This is the simplest VGS(th)(VTO)-tester.
VTO of right side NMOS-FET is measured by left side NMOS-FET circuit which works as relay switch.
for example)
VTO of tested FET=1V
RL=0.1kohm(Id=~100mA) VTO=1.77V
RL=1kohm(Id=~10mA) VTO=1.25V
RL=10kohm(Id=~1mA) VTO=1.08V
RL=100kohm(Id=~0.1mA) VTO=1.03V
RL=1Mohm(Id=~0.01mA) VTO=1.01V
VTO of FET depends on Id.
