Electroscope used as sensitive voltage detector

[neazoi]

Hello,
I would like to make a very sensitive voltage detector, to measure the voltage out of the detector of a crystal set or generally detector diodes without loading them much.
I have found this simple circuit
**broken link removed**
Which can measure also both polarities.

1. Would this be suitable (any better solution)?
2. Does R2 control the meter zero position?
3. Can I increase the 1M resistor to 10M to reduce sensitivity (and load the circuit under measurement less)?
4. Can I connect a set of antiparallel 1n4148 (or other?)at the fet gate, to protect the fet from overvoltage?

Also what about this circuit https://www.circuitstoday.com/wp-co...ier-Transistorized-DC-Voltmeter-using-FET.jpg from that page https://www.circuitstoday.com/dc-voltmeters will it be any better or not?

 

[Audioguru]

The circuit does not have high voltage sensitivity because the Jfet is simply a high input resistance (>100M?) voltage follower.
The Jfet has an extremely low input current, just leakage. The 1M resistor reduces damage caused by static electricity.
The meter zero is affected by how much is the Jfet leakage current which can be high or low.
R2 reduces the voltage sensitivity.

Adding diodes to the input will load the signal because the diodes begin conducting with very low voltages.

 

[neazoi]

The circuit does not have high voltage sensitivity because the Jfet is simply a high input resistance (>100M?) voltage follower.
The Jfet has an extremely low input current, just leakage. The 1M resistor reduces damage caused by static electricity.
The meter zero is affected by how much is the Jfet leakage current which can be high or low.
R2 reduces the voltage sensitivity.

Adding diodes to the input will load the signal because the diodes begin conducting with very low voltages.

how about a circuit like this one http://www.next.gr/uploads/135-9576.png

 

[Audioguru]

how about a circuit like this one http://www.next.gr/uploads/135-9576.png

It has some voltage gain so it can show 0.5V full scale.

The meter will show zero when there is no input only when the Jfets are exactly the same but they probably will not be the same.

 

[neazoi]

It has some voltage gain so it can show 0.5V full scale.

The meter will show zero when there is no input only when the Jfets are exactly the same but they probably will not be the same.

Have you got any more sensitive circuit to show, yes not very complicated?

 

[Audioguru]

An opamp with Jfet or CMOS inputs can have an extremely high input resistance and extremely high sensitivity and is a simple circuit.

 

[neazoi]

An opamp with Jfet or CMOS inputs can have an extremely high input resistance and extremely high sensitivity and is a simple circuit.

Could you suggest a suitable opamp/schematic?
Thanks a lot