Is a MOSFET really a good current source???

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 | | | ||-+ ||

[me too]

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

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.

--
Thanks,
Fred.

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

--
Thanks,
Fred.

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

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 _|_ | | --- --- ||-+ _|_ | | ||

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

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:

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.

--
Tony Williams.

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

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,

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.

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,

| d | '--| ,--|

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!!!

fig 5

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

page 3

thanks for all the feedback,

Daceo

or was that fig 3, oops