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phase inversion


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A common-emitter transistor inverts a signal because of the action of Ohm's Law at its collector resistor:
1) In an NPN transistor, its base is a P type and emitter is an N type of silicon so a positive-going signal at the base increases its base current.
2) An increase of base current in a transistor causes an increase in its collector current.
3) The collector resistor is connected from its collector to the positive supply, so when its collector current increases then Ohm's Law shows that the voltage across the collector resistor increases with its increasing current and therefore the collector voltage is reduced.
Since the collector voltage is reduced, its signal is heading in a negative direction and is inverted from the signal at the base.

The opposite occurs when the signal at the transistor's base is negative-going which decreases its base and collector currents.

Good articles about how transistors work are in our Articles section which is reached by the button at the top of this page. More transistor articles are found with a search on Google.

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