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Ultra simple mosfet voltmeter schematic

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neazoi

neazoi

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Hi I would like to build a very simple mosfet voltmeter. Mosfet, so that it does not load the measuring circuit. It will use an analogue meter.
Can I use the 2n7000 to build this https://www.youtube.com/watch?v=YkZuBu-EUhc ???
What if I want to also measure negative voltage as well? Shall I just connect the ground to the gate and the negative voltage to the source, will that be ok?
 
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Look at FET Id=f(Vgs) curve and see if it's of any use for your purposes. The idea of using a calibration series resistor raises doubts about the author's analog design skills. Might be copy-and-past error while modifying an original common drain circuit.
 
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    neazoi

    neazoi

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FvM said:
Look at FET Id=f(Vgs) curve and see if it's of any use for your purposes. The idea of using a calibration series resistor raises doubts about the author's analog design skills. Might be copy-and-past error while modifying an original common drain circuit.
Click to expand...

I know I said mosfet, but this seems ok http://www.simplecircuitdiagram.com/2010/01/25/fetvm-fet-voltmeter/ plus it can measure negative voltages as well. Any new replacement for the old 2n4340? They use is because of it's low noise low leakage, but it's quite expensive.
Maibe a better option would be this http://www.next.gr/meter-counter/voltmeters/fet-voltmeter-l13908.html since the FETs are matched?
 

Differential FET amplifier with resistive degeneration (the 500 ohm variable resistor) is a different topic and has nothing in common with the post #1 circuit. If you omit the diode, the output can be bipolar, no need to flip the input wires.
 

FvM said:
Differential FET amplifier with resistive degeneration (the 500 ohm variable resistor) is a different topic and has nothing in common with the post #1 circuit. If you omit the diode, the output can be bipolar, no need to flip the input wires.
Click to expand...

You mean something like this?
(but by including a series 1M with the gate and removing the input circuit up to the 100pf)
 

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