Brandeis University Physics 29a
Fall 2018 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 Physics 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 a coaxial cable, not a ×10 probe. Configure the oscilloscope for a ×1 probe and AC coupling. 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: Design an amplifier with a single 2N3904 transistor that will provide a gain of at least one hundred for audio signals, but a gain of zero for 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 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 3: 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 4: 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. Connect a speaker to the secondary winding of the transformer. Voicing a loud 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 potential higher than your power supply voltage?

Part 5: Remove the speaker transformer and restore the collector resistor to your amplifier. Design a second amplifier to take the audio output of your first stage and make it louder. The second amplifier should provide a voltage gain of around ×10, but not ×100. The main purpose of the second amplifier is to provide a larger collector current for the transformer primary. What is the quiescent current in the collector of your second amplifier? How does this current compare to that flowing in your first amplifier? Voicing a loud tone into the microphone, what is the amplitude of the signal on the collector?

Part 5: Disconnect the microphone from your amplifier. Connect your function generator to the input of your first amplifier. Reduce the amplitude of the input until the noise your speaker is making is tolerable to your neighbors. Measure the gain of your two-stage amplifier for frequencies 10 Hz to 1 MHz and plot on a graph. Attempt to explain any variation in gain to your instructor.