Working on a guitar pedal buffer using a J113 for the guitar tuner pedal @rpschultz. We were previously using 9V to supply the J113 but found that we got better noise rejection from the ESP32 by putting the J113 buffer on the same 5V TO-92 regulator that were powering the LM358 op amp with.

At 9V we were biasing the J113 with 3.3K to GND. At 5V I switched the bias resistor to 2.2K and it seems to work fine. Does that seem sane? Was it necessary?

 

How does one calculate the bias resistor for a JFET?

 

Building JFET Preamplifiers for Musical Instrument Use. : 9 Steps (with Pictures) - Instructables

 

Thanks for the article. First thing, the article talks about the MAX Iss, but the datasheet (J113) shows the MIN Idss = 2mA.



I have a spice model, this config shows an Is of 1.1mA using a 3k resistor, which is less than the MIN current. Do I need to lower R3 to get it above 2mA?

 

The 2mA figure is for the case where the gate and source are shorted together.

Do I need to lower R3 to get it above 2mA?

Yes. 2mA through 3.3k would drop 6.6V across R3. That ain't gonna happen with only a 5V supply.

 

OK. That article says to run at 60% of the MAX Ids, which isn't on the datasheet. But you're saying I want the current through R3 to be more than 2mA? To get it above 2mA, R3 needs to be <1.5k or so.
AI says max Ids current on J113 is about 20mA. So running at 60% of that would require lowering to 250R or so.
Using a standard 1k would give 3.2mA which is more than the min and a lot less than the max.

 

To increase the drain current further you need to raise the gate voltage :-

 

This is how I currently have the buffer in my tuner pedal hooked up:

 

Am I looking at this chart right? If I change R4 to 1K, I get this:


Do I figure Vds by subtracting the source voltage (between 3 - 3.5V) from the supply voltage on the drain (5V)? If so, that'd be 1.5 - 2V.

And if that's the case, the current through R4 (the drain current?) would be 3 - 3.5mA?

...and if both of those assumptions are right, that'd mean that the buffer would be operating in this blue square area?



Is that correct or am I missing the mark entirely?

 

Here's the LTspice file.

 

Here's the LTspice file.

Yes, the J113 is operating in that blue area.

 

The article above, the author is likely using a 2N5457. The resistor biasing calculation is correct for the application.
The selection was -3dB cut off frequency 82.4 Hz for his guitar humbucker. The choice of gate resistor is said to be
between 100k and 1M... He says that he uses an active pick-up and he uses 100k resistor.
The audio electronics talk about what exactly they mean can confuse the concept however the simulation circuit
can be made to produce perfect pitch. A good replicator needs to makes the circuit very close to original and with the correct parts
and source voltage. Happy to see the progress with simulation. This video starts off with dit dit dah, the gain for this application
works for his design goals from 500 to 3K.