Today I breadboarded the CCS I will use for the transconductance tester jig which is an addition to my curve tracer:
Bias circuit is a classic from fixed bias amplifiers. I had the 80V available from the curve tracer circuit. The meter is an external panel AC voltmeter which is a trueRMS meter that will measure accurately the 1kHz signal.
The MOSFET CCS is a simple cascoded which can help setting the valve current and operating point. I source it with my bench variable HT power supply, which also helps in setting out the operating point.
We know that we need to have a small AC signal in the grid to increase the accuracy of the Gm test as the transconductance is given by:
So can’t feed with 1Vrms, so will use 100mVrms. If a valve has a transconductance of 1mA/V, then the variance in the anode current will be of 100uA (rms). This represents a challenge to measure accurately using an anode resistor of 10 ohms for example since the developed AC voltage across the resistor will be 1mV (rms). Therefore we will use an anode resistor of 100 ohms which will help capturing small transconductance values as this one.
Edit:
Found that the CCS bypass was omitted in my first circuit. Also the sensing resistor was reduced to 10 ohms to accommodate the AC true-rms meter I have. See updates on this post here
