Astable Multivibrator: 2N7000 MOSFET IDSS and Vthr Tests

Building an astable multivibrator from MOSFETs for longer time constants and more reliable operation suggests I should know a bit more about their operation with minuscule currents and low voltages. I have a small stock of low-threshold ZVNL110A MOSFETs, but using something less obsolete seems in order.

Dirt-cheap 2N7000 MOSFETs have a maximum IDSS around 1 µA at room temperature, which would be way too high in this situation; there wouldn’t be much difference between their ON and OFF states.

The test setup is simplicity itself:

2N7000 IDSS - calibration

2N7000 IDSS – calibration

The initial reading from a 4 V bench supply was 0.00 µA on the Siglent SDM3045, my best low-current meter, so I put a 10 MΩ resistor across the drain and source terminals:

Astable - 2N7000 - IDSS cal

Astable – 2N7000 – IDSS cal

Close enough, particularly given the silver fourth band on that old carbon composition resistor and its no-doubt unclean surface.

The rest of the 2N7000 MOSFETs have IDSS ≤ 10 nA, which you can’t distinguish from zero on that scale.

The 2N7000 datasheet specs give a threshold voltage from 0.8 to 2.5 V for 1 mA drain current, bracketing a 2.1 V typical value, which would be too high for a nearly dead lithium cell.

I calibrated the VGS(thr) current at 11 µA with a 348 kΩ resistor:

2N7000 MOSFET Id cal

2N7000 MOSFET Id cal

Which produced 11.49 µA at 4 V, just as it should, so I plugged in a MOSFET and twiddled the trimpot for a nice round 10 µA:

2N7000 MOSFET Vthr test

2N7000 MOSFET Vthr test

Most transistors conducted 10 µA with the gate at 1.42 V, with a few outliers spanning 50 mV on either side. Close enough and low enough!.

Now, to conjure an astable.

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