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Small audio amplifier without quiescent current

This transistor amplifier provides up to about 1 watt to an 8 ohm speaker. It is therefore also good for radio receivers. Although it shows a somewhat larger circuit complexity, it requires only inexpensive standard components. So the construction costs are not higher, as with other amplifiers of this power class. It is just only a little more work to build the circuit. But in addition to the excellent sound quality it has even more advantages. The amplifier operates namely without bias through the end transistors. Thus, a thermal run-up is safely avoided, so that at this power heatsinks for the final transistors are not essential. Also eliminates any otherwise necessary adjustment of the quiescent current. The bias-less functioning is especially possible owing to the high open-loop gain. It is achieved by the differential amplifier and the following driver, which acts to a constant current source, wich is formed by a current mirror circuit. Thus, a very effective negative feedback is possible, which largely eliminates the crossover distortions of the final transistors.

The voltage gain is given by negative feedback, namely the ratio of the 10kΩ resistor to the 100Ω resistor in series with the 10μF capacitor. It is therefore 40dB at the given dimensioning. If a smaller amplification factor is required, the value of the 100Ω resistor should be increased accordingly. On the other direction, an increase in the amplification factor is limited. As the resistance value more decreases, the distortion increases rapidly and possibly resulting in a tendency to oscillate. However, any damage to the circuit is not to be feared by such experiments, even if the 100Ω resistor is replaced by a wire jumper.

The center voltage at the circuit output, so at the point where the two 2.2Ω resistors are connected together, is set by the two 33kΩ resistors to half the supply voltage. This is always, even with fluctuations in the supply voltage, given a maximum Aussteuerbarkeit. While with simpler circuits due to component tolerances the center voltage and also the gain can vary more or less so that either adjustable elements or closely tolerated components are necessary, one here do not need to worry about something like this. Similar to amplifiers with ICs, the circuit is ready for use immediately after assembly, under the assumption that everything is properly connected. Adjustable resistors for balancing and heat sink usually cost more space, as the few components additionally required here. So the circuit tends to be more space-efficient, as is the simpler circuits of this power class.

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