ECE 2274 MOSFET Voltmeter Richard Cooper Pre-Lab for MOSFET Voltmeter Voltmeter design: Build a MOSFET (2N7000) voltmeter in LTspice. The MOSFETs in the voltmeter act as switches. To turn on the MOSFET. For design assume Vg = 2.1Vdc when it begins to switch on. Design the Rd value, such that the maximum drain current when the MOSFET in on is 8mA for Vdd = 10Vdc Vds SAT = 100mV. Design the input voltage divider resistors values such that Vout turns on at approximately at Vin = 6Vdc, Vin = 10Vdc, and Vin = 14Vdc.Choose input resistors (Rg1, Rg2) values that are in the 10kΩ to 150kΩ range. To find the voltage divider resistors Rg1, Rg2, to you must create three input voltage circuits thresholds when Vin= 6V, Vin=10V, and Vin=14V. The calculation steps will be identical to the first voltmeter, but the input voltage (Vin) will be the only difference. Design the voltage divider (Rg1, Rg2) so that the gate voltage is 2.1Vdc with the desired Vin voltage. Vin Rd1 Id Vin DC Iin Rg1 G D Vout M1 DC Vdd Rg2 S Page 1 of 7
Show all work Fill in table below from your circuit simulation. All currents are derived for a voltage across a know resistor. Set Rg1, and Rg2 for Vin = 6Vdc threshold circuit. Vin Voltage when MOSFET starts to V G Voltage of gate when MOSFET starts to Vin Voltage when MOSFET current Id reaches steady state V G Voltage of gate when MOSFET current Id Vds sat Voltage when MOSFET current Id Rd1 Drain current limit resistor Value Set Rg1, and Rg2 for Vin = 10Vdc threshold circuit. Vin Voltage when MOSFET starts to V G Voltage of gate when MOSFET starts to Vin Voltage when MOSFET current Id reaches steady state V G Voltage of gate when MOSFET current Id Page 2 of 7
Vds sat Voltage when MOSFET current Id Set Rg1, and Rg2 for Vin = 14Vdc threshold circuit. Vin Voltage when MOSFET starts to V G Voltage of gate when MOSFET starts to Vin Voltage when MOSFET current Id reaches steady state V G Voltage of gate when MOSFET current Id Vds sat Voltage when MOSFET current Id Run a DC Sweep simulation on Vin from 0V to 20V in increments of 100mV. Plot Vds for each voltmeter circuit Print out your plot. Remember that a MOSFET is considered to be on after V DS voltage is approximately 0.2 volts. From the LTspice plot Vds of when the MOSFET is fully on. What input voltage did each on the MOSFETs reach steady state? Required Attachments: (3 plots) (6V, 10V, 14V) Vdd = 10Vdc, DC Sweep, varying Vin plotting Vds, plotting Id, and plotting Vg for each input voltage divider plots. Three traces per plot (Vds, Id, Vg). Page 3 of 7
Lab Exercise MOSFET Voltmeter 1. You must test the 2N7000 MOSFET with the curve tracer before build your experiment. Setup the curve tracer to N-FET, Vds max = 10V, Is Max = 10ma, Vg/step = 0.1V, Offset = 1.8V, N Steps = 10, Rload=10, and P max = 0.5W 2. Measure with DMM Build the voltmeter you designed in pre-lab. Manually adjust Vin to find the turn on voltages and saturated voltages of each MOSFET design. Record the voltage at which MOSFET turns on and saturates (where first begins to current and flattens out). Fill out tables. 3. DC sweep Sweep the circuit using the Basic DC sweep Vin from 0 to 20V. Plot the Id current in ma not amps. Add formula data to output section of the Sweep step Load BasicDCSweep > open Step Setup Tab > Open Add Step Tab > Add Step > Processing > Analog Signal > Formula. Divide voltage across Rd by Rd and scale to ma. Drag this new step into the sweep loop just below the DMM step. Open the sweep step > Sweep Output Tab > Add > Processed Data vs. Voltage X-Axis. Run sweep. (3 plots) Vin vs. Id ( 6V, 10V, 14V) Run a DC sweep of Vin from 0Vdc to 20Vdc and turn in the following plots for each on voltage (6V, 10V, 14V) range with your lab report: Vin vs. Id, Measure the voltage across a known resistance (Rd) and calculate the scale factor for the current plot in ma, write the scale factor on the plot. You will have a total of 3 plots to be turn in with your lab report. Remember to label each plot. Also, remember Id is the current in ma. How does it compare with your design? Label all plot. Page 4 of 7
DATA SHEET Name: Name: Instructor: Class day and time: Date: Bench number: MOSFET Voltmeter 1. You must test your 2N7000 MOSFET with the curve tracer before build your experiment. Set curve trace to N-FET, Is Max = 10ma, Vds max =10V, Vg/step = 0.1V, Offset = 1.8V, Rload=.10, N Steps = 10 2. Measure with DMM Vdd= 10Vdc Manually adjust the Vin voltage where observing Id (voltage across Rd). Fill in table below. Show all work Fill in table below from your circuit simulation. All currents are derived for a voltage across a know resistor. Set Rg1, and Rg2 for Vin = 6Vdc threshold circuit. Measure with voltmeter Vin Voltage when MOSFET starts to V G Voltage of gate Vin Voltage when MOSFET current Id V G Voltage of gate when MOSFET current Id Vds sat Voltage when MOSFET current Id Rd1 Drain current limit resistor Value Page 5 of 7
Set Rg1, and Rg2 for Vin = 10Vdc threshold circuit. Measure with voltmeter Vin Voltage when MOSFET starts to V G Voltage of gate when MOSFET starts to Vin Voltage when MOSFET current Id V G Voltage of gate when MOSFET current Id Vds sat Voltage when MOSFET current Id Set Rg1, and Rg2 for Vin = 14Vdc threshold circuit. Measure with voltmeter Vin Voltage when MOSFET starts to V G Voltage of gate when MOSFET starts to Vin Voltage when MOSFET current Id V G Voltage of gate when MOSFET current Id Page 6 of 7
Vds sat Voltage when MOSFET current Id 3. DC sweep. Compare the DC sweeps of your Pre-Lab with the results of your experimental DC Sweeps. What conclusions can you make? Required Attachments: (3 plots) Vin vs. Id ( 6v, 10v, 14v) DC Sweep Vin vs. Id (voltage across a resistor Rd) Add a Formula step to plot the current in ma thru the Rd resistors as a current in ma use a scale factor to convert the voltage to a current on the plot. Page 7 of 7