Low Noise, Single Supply, Electret Microphone Amplifier

Low Noise, Single Supply, Electret Microphone Amplifier
Low Noise, Single Supply, Electret Microphone Amplifier
Design for Distant Acoustic Signals
Donald J. VanderLaan
November 26, 2008
Abstract. 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 microphone
1 Introduction
Amplifier 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 Objectives
The 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 Issues
3.1
Single supply driven
This 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 compatibility
The 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 characteristics
This 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 characteristics
The 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 Results
4.1
Electret microphone biasing network
Operation 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 Ω. The electret
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 be
reasonably 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 immunity
Twisted 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 amplifier was then tested with signals of three different frequencies. The test setup
used a function generator to generate the signals, which drove an 8 Ω speaker. The acoustic
signal was then picked up by the electret microphone and amplified by the amplifier. The
amplifier’s output signals are displayed on the scope, and a spectrum analyzer was used to
verify the spectral content. Figures 4 and 5 depict the amplifier’s output when the speaker
was driven with a 500 Hz signal, figures 6 and 7 concern a 2000 Hz signal, and figures 8
and 9 were taken when the speaker was driven at 3500 Hz. The spectrum analyzer plots are
far more useful, and indicate the amplifier’s output does contain an amplified version of the
tone from the speaker. When analyzing the spectrum analyzer plots below, it is important
to note that the large amount of spectral content present in the plots was not generated from
the speaker during the tests. The amplifier was used to measure the ambient noise of the
testing environment, with the result shown below in figure 10. The noise floor was also
taken by disconnecting the microphone, just to prove the spectral content is in fact acoustic
noise. The noise floor is depicted in figure 11.
As a side note, the tones on the spectrum analyzer plots are all greater then 15 dB above
surrounding spectral content. This provides support that matched should work on these
tones, and by superposition, any signal that could be broken down into these tones.
Although the source was not swept continuously, it is not a stretch to expect similar
behavior throughout the entire audio band.
Figure 4 – Amplifier picking up 500 Hz tone from speaker.
Figure 5 – Amplifier picking up 500 Hz tone from speaker.
Figure 6 – Amplifier picking up 2000 Hz tone from speaker.
Figure 7 – Amplifier picking up 2000 Hz tone from speaker.
Figure 8 – Amplifier picking up 3500 Hz tone from speaker.
Figure 9 – Amplifier picking up 3500 Hz tone from speaker.
Figure 10 – Ambient acoustic noise in the testing environment.
Figure 11 – Electrical noise floor of the spectrum analyzer.
To test the amplifier’s ability to reject common-mode noise, a 2V peak to peak signal
was added to both of the microphone’s lines. Figure 12 shows the common-mode noise and
the resulting output. The output is practically flat, despite the incredibly large noise signal.
Figure 12 – Common-mode noise rejection capabilities. The top waveform is the
common mode signal present on the microphone’s lines. The lower waveform is the
output.
4.3
Direct microcontroller compatibility
A final stage needs to be added, one that will produce an output that can be directly
connected to the analog-to-digital converter of a microcontroller. The design set forth here
will not assume a particular microcontroller, and will allow the user to tweak both the bias
point and voltage swing using potentiometers. Thus a single design can be mass-produced,
yet will find application in a wide variety of areas thanks to the ability to set the output
characteristics.
The final stage will re-set the bias point, depending on the setting of a potentiometer.
Further, the AC swing will be separately adjustable through the use of another
potentiometer. To insure a very stable bias point, the potentiometer will act as a voltage
divider to the 8.5V zener line that is already employed by the initial bias stage. This will
prevent fluctuations on the power supply from altering the output’s DC offset. By rotating
the potentiometers, one can adjust both the DC offset and the AC gain. Figure 14 depicts
the final stage of the amplifier.
4.4
Final testing
The DC offset and AC gain of the final stage was set to the requirements of the
Microchip’s most common microcontroller, the PIC.
The PIC’s analog-to-digital
converters accept an input signal in the range of 0 – 5V. Thus the DC offset was adjusted to
2.5V by turning the 100KΩ potentiometer. The AC swing increased to reduce digitization
errors by adjusting the 500KΩ potentiometer.
The resulting AC gain, from electret
microphone output to final stage output was measured as 325. Note that this number is not
obtainable from the waveforms displayed in figure 13 below due to significant noise pickup
of the oscilloscopes probes. This noise is not actually present on the microphone lines, or
else it would have been amplified and present on the output as well.
Figure 13 – Complete system gain. Note the visible noise on channel 1 is artificial – it is
due to the oscilloscope probe’s picking up noise and is not actually present in the circuit.
5 Component selection
The schematics often have components drawn in red. These are optional and can be
used to limit the bandwidth of the amplifier – perhaps to reduce high frequency noise or
block low frequencies that could slowly drift the signal above the microcontroller’s limit.
The capacitors can be selected by setting the time constant to match the critical frequency
desired. The time constant is determined by the capacitance and the resistance seen looking
in through the capacitor’s terminals. For the design here, this can be done by inspection.
For an Op-Amp, the LF 411 is suggested due to low noise characteristics and its ability
to operate either single or dual supplied. A second supply could be added to the design
presented here to allow for negative DC offsets – but this case finds far fewer applications.
6 Conclusion
This application note presents an amplifier for use with electret microphones. The
overall gain is adjustable over the range of 0 through 500. A gain of 325 was determined to
be ideal for our application. Another feature is the adjustable DC offset, which can be set
anywhere from 0 – 8.5V. The amplifier is quite versatile, and could be adapted to other
microphones, such as a condenser microphone; simply by bypassing the first stage (a series
capacitor would be necessary). The amplifier was designed completely by modifying the
simple differential amplifier presented in all undergraduate electrical engineering courses to
be single supplied, immune to supply oscillations, and offer an adjustable AC gain and DC
offset allowing the user to tailor it to a specific application.
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