EE1.labA: B1 Microphone pre-amplifier construction
Aim of the experiment
This experiment will introduce you to some of the essential skills necessary to construct and test an electronic circuit. You will identify the various components, read their value and place them into a printed circuit board. You will develop the necessary skills to solder them in place.
You will assemble your circuit in a logical sequence, testing each section as you go. This procedure is useful when constructing a prototype circuit, it is less likely to be used in mass production.
You may collect your kit of components from the first-year lab technician a few days before you do the experiment. Identify the components and check them against the parts list. The Appendix contains some useful information on component identification. Use the PCB layout in Figure 2 to locate the position of the components. Do not assemble them yet, as all construction and testing must be carried out during formal lab sessions. You should be ready to assemble the first group of components when you start the lab.
Although it is not necessary to understand fully how the circuit works, it will make testing easier if you have some idea. The description below is included to help you.
Figure 1. Circuit diagram.
This simple microphone pre-amplifier can be used to measure sound pressure. Sensitivity is adjusted by a variable resistor which controls the gain. The microphone can be activated by making an appropriate noise (i.e., talking or whistling) near the circuit. The LEDs are designed to indicate different voltage levels at the output of the amplifier. The microphone pre-amplifier circuit can be divided into three basic stages which are described in more detail below.
1. Microphone coupling
The resistors and capacitors surrounding the microphone in the first stage are designed to provide the microphone capsule with a DC power supply. The DC voltage is then isolated from the amplifier by capacitor C4, which at the same time admits the AC signal generated across the microphone terminals. Together these components constitute a simple kind of band-pass filter whose cut-on and cut-off frequencies are determined by the resistor and capcitor values.
2. Audio pre-amplifier
The second stage amplifies the small AC signal from the microphone up to a line level, where it could be interfaced to other audio devices. The network of resistors is designed to set the correct DC operating point for the transistors, as well as determining their AC gain. Thus, the gain of this amplification stage can be adjusted by the potentiometer PR1 whose central connection acts effectively as a ground for AC signals by the action of capacitor C1.
3. Voltage-level comparators
The third stage of the circuit uses the audio signal to determine the voltage level across a capacitor (C6). The operation of three coloured LEDs is switched by comparing this voltage to three different reference voltages.
With zero AC signal coming from the amplifier, the open-collector output of comparator U1a is off and C6 is charged up by current through large resistor R18. The voltage depends on that across D1, the potential divider R16 and R24, and the gain circuit R5 and R7 (and R19). When an AC signal turns the comparator on, it then discharges the capacitor and reduces the voltage across it.
The three further comparators controlling the LEDs (U1b, U1c and U1d) are switched on and off depending on whether the voltage across C6 dips below their corresponding reference levels. These reference voltages are determined by the voltage across the Zener diode D1 and the potential divider resistors R11, R12, R13 and R14.
The experiment is divided into three parts, corresponding to each of the three stages just described. It is necessary to complete each part before moving onto the next. You should record any tests, measurents or modifications made to your circuits during construction in your log book.
It is essential that the microphone, the semiconductor devices (chip, transistors and diodes) and the electrolytic capacitors (C1, C2, C3, C4, C5 and C8) are placed with the correct polarity.
Figure 2. Component and track layout.
Part 1. Microphone coupling
Solder the components for the first stage onto the printed circuit board that you have been given, then attach two leads to your PCB to provide the 9 Volt power supply. Set a bench power supply to 9V and low current limit. Apply this supply to your board and check the supply is not excessive (i.e., <5 mA).
Using an oscilloscope, check the microphone output at P1 (across C4 and MIC1). You should see an alternating signal that responds to any sounds you make. Record the typical voltage level for speech while talking normally at a distance of about 20 cm from the capsule. If no output is detected, switch off the power supply and check the location and polarity of all the components. Also check for short circuits due to excess solder. Continue fault finding until the microphone is working correctly.
Part 2. Audio pre-amplifier
Assemble and solder the components for the second stage of the circuit. Test the output of amplifier at P2 by observing the signal on an oscilloscope. Make sure that the shape of the signal is similar to that observed at the lower level from the output of the microphone. The alternating signal should have both positive and negative parts (viewed on AC coupling) and not appear clipped or distorted.
Record the typical voltage level for speech as before and make an estimate of the gain. Vary the value of the resistance PR1 and observe how it affects the gain. Finally, adjust the gain to be approximately half that given by the maximum setting.
Part 3. Voltage-level comparators
Assemble the components for the third stage of the circuit. Be careful not to overheat the legs of the chip and ensure the correct orientation of all the diodes. Once you have reconnected the power supply, you should observe that the LEDs respond to sounds of different intensities.
Attach an oscilloscope probe across C6 in the circuit (you may need to hook onto the leg of a connected resistor) and observe the DC voltage. When quiet, the voltage level should float at a constant value. Record this voltage. How does it compare to the voltage across D1? Also observe how the voltage across the capacitor discharges through the comparator U1a as the sound level increases.
Evaluate your circuit's performance by probing it with some tests, noting any results in your log book, together with a summary of what you have achieved and learnt from this experiment. You have now completed the construction experiment: you can try to listen to the amplifier output on some headphones and you can adjust the gain as you please!
Illustration of a complete circuit
Figure 3. Photograph of a populated circuit board.
© 2003-8, written and maintained by Philip Jackson, last updated on 16 Oct 2008.