Electrical – NC JFET switch with only one voltage source


The below circuit functions as a NC switch. At Vgs = 0V current flows through the channel of J1 (the JFET). When S is closed J1 will open and
no current flows through the channel anymore. Right now the circuit has two voltage sources in it.

My question: Can we design a JFET switch circuit with only ONE voltage source?

enter image description here

Best Answer


simulate this circuit – Schematic created using CircuitLab

Yes, you can, but it's a bad idea. If you pull down the datasheet for the JFET used in the example on learning-about-electronics, you'll find that it's designed to be an RF amplifier. (Note for example the Iss for zero volts Vgs is maximum 20mA ... ). By holding Vs a few volts above ground with a small 5.1 V zener (switching to a much higher Vss), you can now have a reasonably high Vds even with a series LED and can pull the gate "well below Vs" to switch off the transistor. Sometimes you will want a resistor between gate and source (as in this example) ... there is a small finite capacitance between the gate and source, and adding a resistor will help carry away that gate charge. Even if you leave it out, there will naturally be some leakage which will dissipate it naturally.