Measuring FET pinch off voltage?

DimebuGG · Post by **DimebuGG** » 16 Apr 2009, 14:24

How am I suppose to measure FET's pinch off voltage?. Is this referred as the cutoff voltage(Vds)?..I'm assuming the V+ at the drain while the common(GND) is at the source. Am I right?..

B Tremblay · Post by **B Tremblay** » 16 Apr 2009, 15:10

http://runoffgroove.com/fetzervalve.html#11

DimebuGG · Post by **DimebuGG** » 16 Apr 2009, 15:15

B Tremblay wrote:http://runoffgroove.com/fetzervalve.html#11

spot on!..

shame on me,i didn't notice it earlier..thanks..

oops, i'm about to use 24V DC as my supply voltage..what's the proper value between 1M-10M resistor shall be used?..sorry

briggs · Post by **briggs** » 16 Apr 2009, 17:03

B Tremblay wrote:http://runoffgroove.com/fetzervalve.html#11

That is one of my favorite articles. It's got a lot of very useful info there

DimebuGG · Post by **DimebuGG** » 18 Apr 2009, 02:54

I have a question Mr. B Tremblay.

Does the selection of the appropriate Drain and Source resistors applies to each stage of a high gain FET circuits?

B Tremblay · Post by **B Tremblay** » 18 Apr 2009, 11:37

If you've determined the optimal values with the calculator on the Fetzer Valve page after measuring several FETs, then you could use the same values for each of the stages, provided that you use FETs with reasonably close Vp and Idss numbers. There will usually be a group that are close.

DimebuGG · Post by **DimebuGG** » 18 Apr 2009, 12:11

Yeah, I did. I kinda keep the individual values of Vp and Idss of each FET and marked them accordingly. These are J201's and 2N5458's. Strangely, unlike those high gain FET emulation circuits usually have an Rs at 1.5K - 4.7K but to those I tested never reached 500 ohms at all. My supply voltage again is 24V DC.

B Tremblay · Post by **B Tremblay** » 18 Apr 2009, 16:15

I'm not quite understanding what you mean. What was the average Vp and Idss for each type of FET?

DimebuGG · Post by **DimebuGG** » 19 Apr 2009, 00:14

Well, I made a mistake in J201's Idss, so here the corrected values:
I tested only 4 devices of each type.
J201
Vp(ave.)= -0.8875, Idss= .6125

2N5458
Vp(ave.)= -1.86, Idss= 2.705

Actually, I have the individual Vp and Idss of each FET.

J201 Vp / Idss
A. -0.76 / 0.55
B. -0.85 / 0.47
C. -0.97 / 0.82
D. -0.97 / 0.62

2N5458
A. -2.49 / 6.2
B. -1.57 / 3.1
C. -1.35 / 2.5
D. -2.03 / 4.6

B Tremblay · Post by **B Tremblay** » 19 Apr 2009, 12:42

If you plug those numbers into the calculator, then you can see how two FETs with close measurements will perform. For example, I put in your first J201 with Vp of -.76 and Idss of .55 and with a 24V supply, the gain will be 24.1dB with Rd of 36k7 and Rs of 1k1. Of course, you will need to decide whether you use 1% or 5% resistors to get close to those values.

What is your overall design goal?

DimebuGG · Post by **DimebuGG** » 19 Apr 2009, 16:00

Oh yes, I already had their respective Rd and Rs. It's not really a stompbox(well maybe yes) but I was thinking then of creating a dual channel SS preamp in a DD size enclosure. It's basically based from some high gain FET circuits and maybe the Fetzer Valve itself as the clean channel. That Fetzer Valve article is a very interesting and informative read. Thanks man!. I should have read it before!!