Low-Drift High-Impedance JFET DC Voltmeter Circuit

 

This is a design circuit for the voltmeter with 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 is the figure of the schematic circuit diagram:

This circuit uses a pair JFETs, 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.