A very attractive solution even today for a single low-power amplifier with a single integrated circuit is the one that makes use of LM386 from National Semiconductor. Besides the low cost, possibility to operate with voltages from 4 V to 12 V, this IC has the gain programmed by an external component, being able to be between 20 and 200.
The range of output powers goes from 200 mW to 700 mW with loads of 8 ohms and the quiescent current with 5 V of power is only 4 mA. Applications for this amplifier include warning systems, alarms, intercoms, receiver outputs, computer sound systems, ultrasonic drivers, servo control, etc.
In figure 1 we have the circuit for the application with voltage gain 20.
To obtain a voltage gain of 200 times simply connect between pins 1 and 8 an electrolytic capacitor of 10 uF with the positive armature on pin 1. The printed circuit board for this amplifier is shown in figure 2.
There is no need for a heatsink for the integrated circuit, and if a source is used, it must have excellent filtration so that snoring does not occur.
MATERIAL LIST
Semiconductors:
CI-1 - LM386 (-N, -1 or -3) - integrated circuit
Resistors: (1/8 W, 5%)
R1 - 10 ohm
P1 - 10 k ohm - volume control, potentiometer, optional
Capacitors:
C1 - 47 nF - ceramic
C2 - 220 uF x 12 V - electrolytic
Several:
FTE - 8 ohm x 10 cm - speaker
Printed circuit board, power supply, wires, solder, etc.
