Download Microphone Preamplifier

ECE331: Electronic Circuits
LABORATORY #1:
University of Toronto
Winter 2012
Microphone Preamplifier
INTRODUCTION
Transducers such as a microphones produce electric signals that are too small for direct reliable
processing and must first be amplified. Microphones have an output impedance of typically
200  and output signal levels around 1 to 10 mV. A preamplifier is used to boost the audio
signal level from the microphone to a level suitable for driving the A/D converter (typically to
the standard 1 V peak-to-peak). Hence, in function, the preamplifier is simply an audio
amplifier1. However, because the preamplifier is the first gain stage in the system, its noise
contribution directly adds to that of the system2. Therefore, the preamplifier should not introduce
any appreciable noise while amplifying the signal.
Using a general-purpose opamp in the inverting and noninverting amplifier configurations,
shown in Fig. 2.5 and Fig. 2.12 of the text, may not be quite adequate. First, these opamps
generally introduce considerable noise into the amplified signal. Second, audio systems may
require gains as large as 60 dB at frequencies in excess of 3 kHz, which might be difficult to
achieve using some general-purpose opamps. For example, the classical 741 opamp (discussed
in Sections 9.3-9.6 of Sedra & Smith) has an open-loop dc gain of 107.7 dB and a unity-gain
frequency of about 1 MHz. Using this opamp in the inverting configuration of Fig 2.5 in Sedra &
Smith results in an amplifier with a 3-dB frequency of only 1 kHz (see equation 2.35 in Sedra &
Smith).
Instead, a good microphone preamp can be made from simple parts available in your lab kit by
using a common-emitter BJT configuration, as discussed in Chapter 6.8 of Sedra & Smith 6th
Edition. If necessary, after the signal has been amplified to around 100 mV peak-to-peak (or
more), a standard opamp in the inverting or noninverting configuration may follow the commonemitter amplifier, as shown in Fig. 1, to boost the signal level to 1 V peak-to-peak. However,
you do not need to build the opamp-based gain stage in this lab.
DESIGN CONSTRAINTS
You need to design your pre-amplifier to use a -2.5 and +2.5 V power supplies: a single 5 V
power supply is also acceptable, but you will need to generate a separate bias voltage.
PREPARATION - SPICE ASSIGNMENT
Before building the circuit, SPICE simulations should be carried out to ensure correct operation
of the circuit you designed. As you will be building the circuit you simulated, you are required
to bring printouts for all simulations, along with a schematic or netlist printout.
The following SPICE assignment for the preamplifier stage should be completed as part of your
lab preparation:
1. Perform a DC operating point analysis to:
a) verify that the transistor is properly biased, by measuring bias voltages
1
2
An audio amplifier is an amplifier specifically designed for use at frequencies less than 100kHz. Note that the
range of audible sound is typically from 50Hz to 20kHz.
The noise contributed by each gain stage in a cascade decreases as the gain preceding the stage increases,
thereby making the first gain stage in the cascade the most critical block in terms of noise.
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ECE331: Electronic Circuits
University of Toronto
Winter 2012
b) measure the bias currents in the circuit, especially for the common-emitter amplifier
2. Perform a transient analysis to:
a) determine the maximum linear output voltage signal swing at the output, and the maximum
linear input voltage swing. What determines the maximum tolerable input signal swing
before distortion occurs?
3. Perform an AC analysis (over the frequency range 1 Hz to 100 MHz), plot the magnitude and
phase response of the preamplifier, and evaluate its midband gain as well as low and high 3dB frequencies.
Vcc
Vcc
Vcc
CommonEmitter
BJT
Opamp-Based
Amplifier
Figure 1 - A possible microphone preamplifier block diagram
HINTS

Two methods of biasing a common-emitter amplifier are shown in Fig. 2. Either of the
two configurations is acceptable for implementation in this lab, but you should design the
amplifier in spice and choose the resistor values before coming to the lab.
Figure 2 - Two method of biasing a common-emitter amplifier

Note that, if you use a microphone instead of the signal source to produce the input
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ECE331: Electronic Circuits
University of Toronto
Winter 2012
signal, DC bias must be applied to the microphone in order for it to produce a signal.
(Make sure the polarity of the microphone is correct!)

The most difficult part of the common-emitter design is likely to be keeping the device in
the active region while providing sufficient gain and output signal swing. Apply the
design methodology discussed in teh lecture and in teh tutorial. If the circuit doesn't seem
to be working, check the bias voltages on the BJT.

You MUST use DC-blocking capacitors to connect signal nodes which are biased at
different voltages. Be sure to selct the capacitor values such that they allow the full
audible range of frequencies (10 Hz to 20 kHz) to be passed.

Don't forget to decouple your power supplies with bypass capacitors and wire your
board neatly to reduce noise. This is very important! This is so important, in fact, that
if you fail to do it you may hear extra tones that will prevent you from hearing anything
spoken in the microphone. In order to get the greatest benefit from your capacitors, try
using a combination of capacitors of different sizes and types. For example, you could
use a 100 F electrolytic cap in parallel with a 100 nF tantalum cap.
IN THE LAB
Build the preamplifier circuit on your breadboard. Feed the input signal to the preamplifier and
also into channel one of your oscilloscope. Feed the output signal from teh pre-amplifier into
channel two. In order to obtain full marks for this segment of the lab, you should be able to
show your TA the following:
1. When the input is a 1 kHz sinusoid of small amplitude (~10mV peak-to-peak), the output
should be a sinusoid with identical frequency and phase but greater amplitude. Set the
vertical scale so that you can see both signals at the same time. Now the screen of your
oscilloscope should look like that in Figure 3.
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ECE331: Electronic Circuits
University of Toronto
Winter 2012
Figure 3 - Input and Output of Preamplifier
SUGGESTED COMPONENTS
You will need a NPN BJT transistor for the pre-amplifier stage. You can use the 2N3904
transistor that should have came with your electronics kit, otherwise order samples from
www.onsemi.com or visit your local electronics store.
If you wish to also build the second amplifier stage based on the opamp, you will also need a
rail-to-rail operational amplifier; remember that your closed loop gain at around 4 kHz should be
at least 40 dB, so pick an amplifier with an appropriate DC gain and unity gain bandwidth.
Make sure your amplifier can also sink/supply the required amount of current. Use the resources
of online suppliers to search for a part that meets your needs. You can use the 741 opamp, but
be aware that this amplifier's output cannot swing to the negative supply rail, so you might have
problems realizing anything more than 1 V peak-to-peak swing. Remember to bring complete
data sheets for the components you are using, even suggested ones.
MARKING SCHEME (OUT OF 10)
Preparation: (4 marks)
•
Clearly labelled schematic (1 mark)
•
Simulation with DC operating points (1 mark)
•
Simulation with Transient analysis (1 mark)
•
Simulation with AC analysis (1 mark)
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ECE331: Electronic Circuits
University of Toronto
Winter 2012
In Lab Performance: (6 marks)
•
•
Circuit operation showing proper amplification (3 marks)
Lab related questions (3 mark)
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