Common-base circuit

As its name implies, the common-base circuit, shown in general form by Fig. 1, has the base at signal ground.
The dc bias on the transistor is the same for this circuit as for the common-emitter circuit. The difference is that the input signal is applied at the emitter, instead of at the base. This causes fluctuations in the voltage across R1, causing variations in I_B. The result of these small current fluctuations is a large change in the dc current through R4. Therefore amplification occurs.

Fig. 1: Common-base circuit configuration.

Instead of varying I_B by injecting the signal at the base, it’s being done by injecting the signal at the emitter. Therefore, in the common-base arrangement, the output signal is in phase with the input, rather than out of phase.
The signal enters through 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 through C3.
The common-base circuit provides somewhat less gain than a common-emitter circuit. But it is more stable than the common-emitter configuration in some applications, especially in radio-frequency power amplifiers.