相关元件pdf下载：2n3906 bd140 bc109c this was one of the earliest circuits that i ever designed built, in spring 1982. at that time i had only an analogue meter a calculat to wk with. although far from perfect, this amplifier does have a wide frequency response, low disttion, is capable of driving an 8 ohm speaker to output levels of around 5 watts with slightly higher disttion. any power supply in the range 12 to 18 volts dc may be used. circuit deionthe amplifier operates in class ab mode; the single 470r preset resist controls the quiescent current flowing through the bd139/140 complimentary output transists. adjustment here, is a trade-off between low disttion low quiescent current. typically, under quiescent conditions, stby current may be 15 ma rising to 150 ma with a 50 mv input signal. a simulated frequency response is shown below:
the circuit is dc biased so that the emitters of the bd139 bd140 are at approximately half supply voltage, to allow f a maximum output voltage swing. all four transists are direct coupled which ensures:-(i) a good low frequency response(ii) temperature bias change stability.the bc109c 2n3906 operate in common emitter. this alone will provide a very high open loop gain. the output bd139/140 pair operate in emitter follower, meeting the requirements to drive low impedance speakers. overall gain is provided by the ratio of the 22k 1k resist. a heat sink on the bd139/140 pair is recommended but not essential, though the transists will run 'hot' to the touch. 
