This is an emitter follower or buffer amplifier circuit, where the output is simply equal to the input minus a diode drop (about 700mV). The advantage of this circuit is that the transistor can provide current and power gain; the transistor draws little current from the input. It provides low output impedance to any circuit using the output of the follower, meaning that the output will not drop under load.

The capacitor and the 800 ohm resistors bias the transistor's base at 2.5 V, so that the average value of the input is moved up to that level. The base-emitter junction acts like a diode, so that the emitter will be a diode drop lower than the base. The collector current will be 100 times the base current, so that any current drawn by the load will be provided by the power supply through the collector, not by the input through the base.

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