IAN LANG ELECTRONICS
No 7. The Audio Amplifier .
The job of the audio amplifier is to take the audio signal recovered by the detector and amplify it to a level whereby we can hear it. The audio amplifier has to be linear or else we hear nothing but an unintelligible noise. In days of yore we would take several transistors, possibly making Darlington Pairs (which you can read about here ) and finally output to the loudspeaker through an impedance
matching transformer. Now, we can do the same thing with just one integrated circuit which can provide a 0.5 W output to a speaker with a characteristic impedance of 8 ohms directly. This is the LM386 which is made by many firms including National Semiconductor who provided the schematics below:
How to Count Pins on an Integrated Circuit.
On an IC there is either a dimple or a notch or both. If there is a dimple, hold it away from you so that it is in the top left hand corner, if there is a notch , hold it facing away from you. In either case you should be looking down at the top of the IC. Now start counting from the top left hand corner downwards, and the top left hand corner is pin 1. If the IC has 8 pins, the one in the bottom left hand corner is pin 4, if 12 it is pin 6 and so on. Up the right hand side you count upwards, so the bottom right hand corner in an 8 pin IC is pin 5, on 12 pin 7 and so on. On an 8 pin then, the top right is pin 8.
As you can see the signal from the detector is capacitively coupled to the inverting input at pin 2. This reduces overloading. There is a potentiometer to act as a volume control. The non-inverting input at pin 3 is referenced to ground. Pin 4 is the ground connection, and output is achieved at pin 5 via an LR filter to reduce noise and a large capacitor in series with the speaker helps a fidelity of tone. Pin 6 is the supply voltage and has a capacitor to ground to remove stray AC interference, pin 7 is optional and is there to reduce a whistle created within the amplifier itself under certain signal conditions. Between pins 1 and 8 is a 10 microfarad capacitor; this allows a gain of 200. Without it the amplifier will have a gain of 20, and by combining resistors and capacitors we can vary the gain. We can in fact dispense with all components except those on pin 5, and even then the ferrite bead and 47 ohm resistor are not necessary, but we risk a higher signal to noise ratio at the speaker.
Most commercial receivers will now use an LM386 or something similar (meatier ones up to 30W are available) as the cost of the chip is actually lesser than that of a transformer.