Brandeis University Physics 29a
Spring 2019 Kevan Hashemi

Lab 7: Audio Amplifier

Part 1: An electret condenser microphone has two terminals. The CMA-4544PF-W microphone data sheet is on the 29A web page. Consult the data sheet so you can identify the two terminals. Connect Terminal 2 to 0 V and Terminal 1 to +10 V with a 33 kΩ resistor. Connect Terminal 1 to your oscilloscope with ×10 probe. You should be able to see the microphone audio signal on the screen. Using your own voice, measure the frequency of a deep tone, a medium tone, and a high tone.

Part 2: Devise a procedure for measuring the output impedance of the microphone circuit for audio signals. Explain your procedure to an instructor. Measure the output impedance of the microphone for audio signals.

Part 3: Build an amplifier with a single 2N3904 transistor that provides a gain of ≥100 for audio signals, but blocks any constant voltage applied to its input. Draw a circuit diagram of the amplifier and explain how it works to an instructor. What is the theoretical input impedance of your amplifier for audio signals? How did you choose the value of the amplifier's input capacitor? What is the quiescent voltage on the collector of the transistor? Build the amplifier and connect it to the microphone. Deliver a steady, medium tone to the microphone with your voice and measure the amplitude of the signal at the input and output of your amplifier. What is the gain of your amplifier for this tone?

Part 4: Devise a procedure for measuring the input impedance of your amplifier for audio signals. Explain your procedure to an instructor. Measure the input impedance of the amplifier.

Part 5: Connect a speaker to the output of your amplifier with the help of a 1000-μF capacitor. What is the voltage gain of your amplifier for a medium tone with the speaker attached? Why does the gain decrease with the speaker attached?

Part 6: Replace the collector resistor of your amplifier with the primary winding of a speaker transformer. Our speaker transformer is the 42TM013-RC, which has turns ratio of 11.5. Its data sheet is on the 29A web page. Connect a speaker to the secondary winding of the transformer. Voice a tone into the microphone. What is the amplitude of the signal on the collector? How is it possible for the collector to rise to a voltage higher than your power supply voltage? What collector resistor would give you the same amplitude signal as the transformer with speaker? Using the transformer equations, what is the effective resistance of the speaker?

Part 7: Remove the speaker transformer and restore the collector resistor to your amplifier. Build a second amplifier that takes the output of the first amplifier and delivers it to a speaker so that the audio is loud and clear. The second amplifier should use one transistor only, and it may or may not use the speaker transformer. What is the maximum amplitude audio signal your circuit can deliver to the speaker terminals? Move the speaker near the microphone. You should hear a high-pitched whining noise. Explain to your instructor how this noise arises.