A typical transistor can pass signals at millions of cycles per second. One could even use a rotary potentiometer controlled by an electric motor to make an amplifier, but the practical frequency ranges would be pretty small. One of the most widely used ones in recent history was vacuum tubes. There are alternate technologies to transistors. That's how the basic common emitter amplifier works. The bias point is determined by the resistors attached to the base, and possibly to the emitter.īy adding a resistor RC to the collector, the voltage across RC becomes. ICQ is the collector current at the bias point.
Re is the base-emitter resistance used for the linear approximation at IC=ICQ When applying small AC voltage signals to the base, around some DC bias point, we can form a linear approximation treating the base-emitter junction as a small resistor having resistance. N is an ideality factor, usually between 1 and 2 Is is the base emitter diode saturation current The base current is related to the base-emitter voltage by.īeta is the transistor current gain, typically on the order of 100 A transistor in a common emitter amplifier only outputs a voltage because of the attached resistors. The current through the collector is proportional to the current through the base.
To be clear, a BJT transistor is a current controlled current source. The input signal Vin is just a control signal that determines how power from VCC comes out. All of the power for the output signal comes from the power supply VCC. The output of a common emitter amplifier is a voltage signal that is proportional to the input voltage.