Lab #7
Follow the instruction and procedures in the Student Manual, unless specified differently
in this note.
Reading:
p.142 – 162 in SM
Ch. 3.01 – 3.10 in AE
Lab #7
7-1 Follow the instruction. Make a semi-log plot of ID vs VGS curve based on your
measurements. Measure IDSS and VP for 3 – 4 samples of 2N5485. Pick one of the FET’s
in 7-2 and 7-3 and use the specific numbers that you measured for the sample. Plot the ID
– (VGS-VP) curve for the sample you chose (linear plot) and confirm its quadratic nature.
7-2 Follow the instruction (measure ID and VDS for 20 Ohm < R < 200k Ohm). Identify a
region for a good current source. Skip 7-2 (b).
7-3 Follow the instruction (read p.159 carefully). You measure gm from your experiment.
You can find the manufacturer’s range of gm for the 2N5485 on p.592 of the lab manual.
Compare your value with the manufacturer’s one, considering the fact that the
manufacturer’s value is for VGS = 0. Skip 7-3 (c).
7-4 Follow the instruction. In (b), explain why adding ½ vDS would improve the situation.
For (c), conduct simulation and experiment. Use around 1 KHz 2 Vpp for message
(modulation) and 10KHz 2 Vpp for carrier (vin). See the instruction for the simulation.
The figure above is the schematic for an “amplitude modulating” circuit. After building
this circuit on the breadboard, construct this circuit in Pspice.
Hint: Vmessage and Vcarrier are sinusoidal voltages (use VSIN not VAC) therefore you
will need to specify the amplitude, the offset (0) and the frequency. The analysis type for
this simulation is transient. The final time is around 20 periods of the carrier frequency.
You can (but do not have to) replace the potentiometer with regular voltage dividing
network composed of two resistors.
Caution: 1 MOhm resistor should be 1 MEG in simulation (not 1 M).
Part 1: Simulate the circuit with the signals suggested in the instruction.
Part 2: What happens to the AM signal as you change the potentiometer ratio from 0.1 to
0.5 to 0.9? Record the percentage modulation (modulation envelope divided by the max
amplitude) for the three potentiometer ratios. Extra credit: explain why the percentage
modulation is max for potentiometer ratio of 0.5.