Brandeis University Physics 29a
Fall 2016 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 or headphones. Assemble the tuner of the transistor radio, which consists of R1, C1, C2, 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 500-1600 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 1550 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 and R2. Do not add C3 or C4 yet. What is the average value of the voltage at B? What is the quiescent current flowing into Q1's collector? Deliver to A a 10-mVpp sinusoid of source impedance 100 Ω using your function generator, two resistors, and a capacitor. Look at the voltage on the collector of Q1. Adjust the frequency of the sinusoid until the amplitude at the output is largest. What is the gain of the amplifier? Add C3. What is the gain now? Use the output amplitude dial on your function generator to modulate your signal amplitude from 10 mVpp to 20 mVpp at around 1 Hz. How much does the voltage on B change with your modulation? Why does B change with your modulation? From your knowledge of transistors, how much do you expect the voltage at B to change with your modulation?

Part 5: Disconnect the voltage source from A and connect the antenna in its place. Complete the radio circuit. When you have all components loaded, your speaker should be making a sound. 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 of your observations to your instructor.

Part 6: Remove each of the following components in turn: C1, C2, C5, 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.

Part 7: What is the input resistance of Q1 for small signals? What resistance must the tuner tank circuit attain in order to force the antenna signal into the base of the transistor? What is the input resistance of Q2 to audio frequencies? Predict what would happen to the amplitude of the audio signal at B if you were to remove C4. Remove C4 and see if you are correct.