# Schmitt trigger using JFET

I want to make a Schmitt trigger using a JFET instead of a bipolar transistor. I started by trying to understand how this circuit worked using a BJT and then moved on to try and replace them with JFETs. See circuit and output desired using BJT below.

I looked up how to replace a BJT with a JFET but I couldn't find anything useful. I tried using capacitors in series with R3 (see below) and I got a square signal, but it is centered on the peak of my input triangle signal which means the two threshold values are the same.

• Try a higher supply voltage (10V?), and more voltage drop at R4. R3 and R5 can be many times larger (10s kohm?), and will need to be a lowish ratio (R3 > R5) to get the gate voltage down. The biggest difference from BJTs is the negative Vgs, and lower gm. Both which mean needing higher input voltages, or adjustments related to that. Dec 20, 2022 at 17:23

The schematic is quasi the same, but some points (as pointed out @Tim Williams) must be taken into account.

Just tried this one ... Good luck. This function with JFET are "very rare" ...
I noted that the voltage at Vd1 is more "expressive" (full swing voltage).

• Nice way of illustrating the result. +!
– jonk
Dec 21, 2022 at 4:51
• @Jonk Thanks. It is the most significative when using the interactive mode of the simulator to see "hysteresis". Dec 21, 2022 at 5:13
• I didn't quite understand, with BJTs one transistor is always off ( if Q1 on then Q2 off and vice versa) but with JFET if J1 is off it behaves like a short circuit therefore Vout is V1*R4/(R4+R3+R6) ? but what happens when J2 is off ? And changing the value of R6 will change the offset of Vout got it ! Dec 21, 2022 at 15:27

Here is another way of looking at the JFET circuit provided by @Antonio51, showing operation at 1 MHz. I added a small capacitor to speed up the transitions.

I also simulated a Schmitt trigger circuit using NPN transistors and 5V logic levels. The little speed-up capacitor enhances the blip on the top of the waveform. This is for 500 kHz:

• I have 2 questions : how do you choose the speed up capacitor value ? And with the JFET circuit why do you add R5 (1 Ohm) is it to get the current that passes through the source ? Dec 21, 2022 at 15:20
• I just tried several values for the capacitor and observed the effect. It may not be really needed, unless you will be running at high frequency. The one ohm resistor was from @Antonio51's answer, and probably is not needed. Dec 22, 2022 at 19:45