Dependent Sources and MOSFET's

In this experiment, the functionality of dependent voltage sources was explored via the use of a ZVN2210A MOSFET. By connecting a constant 5V power supply to a 100Ω resistor (with an inline ammeter, to measure current delivered), and then a MOSFET before grounding the resistor, it was possible to determine the effects of the dependent voltage source (the MOSFET). By connecting a variable voltage source to the MOSFET's gate, it was theorized that we could map the amount of current allowed through the circuit as the voltage supplied to the MOSFET's gate varied. This would practically demonstrate the functionality of a dependent voltage source, as well as help us better understand how n-channel MOSFET's work. The setup was as follows:

**NOTE** The voltage source VsubG was the Analog Discovery tool. It was also the voltage that was varied throughout the experiment.

Above is what the actual setup looked like. Below is a closeup of the resistor and MOSFET connection, which was all connected via a breadboard for ease. 

Key to understanding the picture above:

The LEFT red alligator clip was the constant 5V power, which was fed through the resistor and into the Drain pin of the MOSFET.

The RIGHT alligator clip was the variable power signal, which was fed from the Analog Discovery tool into the MOSFET's Gate pin.

The White alligator clip was connected to the Supply pin of the MOSFET, and it went directly to ground (to complete the circuit).

Also, the resistor was measured to be 98.0Ω ±0.1Ω, which varies slightly from its self-proclaimed 100 Ohm resistance.

Data:

Analysis:

It was observed that there was some sort of threshold voltage that applied to the MOSFET; below 1.53 Volts, the gate was not actuated, and zero current flowed through the transistor. This implies that in order to function, the MOSFET has a minimum potential rating of 1.53V. It also appeared to allow current through the transistor in a piecewise fashion. From 1.53V- ~2V it took drastic changes in voltage to create hardly any current. Then, from ~2V to ~2.5V very little changes in voltage resulted in drastic (and quite linear) changes in current allowed through the transistor. Beyond ~2.5V drastic changes in voltage once again resulted in very little change in current. That creates the idea that perhaps these particular MOSFETs have a functionality range of 2-2.5V, and will behave predictably (linearly) throughout that range. This data suggests that the MOSFET behaves as a dependent current source, as it allows certain amounts of current to flow through it based on other independent voltages. With this data, it is possible to estimate the conductance of the transistor, by taking the slope of the linear portion of the data (which is the resistance of the MOSFET), and simply inverting it. It should be noted that you could also simply take the slope of the linear portion of the data once the axes have been inverted. Both methods lead to an estimated conductance (G) of approximately 81.9 Mhos.

Conclusion:

MOSFETs appear to be dependent current sources, which have both threshold voltages, and linear voltage ranges. If used correctly, they can reliably control the amount of current that is allowed to flow through a circuit, simply by applying a varying voltage signal to the gate of the transistor.