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Practical Transformerless Complementary-Symmetry Audio Output Amplifiers

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A practical complementary-symmetry amplifier is described in which both positive and negative feedback are used; bootstrap positive feedback to increase output power and negative feedback to reduce distortion. The basic amplifier circuit can be used to obtain power outputs varying from tens of milliwatts to tens of watts at good efficiency levels. Because of the circuit simplicity and wide power output capabilities, this amplifier is practical for use in portable radio receivers, portable megaphones, automobile receivers and various other portable audio equipments. The amplifier consists of a Class A driver stage and a direct-coupled complementary-symmetry output stage. As no transformers are used the amplifier can be made relatively compact and light in weight. Design parameters and data on performance of a 100-milliwatt, a 3-watt and a 12-watt amplifier are presented and discussed.

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