Circuit : Andy Collinson
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This circuit uses a JFET differential amplifier to measure DC voltages on an analogue meter. It uses a Wheatstone bridge circuit to minimise drift and input range has selectable ranges.
Notes
R7 and R8 form a potential divider across the power supply and bias the gate of Q1 at 1/3 supply
voltage. Q1, R5 and Q2, R6 form a Wheatstone bridge circuit, the meter M1 and preset PR1 are
at the centre of the bridge. With no input signal, VR1 is used so that M1 reads 0V. Q1 and Q2
should be matched as evenly as possible with values drain source current matched within 10% or
better.
Setup
With the range switch S2 at position 1, apply a calibrated 0.5V DC voltage to the input terminals.
Note that this circuits ground is actually at 1/3 the supply voltage which is why it is battery
powered and isolated from other circuitry. With a 0.5V input signal adjust PR1 so the meter reads
full scale. Remove the input signal and make sure reading is zero, if not adjust VR1 again and
repeat procedure.
R1,R2 and R3 should be 1% tolerance resistors and set the input impedance at 11.1M. Any analogue meter responds much faster than a digital multi-meter and can easily detect more than 20 changes per second. For any input signal that changes faster than this then an oscilloscope is required.
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