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MSU ECE 480 - Low Noise, Single Supply, Electret Microphone Amplifier Design for Distant Acoustic Signals

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Low Noise, Single Supply, Electret Microphone Amplifier Design for Distant Acoustic SignalsDonald J. VanderLaanNovember 26, 2008Abstract. Modern day electronics are often battery powered, forcing the design to be single supplied. Electret microphones are small and affordable, yet require additional circuitry. The amplifier described herein is low noise, relatively immune to supply oscillations, and can operate single supply with an electret microphone.Keywords: Microphone amplifier, low noise amplifier, single supply design, biasing, electret microphone1 IntroductionAmplifier design greatly varies depending on the type of signal to be amplified, what is available to power the amplifier, and what frequency response is desired. For high frequency applications, the amplifier will almost certainly use transistors instead of operational amplifiers (Op-Amps). In addition, high-frequency amplifiers should be impedance matched on their input and output. However in the audio band, impedance matching is not a concern. Due to the low frequency range the audio band covers, either Op-Amps or transistors can be used, although the implementation here was Op-Amp based. The frequency range of audio band amplifiers often requires large value electrolytic capacitors, which can introduce distortion due to their inferior quality.Modern day electronics run off batteries, making single supply designs far superior to those requiring both a positive and negative supply. Though dual supply designs are simpler, the lack of the negative supply requirement will usually make the more complicated single supply amplifier preferable.2 ObjectivesThe objective is to design and build a circuit to amplify the signal from an electret microphone. The amplifier should be robust against noise, powered by a single supply, and produce an output compatible with the data acquisition system of a PIC microcontroller.3 Issues3.1 Single supply drivenThis is one of the central features of this design. All stages of the amplifier must operate without a negative supply voltage. This objective will likely put a constraint on the model of Op-Amp used.3.2 Electret microphone compatibilityThe input signal will be generated from an electret microphone. The electret microphone is different from the typical dynamic microphones used in that it includes a transistor (usually JFET) pre-amp built into the package. The transistor needs to be biased, so the electret microphone must have a DC voltage across it – even without any acoustic input. This DC voltage must be provided by the external circuit.3.3 Versatile output characteristicsThis design will allow the user to condition the output signal to arbitrary requirements. The user will be able to adjust the final DC offset and voltage swing by adjusting two potentiometers. The versatility of this will allow it to be used with an arbitrary microcontroller. The output DC offset should be half the maximum voltage accepted and the voltage swing should be sufficiently large such as to minimize digitization errors.3.4 Low noise characteristicsThe electret microphone may be mounted a distance from the actual amplifier. The longer the microphone leads the more noise that will likely be picked up. One technique would be to use a third wire to ground the shield, as the common XLR connector uses.[1] This application note assumes the cheaper twisted pair wiring configuration is used, and noise mitigation is a central issue.4 Design and Results4.1 Electret microphone biasing networkOperation of an electret microphone requires a DC voltage offset across the microphone’s connectors. This bias voltage is needed to power the simple transistor amplifier that is built-in to the electret microphone housing. Electret microphones vary, but the component used in this design had an output impedance of 1200 . The electretΩ microphone’s gain is directly related to the bias voltage. Therefore, any noise on the positive supply used to provide the DC offset will present on the output of the bias network. Further, because the electret’s AC voltage will be very small, a very large gain amplifier is necessary. Any noise on the power line will make it into the amplifier through the bias network and be amplified one hundred fold. To resolve this issue, a zener diode is used to first drop the voltage from the supply to another DC level. Figure 1 depicts the use of a zener to hold the bias steady. The circuit should suppress oscillations on the power line almost up through an amplitude of 3.5V. Immunity to supply oscillations was tested by adding a 2.3 KHz AC voltage source in series with the 12VDC battery. In-band noise was specifically used, because high frequency oscillations would be blocked by the amplifier anyway – it is specifically designed to pass the audio band. Figure 2 shows the results. The top waveform is the supply voltage, which has oscillations far above anything that can bereasonably expected in the real world. The lower waveform is the microphone amplifier output. Clearly the 2.3 KHz oscillations on the power line do not make it through the biasing network.Figure 1 – Biasing network for the electret microphone.Figure 2 – Testing supply line noise immunity.4.2 Common-mode noise immunityTwisted pair wiring will almost certainly be used to wire the electret microphone to the amplifier. It would not make sense to buy the relatively cheap electret microphone, but then turn around and use high quality shielded cabling to connect it – the money would be better invested elsewhere. The further the microphone is mounted from the amplifier the more likely the leads are to pick up noise. Fortunately, if the two leads are kept close together, the noise picked up should primarily be common-mode noise. As the microphone will convert an acoustic signal into a differential electrical signal, it would be wise to use a differential amplifier to remove the common-mode noise. The differential amplifier is a good choice because it amplifies differential signals and blocks common-mode signals. Figure 3 depicts the differential amplifier. For a single sided design, the differential amplifier must pass the bias point; else half the signal will be clipped off. This was insured by adding the capacitor C5. The


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MSU ECE 480 - Low Noise, Single Supply, Electret Microphone Amplifier Design for Distant Acoustic Signals

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