The brief: design a tiny AF amplifier powered by a single 1.5 V cell, yet capable of driving an 8 ? loudspeaker. How can these requirements best be met?A high power output cannot be expected of an amplifier running on a 1.5 V power supply. For example, the quiescent output voltage of the amplifier is ?·VCC = 0.75 V, and so, unless an output bridge is used, the maximum possible output swing Us is only ±0.75 V. This is reduced to ±0.55 V by the saturation voltage of the output transistors. If the battery is no longer fresh, or is under load, this might be further reduced to say Us = ±0.4 or 0.5 V. The RMS output power into an 8 ? load is then P= Us 2 / 2·R ˜ 15 mW. This is not especially loud, but, as long as a high-efficiency loudspeaker is used, entirely adequate for normal indoor listening.
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