# What Does This JFET Do If Vgs is Always 0V

I'm trying to understand this reference circuit that I found in the datasheet for a piezo accelerometer.

I don't understand what the purpose is if the gate and source are common. My best guess after some research is that the JFET provides a known impedance for the output.

• It's a current source if I remember right. Commented Mar 18, 2022 at 1:47
• The most uncalibrated current source in existance. Current depends on the part and on how it came out from the foundry Commented Mar 18, 2022 at 9:21
• Hum ... Function of this current source not really clear ... Commented Mar 18, 2022 at 13:53

It is being used as a current sink.

With the gate shorted to the source the drain current will be Idss which is the maximum current through the FET under normal conditions. Since this current is nearly constant over most of the range of drain to source voltages it can be used as a relatively stable constant current source or sink. The current can also be set to a lower value by adding a resistor in series with the source.

simulate this circuit – Schematic created using CircuitLab

You can get constant current 'diodes' that are basically an FET in a two pin package with the source and gate shorted internally.

• " by adding a resistor between the gate and source." --> " by adding a resistor between the source and ground" Commented Mar 18, 2022 at 4:29
• @jp314 Well... it depends on if you're using it as a source or sink. As a source that point would be connected to the load. Poor wording on my part, while technically it is between the source and gate, that's a bit ambiguous. I think my edit should clear it up. Commented Mar 18, 2022 at 5:17
• That makes a lot more sense. Thanks, @GodJihyo
– Greg
Commented Mar 18, 2022 at 9:54