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Audio Amplifier with Electret Microphone

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Introduction

In this experiment we design and build an audio amplifier that takes the small output voltage from an electret microphone and amplifies it such that it can drive a small loudspeaker. The amplifier is comprised of an AC-coupled non-inverting operational amplifier with a voltage gain of eleven, with an inside-the-loop emitter-follower on its output with AC-coupling to the loudspeaker. The op-amp section provides voltage gain, and the emitter-follower functions as a buffer, providing the current required to drive the loudspeaker. Placing the emitter-follower inside the feedback loop improves its overall performance.

Objective

To investigate negative feedback amplifiers and Class A amplifiers by building an audio amplifier. To show how to bias analog circuits for use on single-voltage power supplies. To show that a positive feedback loop can occur in an audio amplifier with a microphone on its input and loudspeaker on its output if the microphone is placed too close to the loudspeaker. Following completion of this lab you should be able to describe the basic operation of an electret microphone, explain one way to bias op-amp-based circuits for single-supply operation, describe amplifier clipping, and be able determine the DC- and AC-gains of a non-inverting operational amplifier circuit.

Materials and Apparatus

  • Data sheet handouts for OP484, electret microphone, and 2N3904
  • Audio Amplifier Experiment article handout (linked below in the “Theory” section to a wiki version)
  • IC pinout handout
  • Computer running PixelPulse software
  • Analog Devices ADALM1000 (M1K)
  • Solderless breadboard and jumper wires from the ADALP2000 Analog Parts Kit
  • (1) OP484 from the ADALP2000 Analog Parts Kit
  • (1) Electret microphone from the ADALP2000 Analog Parts Kit
  • (1) 2N3904 NPN transistor from the ADALP2000 Analog Parts Kit
  • (1) 8 Ω loudspeaker from the ADALP2000 Analog Parts Kit
  • (1) 47 Ω resistor from the ADALP2000 Analog Parts Kit
  • (1) 68 Ω resistor from the ADALP2000 Analog Parts Kit
  • (1) 100 Ω resistor from the ADALP2000 Analog Parts Kit
  • (1) 1 KΩ resistor from the ADALP2000 Analog Parts Kit
  • (1) 2.2 KΩ resistor from the ADALP2000 Analog Parts Kit
  • (2) 20 KΩ resistors from the ADALP2000 Analog Parts Kit
  • (1) 4.7 μF capacitor from the ADALP2000 Analog Parts Kit
  • (2) 47 μF capacitor from the ADALP2000 Analog Parts Kit
  • (1) 220 μF capacitor from the ADALP2000 Analog Parts Kit

Procedure

  1. Construct the following circuit on the solderless breadboard; note that the +5 V power is to be supplied by the M1K
  2. Refer to the illustration below for one way to install the components in the solderless breadboard; note that the top rows are reserved for 5.0 V and GND connections
  3. Refer to the included section below for details regarding the microphone pin assignments

    The terminals:

    The terminal with a solder mark (connecting to the case) is negative and the terminal without a mark is positive as shown for a typical microphone in figure 1. Case styles and terminal markings may vary from manufacturer to manufacturer. Double check the polarity of your microphone before connecting it to your circuit.

    Figure 1 Electret Microphone terminals

    19 May 2015 16:59 · dmercer
  4. Refer to the illustration below that shows how to connect the components
  5. See the illustration below that shows how to connect an extension cable to the loudspeaker
  6. Run PixelPulse on the computer and plug in the M1K using the supplied USB cable
  7. Update M1K firmware, if necessary
  8. Connect the M1K +5V power supply to the circuit as indicated in the schematic
  9. Verify that sound waves input to the microphone are reproduced by the loudspeaker
  10. Set up PixelPulse to measure voltage on Channel A
  11. Connect the emitter of the 2N3904 transistor to Channel A of the M1K using a wire from the kit and observe the waveform that corresponds to the loudspeaker output on PixelPulse; explain why the voltage is monitored at the transistor emitter rather than directly across the loudspeaker
  12. Set up PixelPulse to source voltage/measure current on Channel B
  13. Remove the microphone and RD and connect Channel B of the M1K in place of the microphone using a wire from the kit
  14. Set up the sine wave sourced from Channel B to swing between 1.0000 V and 1.0400 V using PixelPulse
  15. Observe the amplifier output on Channel A using PixelPulse and verify that the amplifier has an AC voltage gain of eleven
  16. Slowly increase the high level of the sine wave sourced by Channel B using PixelPulse while observing the amplifier output on Channel A
  17. Continue to increase the voltage of the sine wave sourced from Channel B until clipping is observed on the waveform measured on Channel A
  18. Explain the reason for the clipping
  19. Connect the microphone to two long wires and place it back in the circuit along with RD according to the schematic
  20. Move the microphone directly in front of the loudspeaker until audible feedback occurs
  21. Explain why this feedback occurs

Theory

A detailed description of the design and analysis of the audio amplifier is provided in the Audio Amplifier Experiment article handout. Please refer to the handout for the details of the amplifier theory, available at the link below

Theory

Observations and Conclusions

  • An op-amp can be used to amplify the voltage of audio signals and a transistor emitter-follower circuit can be placed following the op-amp to provide current drive to a loudspeaker
  • The emitter-follower circuit can be placed inside the op-amp negative feedback loop
  • Applying an input signal that is too large can cause clipping of the amplifier output signal
  • Positive feedback can occur in an audio amplifier with a microphone input and loudspeaker output if the microphone is placed too close to the loudspeaker
university/courses/engineering_discovery/lab_4.txt · Last modified: 08 Apr 2016 23:01 by jpearson