Brandeis University Physics 29a
Spring 2018 Kevan Hashemi

Lab 8: Transistor Radio

Power Supply: Use your bench-top power supply to provide 10 V to your transistor radio. Do not use the breadboard power supply. Turn the breadboard power supply off. When it is on, it introduces noise into your transistor radio that corrupts the audio signal.

Part 1: Make a radio antenna using a few meters of hook-up wire and a bamboo pole. Place it straight up on your work bench and hold it in place with some masking tape. Make sure you can plug the near end of the wire into your breadboard. If possible, keep the top of the antenna one meter or more from the overhead lights.

Part 2: Collect an air-gap variable capacitor, an inductor of fifty turns of hook-up wire on a 10-mm diameter ferrite rod, a speaker transformer, and a speaker. Assemble the tuner of the transistor radio, which consists of L1, and VC1. Do not connect the antenna to point A. Instead, deliver to point A a 1.0-Vpp sinusoid with source impedance 10 kΩ using your function generator, a resistor, and a capacitor. With a ×10 probe, measure the amplitude of the sinusoid at A. Adjust the variable capacitor to its maximum capacitance. Sweep the sinusoid frequency from 100 kHz to 5 MHz. What it the resonant frequency of your tuner? Adjust the variable capacitor to its minimum capacitance. What is the resonant frequency now? These are the minimum and maximum tuning frequencies of your radio. We would like them to include the range 850-1300 kHz. If your tuning range does not include 850 kHz, add a 47-pF capacitor in parallel with VC1. If your range does not include 1400 kHz, remove five turns from your coil. With your tuner set somewhere near the center of your tuning range, measure and plot the amplitude of A versus frequency, starting 100 kHz below and ending 100 kHz above the tuned frequency.

Part 3: Medium-wave radio stations can broadcast at frequencies separated by as little as 10 kHz. Suppose two stations are broadcasting on two frequencies, and suppose that your antenna receives equal power from each station. Assuming your antenna acts like a signal with 10-kΩ source impedance, what is the minimum separation of station frequencies such that your tuner can select one station and have the amplitude of the other station be half as large?

Part 4: Disconnect your signal source from A. Assemble the first amplifier stage of the radio by adding Q1, C1, R1, and R2. Do not add C2 or C3. What are the average values of the voltages at B and C? What is the quiescent current flowing into Q1's collector? Deliver to B a 10-mVpp sinusoid of source impedance 100 Ω and frequency 1 MHz. Use your function generator, two resistors, and a capacitor. What is the voltage gain of the amplifier from B to C? What is the average value of C? Explain why the average value of C changes when we apply a 10-mVpp sinusoid to the base of Q1.

Part 5: Disconnect the voltage source from B. Add capacitor C2. Connect the antenna A12. Complete the radio circuit. When you have all components loaded, your speaker should be making noise. Adjust the tuning capacitor. How many stations can you detect? Can you improve reception by putting your hand on the coil? What happens if you disconnect the antenna and touch the coil? What happens when you turn the breadboard power supply on? Try to explain each observation to your instructor.

Part 6: Remove each of the following components in turn: C1, C2, C4, and R2. Replace each component before you remove the next. For each component, note the change in the behavior of your transistor radio that occurs when you remove it, and try to explain why the change takes place.