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NPN voltage divider biased, swamped, common-emitter amplifier.
Blue: AC Input signal.
Orange: DC Biased input signal.
Red: Amplified inverted DC biased signal.
Green: AC Output amplified signal.
Inversion: Notice how the input and output signals are out of phase by 180°. This is because the transistor acts as a current controlled resistance (it's original name was transresistance) and the output is taken between the collector resistor and the transistor, making it effectively a voltage divider in principle.
The more current at the base the more current across the transistor thus the less resistance it has, resulting in a smaller voltage drop. The less current at the base the less current across the transistor thus the more resistance it has, resulting in a larger voltage drop.
Voltage divider: The 10K and 47K resistors form a voltage divider to boost the 1V incoming voltage signal into positive voltage only territory at the base of the transistor since transistors can only conduct positive currents (from collector to emitter).
Capacitors: The 10uF coupling capacitors are used to isolate DC from AC. If they are large enough they appear as a short to AC but as an open to DC, so the input and output signals are kept as AC while keeping the DC in the circuit inside of it.
The 100uF bypass capacitor is used to filter out AC, because it looks as an open (R=MAX) to DC, DC signals can only go through the second 470 resistor, but because it looks like a short (R=min) to AC, AC signals will go to ground through it instead of the second 470 resistor (current flows through the path of least resistance).
Both DC and AC do pass through the first 470 resistor.
Swamped emitter: The act of using the two 470 resistors with the bypass capacitor at the emitter is called swamping. This is done to reduce the instability effect of the internal dynamic emitter resistance (r'e) of the transistor on the AC gain while keeping a reasonable amount of gain. The gain can be calculated as Rc/Re, Re being the overall resistance at the emitter and Rc the overall resistance at the collector.
If there was one single 940 resistor with a bypass capacitor, the gain would be very high since r'e is usually very small and AC will ignore the 940 resistor since it's shorted to ground through the bypass capacitor. However, r'e is very dependent on beta, the current passing through the transistor, temperature and manufacture processes so the gain becomes very unstable. If the bypass capacitor is removed and only one 940 resistor is left, AC will see Re as r'e combined with the 940 resistor, this will reduce the effect of r'e on the gain making it much more stable, however the gain will be very low since Re would be much larger.
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