Common Emitter Amplifier - NPN

The common emitter amplifier has many uses as a general purpose amplifier. This emitter typically offers high current gain, medium input resistance and a high output resistance. Due to the internal capacitance of the transistor it lacks the ability to operate at high frequencies. The output of a common emitter amplifier is 180 degrees out of phase to the input signal.

There are four regions of a bipolar-junction-transistor (BJT).

• Saturation: The transistor acts like a short circuit. Current freely flows from collector to emitter.
• Active (Forward-active): The current from collector to emitter is proportional to the current flowing into the base.
• Cut-off: The transistor acts like an open circuit. No current flows from collector to emitter.
• Reverse Active: Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for).