Brandeis University Physics 29a
Fall 2017 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 +5 V with a 3.3 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 using 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? If you use a capacitor on the input of the amplifier, how did you choose its value? Build the amplifier and connect the microphone to its input. Deliver a steady, medium tone to the microphone with your voice and measure the amplitude 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? What is the output impedance of your amplifier? What is the impedance of the speaker? 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, such as the 42TM013-RC, whose data sheet you will find on the Physics 29A web page. Connect a speaker directly to the secondary winding. What is the quiescent voltage on the collector of your transistor? Voicing a medium 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 supply voltage? What power is your amplifier delivering to the speaker? Using your measurement of the speaker impedance for a medium tone, what is the impedance of the speaker primary for a medium tone with the speaker attached to the secondary?

Part 5: Remove the speaker transformer and restore your collector resistor. Design a second amplifier stage to take the audio output of your first stage and make it louder when delivered to a speaker through a speaker transformer. The second stage should provide some gain, perhaps ×10, but should also be able to drive at least ±5 V onto the speaker transformer. What is the input impedance of this second stage? If you use a capacitor to connect the two stages, how did you decide its value? What is the maximum power your two-stage amplifier can deliver to the speaker?

Part 5: Disconnect the microphone from your amplifier. Divide the output of your function generator by a factor of one hundred with a resistor divider, and apply the result to the input of your two-stage 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 in a graph. Attempt to explain any variation in gain to your instructor.