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HP-4140b J113 JFET measurement
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RichardM:
Hi

I have been playing around with an HP-4140b pico amp and voltage source meter as a learning experience.

I had a go at tracing Id-Vds characteristics of a J113 JFET but the results suggest something is perhaps wrong with my setup. I have a triax cable with aliigator clips and BNC/alligator clips for the voltage sources. I don't have anything in a shielded box, just open to all the surrounding noise. The curves I get from the HP-4140b are overlayed with the datasheet and as you can see I get an odd dropoff in measured Id current part way through all the tests for different Vgs values ? Is it beacuse I do not have a sheieded test fixture setup ?

Regards

Richard
Kleinstein:
The currents are not yet that small that one would really need shielding. So the lack of shilding is likely not an issue. Depending on the setup there is a slight chance the FET is oscillating under some conditions. After all the FETs are quite potent high frequency amplifiers and the electronic regulated voltages tend to be not always ideal. With the right amount of inductance one can build an RF range oscillator.

For the JFET it may help to add some resistance (e.g. 10 K) in sereis to the gate. This should not effect the quasi DC measurement, but makes oscillation less likely.
A frist test for the meter would be to check if a resistor reads correct.
G0HZU:
I dug out a J113 and put it into my JFET curve tracer jig and I get the results below if I try and copy your Vds and Id ranges. My jig uses ferrite beads inline with the drain and gate connections to suppress any instability. It supports leaded and SMD JFETs and it is not screened. I do use twisted pair wires for the remote connections to a dual programmable PSU and a DVM.

G0HZU:
Here's a recent plot of a BF256B JFET.

This is a process 50 JFET (general purpose / VHF amp) and I think the J113 is process 51 (chopper).
RichardM:

--- Quote from: Kleinstein on June 26, 2022, 12:02:19 pm ---The currents are not yet that small that one would really need shielding. So the lack of shilding is likely not an issue. Depending on the setup there is a slight chance the FET is oscillating under some conditions. After all the FETs are quite potent high frequency amplifiers and the electronic regulated voltages tend to be not always ideal. With the right amount of inductance one can build an RF range oscillator.

For the JFET it may help to add some resistance (e.g. 10 K) in sereis to the gate. This should not effect the quasi DC measurement, but makes oscillation less likely.
A frist test for the meter would be to check if a resistor reads correct.

--- End quote ---

Thankyou Kleinstein for taking the time to reply. The resistor did the trick. I will see whether I can see any oscillation on the scope, another learning exercise :)

Richard
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