Understanding different transistor source resister values for buffering

[acehobojoe]

for reference, I was reading through this forum.

http://www.diystompboxes.com/smfforum/index.php?topic%3D47572.20

I am creating a buffer that is a 2n5457 much like this one : http://www.beavisaudio.com/techpages/HIW/buffer_jfet.gif

I have however switched to a J201 transistor because of it's availability, yet it seems to be behaving differently.

However, I have omitted the R1 resistor because of some other schematic I saw that didn't have it. It seemed to work without the 1m there, because I remember reading that it is still somehow biasing, but I think it must not be because this new transistor's differences. I'm going to run a few tests, try different things. I think that I'll add the R1, and maybe tweak the 3.3k R3 shown in the jfet picture above.

Any thoughts from anyone on why the buffer can sometimes have one biasing resistor and other times not? Or perhaps it is not a biasing resistor when on it's own. thanks in advance.

[antonis]

The R1-R2 combination form a voltage divider which bias the Fet's Gate at a desired Voltage level..
(half of Vcc when are equal, Norton's equivalent R1//R2 otherwise...)

If you refer to 1 biasing resistor you have to consider it as a pull (up or down) resistor which connects the Gate to an apropriate voltage..
(which voltage should be present on resistor's other end...)

As for R3, it sets buffer's output impedance and limits Fet's current..

[italianguy63]

J201 has higher gain than the 2N5457.  Not really interchangeable.

I had done some testing and found a real close equivalent in 2SK30A.. but I don't remember which "color".  I think it was 2SK30A-G  But, I will have to check my notes at home and see if I can find it (if you are at all interested)!


 MC

[acehobojoe]

I'm interested for sure

[italianguy63]

OK-- hopefully I can find it.

But, I was testing a bunch of 2N5457's and J201's for a project.  Digit (Dino) also had that Guytone project he had that required 2SK30A's.  And, there are different colors (red = R, Yellow = Y, Green = G), each of them had different amounts of gain.  But, one of them was very close in characteristics to the 2N5457.  Basically, an affordable equivalent.

If Dino is reading maybe he can chime in...

I'll look for my data tonight.  MC

[duck_arse]

from the toshiba 2sk30atm datasheet:
Gate-source cut-off voltage [VGS(OFF)] [VDS = 10 V, ID = 0.1 μA] −0.4 - −5.0 V
Note: IDSS(mA) classification - R: 0.30~0.75, O: 0.60~1.40, Y: 1.20~3.00, GR: 2.60~6.50

don't look for colours, they will be letter markings on the real thing ie 2sk30A-Y.

[italianguy63]

Thanks Duck!

It appears the 2SK30ATM-Y is the one...

"O" would be a little more hot tending towards the J201...

MC

[PRR]

> much like this one

That one holds the gate up at half of supply voltage. The source follows gate, will be near half supply. The source can then swing almost 4 Volts up or down. The input cap is essential.

Without R1, gate sits at ground. Source follows, perhaps a few-tenths of a Volt above ground. Source can swing up a long way, but can not swing below zero. Maximum both-ways signal voltage is a few tenths Volt. Input cap is optional.

As guitar is a few-tenths Volt signal, the latter scheme often works. It works better with a high-Vto JFET which follows many-tenths Volt high.

2n5457 in this circuit will self-bias source about 0.8V above ground. This is enough for most guitar inputs.

J201 will bias its source much lower. You *can* measure it in-circuit. If it is maybe 0.2V, soft guitar comes through clean, large guitar is clipped on one side, distorted.

> some other schematic I saw that didn't have it.

Don't believe everything you see. Some days, you shouldn't believe anything until you experiment. The R1+R2 scheme is generally better for buffing. But throw the extra 12 cents and try it both ways.

[acehobojoe]

Don't believe everything you see. Some days, you shouldn't believe anything until you experiment.

Agree, fully.

[PRR]

Today's news-paper comic touches on this:



(Apologies to Mr Pastis for ruining his work.)

[italianguy63]

Joe-- forgot to mention--

Check the pinouts on the SSK30's, it is different from the 2N5447 and J201.  (sorry)

MC