Common base configuration. This diagram shows an NPN transistor circuit.
As its name implies, the common base circuit (above figure) has the base at signal ground. The dc bias is the same as for the common emitter circuit, but the input signal is applied at the emitter, instead of at the base. This causes fluctuations in the voltage across R1, causing variations in IB. The result of these small current fluctuations is a large change in the current through R4. Therefore, amplification occurs. The output wave is in phase with the input wave.
The signal enters through capacitor C1. Resistor R1 keeps the input signal from being shorted to ground. Bias is provided by R2 and R3. Capacitor C2 keeps the base at signal ground. Resistor R4 keeps the signal from being shorted out through the power supply. The output is taken through C3.
The common base circuit provides somewhat less gain than a common emitter circuit. However, it is more stable than the common emitter configuration in some applications, especially RF power amplifiers.