英文原稿

Modest power audio amplifiers for driving small speakers or other light loads can be constructed in a number of ways. 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 and many designs are featured in manufacturers’ application notes. Older designs employed audio interstate and output transformers but the cost 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 500Ω to 80Ω transformer is more efficient than non-transformer designs when providing 100 mill watts 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 rectangular.

Here are a few easy-to-build audio amplifier circuits for a variety of hobby application:

Simple LM386 Audio Amplifier Op-Amp Audio Amplifier

Crystal Radio (and other purpose) Audio Amplifier Simple LM386 Audio Amplifier

This simple amplifier shows the LM386 in a high-gain configure (A=200). For a maximum gain of only 20, leave out the 10μF 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 10μF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum. Figure 1 shows simply LM386 audio amplifier.

Op-Amp Audio Amplifier

Figure 2 shows the circuit of op-amp audio amplifiers .The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present . If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. For example ,an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground , destabilizing feedback via the power supply is greatly reduced .

Crystal Radio (and other purpose) Audio Amplifier

Figure 3 shows the circuit of crystal radio amplifier. Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior(with the exception of the amazing one-transistor reflex ). The TL 431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphone and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68Ω resistor may be increased to several hundred ohms when using high impedance earphones to battery power.