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This is a common-emitter amplifier, which amplifies the input voltage about 10 times.

The capacitor and the 110k and 10k resistors bias the transistor's base at about 1.7 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. Since the transistor stays in forward-active mode, the collector current will be 100 times the base current.

The emitter voltage fluctuates with the input voltage, and so the current across the 1k resistor fluctuates proportionately. Since the collector resistor has the same current across it but has 10 times the resistance, the collector voltage swings 10 times as much (and with phase opposite to the input).

Note that the peak value of the output is not 5 V as this analysis would predict. The actual gain is more like 9.5 times for various reasons. For example, the base-emitter drop is not constant, but varies with the base current.

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Generated Tue Feb 23 2010