This circuit allows measuring the most important parameters of a N-channel MOSFET. The MOSFET gate can be driven either from an NE555 timer or from an OPAMP output, selected by a gate drive switch.
Dynamic mode (NE555 active):
The NE555 provides a square-wave gate drive, with frequency set by its potentiometer. This mode allows observation of switching behavior on an oscilloscope. The gate charge Qg can be estimated from the Miller plateau by multiplying the plateau time with the current at the gate resistor.
Static mode (OPAMP active):
The OPAMP acts as a gate bias source. Starting with its pot fully closed, increase the bias by slowly turning it open while probing the voltage across the 100 mΩ source sense resistor until you see voltage. That voltage is Vgs(th) - in the practical sense, which can differ from the datasheet value that is measured with a very small drain current. With the pot fully open measure the drain-source voltage Vds(on). Measure the voltage across the sense resistor and divide it by the resistor value and you will get Id. Divide Vds(on) by Id and you'll get Rds(on).
