Brandeis University Physics 29a Fall 2018 Kevan Hashemi

# Lab 8: Transistor Radio

Power: Use your bench-top power supply to provide 10 V to your transistor radio. Connect your 0-V to the power supply's ground terminal. Turn your breadboard power supply off and disconnect its power cord.

Construction: Arrange components as they are presented in the diagram. Use columns of contacts for you power supplies. Spread out the components so that you can see how they are connected, but not so much that you need long wires to connect them. Use color coding for +10V (red), 0V (black), and signals (others).

Part 1: Make an inductor out of fifty turns of hook-up wire on a 10-mm diameter ferrite rod. Assemble the tuner of the transistor radio, which consists of L1, and VC1. Do not connect the antenna to point A yet. Instead, deliver to point A a 1.0-Vpp sinusoid with source impedance 27 kΩ using your function generator and a resistor. Adjust the variable capacitor to its maximum capacitance. Measure the amplitude of the sinusoid at A. Increase 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 800-1400 kHz. If your tuning range does not include 800 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 200 kHz below and ending 200 kHz above the resonant frequency. What is the half-power width of your tuner response? How does this width dictate how selective your tuner can be?

Part 2: Load Q1, C1, R1, and R2 onto our breadboard. Place C1 near Q1. Do not load C2 or C3 yet. What are the voltages B and C? What are the quiescent currents flowing through Q1's base, collector, and emitter? Configure your function generator to produce a symmetric, 1-MHz, 10-mVpp square wave with offset equal to the voltage you just observed on B. Connect this signal directly to the base of Q1. Adjust the offset of the signal until the average voltage at C is roughly 6 V. What is the amplitude of the 1-MHz square wave on C? What is the gain of your amplifier at 1 MHz? Increase the amplitude of the square wave on B to 50 mV. What is the average voltage on C now? Add capacitor C3 across R2. Why does the square wave disappear from C? Why is the average voltage on C the same with and without C3? With C3 in place, plot a graph of the voltage on C versus the amplitude of the square wave on B for amplitudes 0 mVpp to 100 mVpp.

Part 3: Make a radio antenna using a few meters of hook-up wire and a bamboo pole. Leave 50 cm of free wire at the base for connection to your breadboard. Place the pole straight up on your work bench and hold it in place with some tape. Disconnect your signal source from B. Add capacitor C2 so as to connect your tuner to the base of Q1. Connect your antenna to A. Adjust you tuning capacitor and look for signs of an audio signal on C. When tuned to Radio 29A, 1.4 MHz, an audio signal should be visible on your oscilloscope. If you see no audio, ask for help from an instructor.

Part 4: Complete the radio. Your speaker should be making noise. You should be able to pick up our own Radio 29A and one or two other stations. Try to identify the stations and estimate their broadcast frequencies. Can you improve reception by putting your hand on the coil? What happens if you disconnect the antenna and touch the coil?

Part 5: Remove each of the following components in turn: C1, C2, C3, C4, and C5. 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. Try to explain the changes to your instructor.