Author Topic: Diode between Gate and Source  (Read 326 times)

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KarenColumbo

Diode between Gate and Source
« on: August 07, 2017, 07:21:37 AM »
I stumbled upon this while sifting through Aron's archive:
http://www.aronnelson.com/gallery/main.php/v/J0K3RX/BF5_sch.jpg.html
Looks interesting - but what's with the diodes D1 - D4 between those JFETs' Gates and Sources?

"In the beginning there was nothing - which exploded" - Terry Pratchett

teemuk

Re: Diode between Gate and Source
« Reply #1 on: August 07, 2017, 09:27:55 AM »
They mimick "grid conduction" of vacuum tubes. Basically, with high-enough grid-to-cathode voltage difference tube's grid begins to conduct and its impedance rapidly decreases. Assuming the circuit is driven from moderately high-impedance source the voltage divider "clamps" the output signal creating clipping distortion at the grid. Essentially the grid behaves like a diode, and that characteristic is replicated in the FET circuit by addition of... ...a diode.
Sometimes you see equivalent junctions of BJTs or FETs used for similar purpose.

PRR

Re: Diode between Gate and Source
« Reply #2 on: August 07, 2017, 05:18:58 PM »
Well, the JFET gate is a diode also.

At first glance, this adds nothing.

The diode specified is not a generic, so may be low voltage, so may do a little.

amz-fx

Re: Diode between Gate and Source
« Reply #3 on: August 07, 2017, 05:29:32 PM »
Paul,

At low currents, the specified diode has a Vf of about 0.25v while the jfet junction will be around 0.7v.

regards, Jack

robthequiet

Re: Diode between Gate and Source
« Reply #4 on: August 08, 2017, 12:23:12 AM »
http://www.aronnelson.com/gallery/main.php/v/J0K3RX/SE-1+Schematic.PNG.html

So here is a gate to drain diode with a MOSFET stage -- is this for a similar reason?

PRR

Re: Diode between Gate and Source
« Reply #5 on: August 08, 2017, 02:51:20 PM »
> is this for a similar reason?

No.