If you need to protect sensitive circuits from overvoltage breakdown conditions, diodes are one way. When the input voltage becomes to high, the diode will conduct and limit the voltage to around 0.7 volt. However, if your circuit is used to measure low-level currents, even a slightly biased diode can siphon off a significant amount of current - seriously degrading measurement accuracy
In order to keep your measurements accurate and protect your measurement circuitry you will need a diode that has a very low current in the non conduction region (below the forward bias voltage). In this case, a JFET connected diode is an alternative. When a JFET is connected as a diode, it exhibits very low levels of leakage current in the non-conduction region.
A JFET connected diode is simple to build. Just connect the source and the drain of an N-Channel JFET together. The gate of the JFET will serve as the cathode and the drain/source will be the anode. Such a connection is shown in the LTSpice schematic below. For this circuit, a 1 Volt sine wave (at 300 Hz) is applied to the JFET diode.
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| A diode can be easily constructed with an N-Channel JFET |
The output of the JFET diode is taken across the load resistor, R1, Just like a regular diode, it will clip the negative portion of a sine wave. Also just like a regular diode, it will not conduct until the driving voltage reaches 0.7 volt. This shortens the duty cycle of the output waveform and lowers the peak voltage of the output waveform.
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| The JFET diode clips the negative cycle of a sine wave |
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Diode Connected JFET Protects Op Amps
Common Circuit Applications (JFET Diode)
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