Audio Amplifiers

The first choice is usually an integrated circuit designed for the purpose. A typical assortment can be seen on this National Semiconductor page. Discrete designs can also be built with readily available transistors or op-amps andmany designs are featured in manufacturers` application notes. Older designs employed audio interstage and output transformers but the cost
Audio Amplifiers - schematic

and size of these parts has made them all but disappear. (Actually, when the power source is a 9 volt battery, a push-pull output stage using a 500 ohm to 8 ohm transformer is more efficient than non-transformer designs when providing 100 milliwatts of audio. ) As a general rule, transformerless low power speaker projects will work better with 4. 5 or 6 volt battery packs of AA, C, or even D cells than 9 volt rectangulars. This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum. The Curiously Low Noise Amplifier takes advantage of the wonderful noise characteristics of the 2SK117 JFET that boasts a noise voltage below 1 nV/root-Hz and virtually no noise current. The noise voltage of the amplifier is only 1. 4 nV/root-Hz at 1 kHz, increasing to only 2. 7 nV/root-Hz at 10 Hz. The noise current is difficult to measure, so this simple utility amplifier can see the noise from a 50 ohm resistor and a 100k resistor, too. (The 1. 4 nV input-referred noise will increase to about 1. 7 nV with a 50 ohm resistor, instead of a short, and a 100k resistor will give an input-referred noise near 40 nV, with very little contribution...

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