold voltage (e.g., 0.5V
at 0.1A), which makes accurate class-AB biasing practical.

I've not measured their drain output impedance. I have some
2sk1058 and 2sj162 complementary MOSFETs (7A, 160V) that'd
be interesting to look at. These have zero tempco at 100mA.

BTW, a nice place to get small quantitites of these lateral
MOSFETs is MCM Electronics, http://www.mcmelectronics.com/
which is part of the Newark and Farnell InOne companies, so
you can cross order.
 
http://www.renesas.com/fmwk.jsp?cnt=power_mos_gen_amp_root.jsp&fp=/pr...

The strong reason these are preferred for audio power amplifiers
is their negative Id temperature coefficient at modest currents
(e.g., above 0.1A, and low gate-threshold voltage (e.g., 0.5V
at 0.1A), which makes accurate class-AB biasing practical.

I've not measured their drain output impedance. I have some
2sk1058 and 2sj162 complementary MOSFETs (7A, 160V) that'd
be interesting to look at. These have zero tempco at 100mA.

BTW, a nice place to get small quantitites of these lateral
MOSFETs is MCM Electronics,http://www.mcmelectronics.com/
which is part of the Newark and Farnell InOne companies, so
you can cross order.

For AB output stages, I don't think there would be that much concern
about fet output impedance.

One think I know from chip design is vertical current flow is more
"sensitive" to silicon defects. FETs with lateral current flow should
be more rugged.
 
W

Winfield Hill

Jan 1, 1970
0
For AB output stages, I don't think there would be that much
concern about fet output impedance.

I agree. In fact, looking at the datasheet curves, it's clear
that the lateral MOSFET acts more like a triode, with a poor
output resistance, whereas the V-FET acts more like tetrode,
with a flat Id vs. Vd plot. An explanation first put forth
by RCA engineers working on Spice models, Wheatley and Ronan,
handles this nicely. They broke the MOSFET into two parts,
including a cascode JFET. See Intersil's AN7260 appnote,
http://www.nalanda.nitc.ac.in/industry/appnotes/Intersil/an7260.pdf

| d
|
'--|
,--|<---,
| |
g |--' |
--||--, |
| |
+-------'
| s

For Vds above 8 volts or so the JFET presents a constant voltage
to the MOSFET, hence the lack of much Vds influence. This also
explains the sharp change and reduction in the Crss plots above
about 8 volts. The MOSFET portion's Crss simply doesn't see the
drain voltage.
 
D

daceo

Jan 1, 1970
0
For Vds above 8 volts or so the JFET presents a constant voltage
to the MOSFET, hence the lack of much Vds influence. This also
explains the sharp change and reduction in the Crss plots above
about 8 volts. The MOSFET portion's Crss simply doesn't see the
drain voltage.

This is all interesting stuff, Crss and all that, bit of a brain
stretch, guess I will have to stay clear of the laterals for this
application. may be interesting to try an amplifier with them one
day ... Was just comparing data sheets for my current preferred
alternative, measured to day on my "curve tracer" at 240Kohm, when I
should happen to spot the Vds/Ids graph. In fact have looked a a few
ST devices data sheets today, and none of them look that good with
respect to Zd.

If in doubt RTFM!!!! now I come to think of it!!!

http://www.rapidonline.com/netalogue/specs/47-0320.pdf fig 5

gosh, I am going to use one of these instead!!!

http://www.farnell.com/datasheets/88093.pdf page 3

thanks for all the feedback,

Daceo
 
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