ECE 201L Circuit Analysis Laboratory
Lab 8

This lab explores the behavior of diodes and the properties of bipolar junction transistors (BJTs)

Attachment: lab8.zip.

Do the following exercises. Report your results by editing the attached Word document and submitting it in Isidore. Submit one report per group.

  1. Construct the full-wave bridge rectifier shown in Figure 1. Use a 1500 Ω resistor for the load. Build one version with 1N4001 diodes and one with light-emitting diodes.


    Figure 1. Full-wave diode-bridge rectifier.

    1. Do the following for each bridge. Measure source voltage on oscilloscope with probes between A and B. Measure load voltage with probes between C and D. Do not attempt to measure both signals on the oscilloscope at the same time, the oscilloscope channels share a common ground and trying to make simultaneous measurements will introduce ground loop problems. Set the frequency of the source to 50 Hz and the amplitude to 10 volts peak-to-peak. Attach oscilloscope images of the source voltage and the load voltage to the report, showing the maximum voltage in each signal.

    2. For the LED bridge: Reduce the input frequency to 1 Hz. Observe the pattern of lights and describe in your report.

  2. Identify the 2N3904 (npn) and 2N3906 (pnp) transistors. Measure the β of the transistors using the hFE setting on the handheld multimeters. Obtain measurements for the transistors connected correctly and backwards (interchange C and E).

    Use a 2N3904 (npn) transistor in the following circuits.

  3. Build another “night light” circuit, this time with a transistor. Make the resistance R1 about 30 kΩ. You can use a 10k potentiometer and a 25k fixed resistance to allow some “tuning”. Make VCC = 9 Volts. Demonstrate your circuit to the teaching assistant.

    Measure the resistance of R1 and the voltage VBB at which the transistor begins to turn on (LED begins to glow).

  4. Replace the LED with an electric motor and demonstrate that you can control the speed of the motor by the amount of light falling on the photo cell.

  5. Build the following circuit. Select RB between 1.5 – 2.5 kΩ and RC between 100 – 300 Ω. Make VCC a nine-volt battery. Connect an analog output channel from the DAQ to provide VBB and connect an analog input channel across RC. See bjt_test.m for an example data acquisition program. Identify the control voltages (and corresponding currents) for the cutoff region, linear region, and saturation region of operation.

  6. Build the following circuit, replacing resistor RC from above with the motor. Run the data acquisition program (see motor.m) to measure the voltage across the motor as a function of the input control voltage. Identify the minimum control voltage needed to fully turn on the motor.

  7. Replace the control signal from the data acqusition system with a signal from the signal generator. Set the signal generator to produce a 1 kHz square wave with a minimum voltage of zero and a maximum voltage equal to the control voltage determined in the above measurement. Vary the duty cycle and observe that you can change the speed of the motor. This is called pulse-width modulation (PWM). Determine the duty cycle that allows the motor to just barely spins. Note that if the motor stops you may need to jack up the duty cycle to get it spinning again before trying to slow the motor down. Capture an oscilloscope image showing the control signal for having the motor turning near minimum speed.

Maintained by John Loomis, last updated 15 March 2010