Audio Amplifier Circuit


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In the first part of lab#1 you will construct a low-power audio amplifier/speaker driver based
on the LM386 IC from National Semiconductor. The audio amplifier will be a selfcontained, battery-operated component. In the second part of the lab you will construct a
microphone circuit using a compact electret condenser microphone cartridge. These circuit
modules are important building blocks of many audio communications systems, and will be
used in our ultrasonic transceiver system.
There are several things you can learn in this lab if you pay attention. By studying this
document and experimenting with the components and circuits in the lab, try to understand
the following:
■ Electrical characteristics of audio speakers
■ Characteristics of condenser microphones
■ Design of single-supply battery-operated op-amp circuits
■ Use of diode limiters/clamps for input protection
■ Use of bias de-coupling capacitors on ICs
■ Active filters for tone control
■ How to choose DC blocking capacitors

We will discuss the analytical aspects of active filter design in lecture. You will have an
opportunity to design an active filter yourself in a later lab.
The objective of the lab is not simply to create a working circuit, it is to learn about
circuits! So, as you progress through the lab, try to understand the role of each component,
and how the choice of component value may influence the operation of the circuit. Please
tinker with component values: that is an especially valuable way to learn. Ask yourself
questions such as: Why is this resistor here? Why does it have this resistance value? Why is
this blocking capacitor 1µF instead of 0.1µF or 10µF or 100µF? Why was this particular opamp chosen? It is only when you can answer such questions that you will truly understand
the labs and progress towards designing your own circuits. In the process you may even find
a better solution; in fact, the present laboratory experiments in this course have several slight