Low-Drift High-Impedance JFET DC Voltmeter

Posted on Feb 4, 2014

The voltmeter shown in the schematic diagram below has very high impedance. The range selector uses conventional voltage divider resistors, but since the following pre amplifier uses JFET, the divider can be designed using very high resistance series resistors. This DC voltmeter circuit also featured with low drift. This circuit uses a pair JFETs,

Low-Drift High-Impedance JFET DC Voltmeter
Click here to download the full size of the above Circuit.

which is configured in a balanced-bridge source-follower amplifier circuit. The Q2 and Q1 must be matched within 10% for Idss that will maintains bridge balance over temperature. Here is the schematic diagram of the circuit:

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