Common Emitter
A common-emitter amplifier is the bipolar-transistor stage in which the emitter terminal is common to the input and the output signal: the signal enters at the base, leaves at the collector, and the emitter sits at (or near) AC ground. It is by far the most widely used single-transistor configuration because it is the only one that provides both substantial voltage gain and substantial current gain at once, giving the largest power gain of the three basic connections. When a schematic shows a transistor amplifying a small signal with a collector resistor, the odds are strongly that it is a common-emitter stage.
The stage works by turning a small base-voltage change into a large collector-current change and then into a voltage across the collector resistor. A rise in base voltage raises the collector current, which increases the drop across the collector resistor RC, which pulls the collector voltage down — so the output is an inverted copy of the input, a 180-degree phase shift. The voltage gain is approximately −RC / (re + RE), where re is the transistor's small internal emitter resistance, about 26 millivolts divided by the emitter current in milliamperes. With the bypassed example above (RC = 4.7 kΩ, re about 23 Ω) the gain is around −200; remove the bypass capacitor and the 1 kΩ emitter resistor takes over, cutting gain to about −4.6 but making it far more predictable and linear. That trade — gain for stability and low distortion, called emitter degeneration — is one of the central tricks of amplifier design.
Qualitatively, the impedances are medium in and fairly high out, which is the reason the stage is called a general-purpose workhorse. The input resistance looking into the base is roughly β × (re + RE), usually a few kilohms to tens of kilohms once the bias resistors are included in parallel, so the stage loads a signal source moderately. The output resistance is roughly RC (the transistor's collector acts like a high-impedance current source in parallel with it), so the stage is not a good direct driver of low-impedance loads such as speakers — an emitter follower is generally added for that. Because the base-collector capacitance is amplified by the stage gain (the Miller effect), the bandwidth of a high-gain common-emitter stage is considerably narrower than that of the common-base connection.
The bias network is half of the circuit, and the amplifier fails without it. The resistor divider R1 and R2 sets the base voltage; the emitter resistor RE turns an unpredictable beta into a predictable current, because the emitter current is simply (VB − 0.6 V) / RE. Coupling capacitors at input and output block the DC levels while passing the signal, and the bypass capacitor across RE restores AC gain. The quiescent collector voltage is deliberately set near the middle of the available supply swing so that the output can move equally up and down before clipping. The most common mistake is forgetting that a bypass or coupling capacitor sets a low-frequency cutoff, so an amplifier that sounds fine at 1 kHz may lose its bass.
Modern relevance is broad, although the discrete stage is now often hidden inside integrated circuits. The common-emitter connection is the gain stage in operational amplifiers' second stage, in audio preamplifiers, in the driver stages of RF transmitters, and in the first stage of countless sensor interfaces. In CMOS the exact analogue is the common-source stage. A frequent misconception is that the configuration name implies the emitter is connected to ground; it only needs to be at AC ground, and with an unbypassed emitter resistor the emitter is not at signal ground at all, but the stage is still a common-emitter stage because input and output share the emitter node as the reference.
Read the common-emitter stage as the transistor's standard way of doing its job: a current-steering valve that inverts and magnifies voltage, whose gain comes from the device but whose stability comes from the bias network and degeneration wrapped around it.