Measuring JFETS for gain

[armdnrdy]

I'm currently working on an existing Mu-amp circuit.

Using JFETs as an amplifier is a bit out of my comfort zone.

I have fifty PN4393 JFETs to work with so...I was thinking to sort them into a few different ranges of gain.

I researched JFET gain, and read about transconductance curves. I read through the articles on the Fetzer valve and "JFETs the new Frontier."

Now I know I'm going to get beat up over this but....here goes:

Is there a quick and dirty way to establish the rough gain (higher/lower) of JFETs?

I'm using a Atmega328 tester which gives me (I) and (VGs)....but ...I'm not sure at what parameters these measurements are taken.

Is there a circuit which establishes gain?

Thanks for the help.

[midwayfair]

Dirtiest way:

Pick a Fet. Let's use the 2N5457, which is good for mu amps. put it on your breadboard with a 1M on the gate to ground, 470R to ground from the source, and 2.7K on the drain to +9V. You should be somewhere near 6V-7V with that on the drain.

Take a pile of FETs. Plug them in and sort by their drain voltage.

Does this give you the exact gain? No. It does, however, let you get a close enough match for a basic amplifier circuit, which is all you needed.

[Davelectro]

I think jfets in a mu amp arrangement don't need to be matched.

[Frank_NH]

If you want to sort your JFETS, build a JFET tester circuit like the ones here at Tagboard Effects:

http://tagboardeffects.blogspot.com/2012/07/greatly-improved-jfet-matcher.html

I've built circuit number two and used it to measure Idss (drain current for zero gate voltage) and Vp (the pinchoff voltage) for all of my JFETs (J201, 2N5457...).  Each JFET's data is recorded and JFET labeled before storing.  This way, I can go back later on and find a suitable JFET for a specific project.

As for gain, I have written for myself a spreadsheet which calculates DC bias and AC gain for a common source JFET amplifier (one of the more common configurations you see in amp sims and overdrives), using prescribed source resistance and drain voltage and the data for the JFET.   An additional resource is the Fetzer calculator at Runoffgroove:

http://www.runoffgroove.com/fetzervalve.html

For mu amps, there are descriptions and equations which could perhaps be placed in a spreadsheet.  But you may want to read this first:

http://www.diystompboxes.com/smfforum/index.php?topic=88401.0;wap2

[armdnrdy]

Thanks for all of the replies.

Using the measurements taken from six different types of JFETs listed on ROG,
http://www.runoffgroove.com/fetzervalve.html#11
it appears that the parameters for higher gain JFETs are:

High VP
Low Idds
High RS
High RD

Davelectro...My goal is not to "match JFETs for this application. All I want to accomplish is to "sort" a batch of JFETs into two categories. Higher gain and lower gain.

Question for midwayfair..please excuse my ignorance on the subject...
"Plug them in and sort by their drain voltage"
Would I be looking for a higher or lower drain voltage reading for a higher gain JFET?

[midwayfair]

Would I be looking for a higher or lower drain voltage reading for a higher gain JFET?

In transistors, when using the same bias network, a lower drain or collector voltage is usually an indicator of higher gain. I'm sure some smart cookie can tell us when it isn't, however!

[Frank_NH]

Re: High gain

Actually, if you look at the Fetzer valve gain (Av) formula

Av = Vo/Vi = 0.54*(Vcc/Vp - 2)

where, Vcc = supply voltage and Vp is the pinchoff voltage (absolute value), it appears that a lower JFET Vp leads to higher gain.  This is interesting as Av is independent of Idss.  Of course, other important DC bias parameters are affected by Idss (e.g. Rs = 0.83*Idss/Vp for Fetzer).   If you look at the examples at the bottom of the Fetzer valve page, you can see gain decrease as Vp increases for different classes of JFETs.

[PRR]

> it appears that a lower JFET Vp leads to higher gain.

Correct.

In a well-designed amplifier, Vbatt is the maximum output and Vp is the maximum input. Then Vbatt/Vp is the gain.

> interesting as Av is independent of Idss.

Pretty true.

> fifty PN4393 JFETs to work with so...I was thinking to sort them into a few different ranges of gain.

For a given type (given active area), in-circuit gain will depend much more on device current and loading than specific device parameters. For this (non-"amplifier") device, I would expect gain of less than 10 for 1K drain load to nearly 100 for 100K drain load.

[armdnrdy]

Thanks for the replies,

I ended up using my existing ROG FET tester to measure the VP of the 50 FETs that I have.

They ranged from .93mV to 2.3V.

I chose 5 JFETs between .93mV and 1.1V for my build. (Catalinbread DLS MKIII)


PRR....

Yes...the original build calls out for MPF4393s. I have never seen this JFET used in an amplifier configuration. The only stomp box circuit that I've seen this JFET used in is the ADA Flanger in the gating (Threshold) circuit.