Resistors and Ohms Law, Dependent Sources and MOSFETs

Resistors and Ohms Law – Voltage-Current Characteristics

In this lab, we wired a simple circuit containing a voltage source and a 100 Ω resistor to examine the relationship between voltage and current. We used WaveForms and Analog Discovery interface to simulate a DC voltage and measured the corresponding current with a digital multimeter (DMM).

Before wiring the circuit, we used the DMM to measure the actual resistance of the 100 Ω resistor. We got a 97.9 Ω reading on the DMM.

We then wired the circuit according to the diagram and took several readings for current as we increased the voltage from 0 V to 2 V in increments of 0.2 V.

Using this data, we created a graph of current vs. voltage. We noticed that the relationship between current and voltage was linear. We fit a linear trendline and obtained the equation for the line. The slope of the line is y=0.1084x. The slope, 0.1084 represents the resistance of the 100 Ω resistor. However, since the current was measured in mA, the resistance is given in MΩ. Therefore, the experimental resistance for the 100 Ω resistor was 0.1084 MΩ or 108.4 Ω. The R^2 value is 1 so the data points fit right on the trendline.

When we compare the calculated values from the graph to the original measured value, we notice that there is a difference of about 10 Ω. This may be due to the resistance in the circuit wires since we initially measured the resistance of the 100 Ω resistor directly, without using any wires in addition to the DMM ones.

Dependent Sources and MOSFETs

In this lab experiment, we use a ZVN2110 MOSFET transistor along with a 5 V DC source and 100 Ω resistor and measure the drain current (I_D) when the gate-to-source potential (V_G) is varied. We use a digital multimeter (DMM) to measure the drain current. To provide the 5 V source and the variable voltage (V_G) we use WaveForms software with Analog Discovery interface.

Here we see the resistor, transistor, DMM, and Analog Discovery interface wired according to the diagram.

Before recording any data, we observed the current readings on the DMM when we varied the voltage. We found that the current reading was zero until a certain amount of voltage was applied. The current then went up as the voltage increased but seemed to top off at a certain point, after which the current would not rise as more voltage was applied.

We started recording data at 1.6 V and increased the voltage in steps of 0.1 V until the current seemed to remain constant.  Plotting the data on a graph we can see that the drain current remains zero until it reaches a threshold voltage of about 1.7 V is applied. The current then varies linearly with respect to the applied voltage. Once about 2.4 V are provided to the circuit, the current begins to top off. We stopped taking measurements after 3.0 V where the current seemed to stay fixed at about 48 V.

After analyzing the data and the graph, we can conclude that the MOSFET behaves like a voltage dependent current source. Also, the slope of the line is significant because it represents the conductance of the circuit. Conductance is the inverse of resistance (g=I/V). Taking directly from the equation of the trendline, we find that the conductance of the circuit is 113.4 mƱ.

To improve this experiment, we could take more readings on the linear region to create a better trendline. The graph seems a bit curvier than linear in the trendline region. Having more points in that region would give a more complete representation of the current to voltage behavior. However, the 0.9876 R^2 value for the linear region shows that our data points fit tightly to the trendline.