2 class days
Lab 7: FETs
Lab Assignment
1. FET Characteristics
HH 7.1. Answer all questions. Measure IDSS and VP for 3 – 4 samples of 2N5485.
Pick one of the FET’s for later use and use the specific numbers that you
measured for the sample. For the FET that you have chosen, make a semi-log
plot of ID vs VGS curve based on your measurements. Plot also the ID vs. (VGS-VP)
curve for the sample you chose (linear plot) and confirm its quadratic nature.
2. FET Current Sources
HH 7.2. Answer all questions. Measure ID and VDS for 20 Ohm < R < 200k Ohm).
Identify a region for a good current source. Skip 7-2 (b).
3. FET as Variable Resistor
Source: HH, Eyler lab 6 (modified)
HH 7.4. Answer all questions.
In (b), explain why adding ½ vDS would improve the situation.
For (c), use around 1 KHz 2 Vpp for message (modulation) and 10KHz 2 Vpp for
carrier (vin).
The little AM radio transmitter in part (d) basically works fine, but a few hints may
help. Try 1 KHz as the modulating frequency, and use an antenna about 1m long
(not just a few inches). Tune the signal generator carefully around 1 MHz,
remembering that the radio is very sharply tuned. Finally, notice that some scope
probes significantly attenuate the transmitted 1 MHz output even when used
properly in the 10X attenuation setting. When set to 1X they give readings that
are completely misleading. Why does this happen? What is the carrier
frequency where you find the modulating signal?