[sdiy] Help with sawtooth convertor

David G Dixon dixon at mail.ubc.ca
Fri Jan 11 01:34:17 CET 2019

> So I would need a negative voltage to turn on a jfet, yes?

No.  The N-JFET rides at or near the negative rail when off, and lifting the
gate to ground turns it on.  You don't want to go any higher than that,

I've never used a P-JFET.

The thing you need to be careful about with JFETs is the turn-off voltage.
Some of them have fairly low gate turn-off voltages (like -10V) but if you
hold them at the negative rail then you'll be fine.  The two JFETs I use
most often are 2N5485 and 2N4391 (or PN4391).  The 5485 is a little more
forgiving regarding turn-off -- it turns off at -4V, while the 4391 needs to
go quite a bit lower to turn off.

In my saw-shaper circuit, I gate the JFET with the signal coming from the
triangle core comparator.  This is a square wave of about 16Vpp square wave
(+/-8V).  I send it to the gate through a backward diode so that it is only
-9V to 0V at the gate.  The gate is connected to ground through a 1M
resistor, and the source is also grounded.  The drain is connected to the +
input of an opamp, such that the opamp is either an inverter (when the JFET
is ON) or a non-inverter (when the JFET is OFF).

Also note that the source and drain of JFETs are completely interchangeable.
I connect them based entirely on layout convenience.

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