Common base

In this circuit the emitter terminal of the transistor serves as the input, the collector as the output, and the base is connected to ground, or "common", hence its name.

This arrangement is not very common in low-frequency discrete circuits, where it is usually employed for amplifiers that require an unusually low input impedance, for example to act as a preamplifier for moving-coil microphones.

However, it is popular in integrated circuits and in high-frequency amplifiers, for example for VHF and UHF, because its input capacitance does not suffer from the Miller effect, which degrades the bandwidth of the common-emitter configuration, and because of the relatively high isolation between the input and output.

The input signal is represented by a Thévenin voltage source vs with a series resistance Rs and the load is a resistor RL.

This circuit can be used to derive the following characteristics of the common base amplifier.

For a given supply voltage, the larger this drop, the smaller the transistor VCB and the less output swing is allowed before saturation of the transistor occurs, with resultant distortion of the output signal.

The output resistance is large, at least RC || rO, the value which arises with low source impedance (RS ≪ rE).

An alternative to the use of the hybrid-pi model for these calculations is a general technique based upon two-port networks.

Figure 3 shows the common base amplifier used as a current follower.

Figure 1: Basic NPN common base circuit (neglecting biasing details)
Figure 2: Small-signal model for calculating various parameters; Thévenin voltage source as signal
Figure 3: Common base circuit with Norton driver; R C is omitted because an active load is assumed with infinite small-signal output resistance