#### Chuck D. Bones

##### Circuit Wizard

Here's a comparison of most of the JFETs we use in pedal circuits.

I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a given part number. All of these parts were in-spec, although a few were right on the edge. The test conditions for Vgs,off vary from one part number to the next. Some spec sheets measure at Id = 10nA, others at Id = 1μA. My measurement conditions were in the ballpark, typically around 150nA. The resulting error in my Vgs,off measurements are a few % at most. Close enough for Rock & Roll.

Some tips on reading the table below:

gm is negative because the JFET inverts the signal.

To calculate voltage gain, multiply gm (in mS) x drain resistor (in K). Example: Rd = 22K, gm = -1.691ms, then gain = 37.2 (31dB). This assumes the source resistor is bypassed by a large capacitor.

The source resistor can be calculated as Rs = -Vgs / Id. Example: 1.66V / 0.2mA = 8.3K.

I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a given part number. All of these parts were in-spec, although a few were right on the edge. The test conditions for Vgs,off vary from one part number to the next. Some spec sheets measure at Id = 10nA, others at Id = 1μA. My measurement conditions were in the ballpark, typically around 150nA. The resulting error in my Vgs,off measurements are a few % at most. Close enough for Rock & Roll.

Some tips on reading the table below:

gm is negative because the JFET inverts the signal.

To calculate voltage gain, multiply gm (in mS) x drain resistor (in K). Example: Rd = 22K, gm = -1.691ms, then gain = 37.2 (31dB). This assumes the source resistor is bypassed by a large capacitor.

The source resistor can be calculated as Rs = -Vgs / Id. Example: 1.66V / 0.2mA = 8.3K.

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