Common Emitter

The Common Emitter topology is shown above. This could also be called a Grounded Emitter.

Note that I have explicitly drawn two nodes for the input and output ports instead of using a ground/common symbol. This is helpful for understanding why we use the name "common emitter."

Remember that a voltage only exists between two nodes in a circuit. It is not clear if you say something like, "there is 3 volts here..." without also saying, "with respect to this other node here," or more commonly, "with respect to ground."

With that in mind...

Our input voltage is between the base and the emitter.

Our output voltage is between the collector and the emitter.

Since the emitter is common to both input and output, we call this the common emitter circuit.

I have purposely left the resistor values and battery values blank for this general overview of the common emitter circuit. This is not a practical circuit that you will see anywhere, but it gives us a starting point to discuss this topology.

I have given each terminal a resistor.

The base resistor connected to a bias voltage. This will almost surely be derived from the supply voltage in any real circuit, as practical examples will show. If you have a basic understanding of transistors, then you know that the base-emitter junction must be forward biased for the transistor to turn "on," or allow current to flow between collector and emitter. The bias voltage provides the dc conditions to ensure that our transistor is "on." The base resistor is required because we cannot connect our input or the transistor base directly to the low impedance voltage source. If directly connected, the voltage source would "win out" and our signal would be lost. Additionally, the bias voltage strike a balance where it is somewhat current limited without being so weak as to cause instability.

The emitter resistor may not exist as an external resistor in a practical circuit (i.e. the emitter may be tied directly to ground) but an internal resistance will always exist that can be modeled as a resistor. In either case, this resistance will usually be much smaller than either the base resistor or the collector resistor.

The collector resistor is our load resistor, and like all load resistors it serves a dual purpose of current limiting and generating a usable voltage for our output port. Just like the bias voltage at our input port, our supply voltage source would keep the collector node at a steady voltage if we did not separate things with a resistor.

This topology allows us to input a small signal voltage and output a larger signal voltage. Input impedance will be medium to low, but we can adjust the base and emitter resistances or buffer the input if this is an issue. Output impedance is medium to high, which, like the input, is less than ideal, but can also be overcome by buffering.

The output voltage will be 180 degrees out-of-phase with the input voltage. More input voltage causes a decrease in output voltage. Less input voltage causes an increase in output voltage. The bias voltage is typically set so that the collector node rests at 1/2 of the supply voltage, allowing for maximum signal swing.

Of the three elementary topologies, the common emitter is really the only one that functions as a useable voltage gain for audio circuits so you will become very familiar with this circuit.