Day Two - Dependent Sources and MOSFETs 3/2/17

Today, we did two labs. One of the labs is Voltage-Current Characteristics, and another is Dependent Sources and MOSFETs.

At the beginning of the class, our warm-up problem is:

The answer is even if the switch is turned ON, there is no change in the circuit because the potential drop remains the same as the potential in a battery is equal to the potential of the light bulb.

After that, we have our first lab - Voltage-Current Characteristics.

In this lab, we have to find the relation between current and voltage under a constant resistance.

The circuit diagram is:

Under the consideration of uncertainty, we measured the resistance of the resistor, and we get 98.7 ohms for a 100 ohms resistor. 

By changing the voltage of Vs, we are changing the current in the circuit and thus changing the voltage drop between the terminals of R.

Measured Data:

As we vary the Vs, the calculated V and I also vary.

We collected five values of voltage and current with different controlled Vs.

The graph does look like the current and voltage have a direct proportional relationship.

We further explore on this topic, and we find that the equation of this graph is:

y = 97.7811x + 0.0442

As the slope of this graph is the resistance of the resistor, we can find that there is some percentage error between our calculated value and measured value, but the relation between voltage and current still holds.

After that, we had our second experiment using MOSFETs.

MOSFETs stands for Metal Oxide Semiconductor Field Effect Transistors, and it is a voltage controlled current source.

Here is the setup of the experiment.

By changing the voltage applying to the gate of MOSFET, we are able to change the current through the drain and source. Increasing the voltage allows a stronger field for current to flow.

Here is the data we collected by changing the Vg.

We analysed the data by plotting the curve out.

From here, we can see that there is no rapid increase until the voltage is close to 0.7 volt.

Therefore, we can conclude that 0.7 volt is the threshold voltage required to operate the MOSFET.

We can also conclude that the MOSFET is a voltage controlled current source because as voltage changes, the current will also change. 

Our calculated g value is around 49.8605 by best fitting the "linear part" of the curve.

Note: the increment is not by 0.3 because we might have picked up a bigger resistance in our experiment, and the difference between the applying voltage and the voltage of the drain is so big that only a small applying voltage will control the current. 

Summary:

In day two, we learned about the relation between voltage and current is direct proportional if a constant resistance is used. We also learned that MOSFET is a voltage controlled current source because as we change the voltage at the gate, the allowed current go through from the drain to the source is going to increase; however, applying low voltage to the MOSFET might not allow current to flow because there is a threshold voltage in the MOSFET.