1.In the beginning of class, we are asked to make a prediction about what will happen if the switch is closed in the circuit shown below.
We make a prediction that both the upper light bulb and the lower light bulb will stay the same.
As we can see from the picture, the brightness of the two bulb stay unchanged. Our prediction is correct. Since the two bulbs are identical, we assume that the power supply (the two batteries) is 3V. So on each bulb, it is 1.5V. When we connect a battery in parallel with one of them, the voltage in the circuit stays the same. Since P=V^2/R, we can say that the power of both bulb stay unchanged as long as the resistance of the two bulbs is constant. Therefore, the brightness of the two light bulb does not change.
2. In this lab, we use WaveForm as power supply that can have various voltage value through a 100 ohm resistor.
Here is the circuit. We are going to measure the current across the resistor.
The set up of the experiment.
Our data table
We measure the real value of the resistor, and it shows that it is 100.9 ohm. We apply different voltages across the resistor with WaveForms and measure the current for each voltage.
The graph of Voltage vs. Current. It is a linear relationship, and it follows the Ohm's law, the relationship between current and voltage, V=IR.
In the graph, the slope 103.6 should be the resistance of the resistor. It's bigger than 100.9 ohm, which means the wires in the circuit has a small value of resistance.
% difference is (100.9-103.6)/100.9*100% = -2.67%
3. In this lab, we are going to use a MOSFET and a 100 ohm resistor. We have a resistor which has a true value of 100.9 ohm. We are going to determine the threshold voltage of the MOSFET. If the voltage is below the gate voltage, there won’t be any significant current. With MOSFET, we can increase the power supply’s current by increasing the gate voltage. A MOSFET and a power supply, can act as a voltage controlled current source in which the drain current is controlled by the gate voltage.
The set up of the experiment. We at first use the regular power supply. After figuring how to use the waveform, we do a new experiment with waveform.
The circuit of the experiment
The data table of our experiment. We connect a 5 V voltage supply in the circuit, and apply different voltages to the MOSFET. As we can see, the current starts increasing significantly between 2.0V and 2.2V. So we guess the gate voltage is 2.2 V.
Gate Voltage vs. Drain Current Graph.
From the graph, we can see the threshold voltage is around 2.2 V. When the voltage is below 2.2 V, it increases slowly. When it is greater than 2.5 V, it increases slowly again. So the starting point which it increases rapidly is the threshold voltage.
The dependent source is a VCCS (voltage controlled current source). The current of it is controlled by the input voltage.
When I try to fit a straight line, I get an estimated value of g for 0.0355.
Summary:
In today’s lab, we review Ohm’s Law and Kirchhoff’s Laws. We learn a little about depend source, and one of it, voltage controlled current source (VCCS).
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