of a
Electret Condenser Microphone
|
Abstract: Electret Condenser Microphones are versatile microphones found in many modern consumer electronics. Their compact size and low cost make them extremely useable. These Microphones do however have one drawback, their output is extremely weak. ECM’s must be amplified to produce a useable signal. Various amplification circuits were examined and it was determined that a transistor amplifier produced the best results. A number of these common emitter voltage amplifier designs were constructed and tested. Introduction: A condenser microphone contains a diaphragm and a plate to which a polarizing voltage is applied. Sound waves hitting the diaphragm cause the capacitance between it and the back plate to change. This in turn induces a voltage on the back plate. The ECM’s used in this lab function much the same as condenser microphones except they have a permanent charge implanted in an electret material. This means they have extremely light diaphragms. The advantage to this design is that ECM’s are essentially immune to vibrations. 
  http://www.kingstate.com.tw/9-5.htm Method: The ECM requires a powering circuit in order to function. These microphones operate in a voltage range of approximately 3 – 12 volts. When powered directly off of 12 volts, the output of the microphone floated above ground with a slight DC offset. This was deemed unacceptable as it could possibly cause damage to the amplification circuit. A 2200 Ohm resistor was added to Vcc to limit current, a 10uF capacitor was added to eliminate the DC offset and a 10000 Ohm resistor was added to pull the output to ground. The following diagram depicts the powering circuit.  http://www.hut.fi/Misc/Electronics/circuits/microphone_powering.html The circuit, as constructed above, was connected to an oscilloscope and the output was examined visually under different stimulus. It was found that the output signal strength of an ECM is extremely low. An initial attempt to amplify this signal using operational amplifiers ended in failure. This experimenter found that op-amp circuits were inconvenient, requiring both positive and negative supply voltages. Operational amplifiers also seemed to attenuate higher frequencies and were extremely susceptible to noise. It was decided to use an amplification circuit that could be powered from the same 12 volt supply. The following diagram depicts a common emitter transistor amplifier.  http://ecircuitcenter.com/Circuits/trce/trce.htm
The gain of the amplifier can be approximated by taking the collector resistance over the emitter resistance. RC = 1800Ohms RE1 + RE2 = 600Ohms therefore the expected gain is 1800/600 = 3. Results: Two gain 3 transistor amplifier circuits arranged in series produced a useable signal. Maximum values of 3 volts were obtained from direct contact with the microphone and more conservative voltages of approximately 1 - 2 volts were obtained from close proximity tuning forks. The output from each stage of the amplifier was indistinguishable using available laboratory equipment from the un-amplified signal other then the 3 fold increase in strength. The system appeared to be noise free over the range at which it was tested. Conclusion: The high component count involved in the construction of transistor amplifiers does at first appear to be difficult to implement. It was, however, found that these amplifiers produce a cleaner signal and have a broader range of operating frequencies. Operational amplifiers, although convenient, don’t seem to perform as well in this case. |