What it shows
Small radios, intercoms and toy speakers use small audio power amplifiers. The LM386 gives gain of 20 or 200 with a few components. The project builds one and measures the voltage gain and how it changes with frequency, so that a student learns to read a Bode-style plot.
The science
The LM386 is a power amplifier with an internal gain of 20. Connecting a 10 µF capacitor between pins 1 and 8 raises the gain to about 200. The output capacitor blocks DC from the speaker and, along with the speaker resistance, forms a high-pass filter. Input and bypass capacitors also affect the response, so gain is lower at very low frequencies and high at speech frequencies.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An LM386N-1 amplifier IC
- An 8 Ω, 0.5 W speaker
- Capacitors 10 µF, 220 µF, 0.05 µF and 0.1 µF
- A 10 Ω resistor, a 10 kΩ volume pot
- A 9 V battery
- A signal generator app on a phone with a 3.5 mm cable and a 10 kΩ potential divider
- A multimeter with AC volts range, or a laptop sound card oscilloscope software
Safety
For this project: Keep the sound level low so that your ears are not damaged during test tones. Use only a battery. An adult must check the wiring before the first power-up.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Study the datasheet circuit for gain 20 and for gain 200 and note the pins.
Build the gain-20 circuit on a breadboard with the volume pot at the input and the 220 µF output capacitor to the speaker.
Add the 10 Ω with 0.05 µF series network from output to ground to stop high-frequency oscillation.
Feed a 1 kHz tone from the phone through a potential divider so that the input is about 20 mV peak, and listen on the speaker.
Measure the input and the output AC voltage with the meter or sound-card scope and calculate gain = Vout divided by Vin.
Repeat at 50, 100, 200, 500, 1k, 2k, 5k, 10k and 15 kHz with the input fixed at the same value.
Add the 10 µF capacitor between pins 1 and 8, and repeat at least five frequencies.
Plot gain in decibels (20 log10 of gain) against frequency on semi-log paper or in a spreadsheet.
Mark the points where the gain is 3 dB below the maximum and state the bandwidth.
Write the report on circuit, tables, graphs and the reason for each capacitor.
What you should see
The amplifier gives about 20 times gain in the basic circuit and up to roughly 150 to 200 times with the extra capacitor, flat over the speech range of 300 Hz to 3 kHz and falling at the ends. The graph shows the bandwidth.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a coupling capacitor placed in series with the speaker?
It blocks the DC level at the output so that no DC flows through the speaker coil.
2What happens if the input is too large?
The output clips at the supply limits and the sound is distorted.
3What does 20 dB mean?
A voltage gain of 10 times, because dB = 20 log10 of the ratio.
4Why does the gain fall at high frequency?
The amplifier has internal capacitances and a limited bandwidth, and the filter network at the output also cuts high frequencies.
Take it further
Add a TL072 op-amp as a microphone pre-amplifier in front of the LM386 and build a small intercom.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each