Common emitter configuration. This diagram shows an NPN transistor circuit.
A transistor can be hooked up in three general ways. The emitter can be grounded for signal, the base can be grounded for signal, or the collector can be grounded for signal. An often-used arrangement is the common emitter circuit. “Common” means “grounded for the signal.” The basic configuration is shown in above figure.
A terminal can be at ground potential for a signal, and yet have a significant dc voltage. In the circuit shown, capacitor C1 appears as a short circuit to the ac signal, so the emitter is at signal ground. But resistor R1 causes the emitter to have a certain positive dc voltage with respect to ground (or a negative voltage, if a PNP transistor is used). The exact dc voltage at the emitter depends on the resistance of R1, and on the bias. The bias is set by the ratio of the values of resistors R2 and R3. The bias can be anything from zero, or ground potential, to +12 V, the supply voltage. Normally it is a couple of volts.
Capacitors C2 and C3 block dc to or from the input and output circuitry (whatever that might be) while letting the ac signal pass. Resistor R4 keeps the output signal from being shorted out through the power supply. A signal enters the common emitter circuit through C2, where it causes the base current, IB, to vary. The small fluctuations in IB cause large changes in the collector current, IC. This current passes through resistor R4, causing a fluctuating dc voltage to appear across this resistor.
The ac part of this passes unhindered through capacitor C3 to the output. The circuit of above figure is the basis for many amplifiers, from audio frequencies through ultrahigh radio frequencies. The common emitter configuration produces the largest gain of any arrangement. The output wave is 180° out of phase with respect to the input wave.