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In electronics, a common base amplifier (also known as ground base) is one of the three basic single-stage bipolar junction transistor (BJT) amplifier topologies, commonly used as a current buffer or a voltage amplifier . In this circuit, the terminal of the emitter of the transistor serves as input, the collector as output, and the base is connected to ground, or "common", hence its name. The analog field effect transistor circuit is the common gate amplifier.
![[Image: 130px-NPN_common_base.svg.png]](https://upload.wikimedia.org/wikipedia/commons/thumb/2/29/NPN_common_base.svg/130px-NPN_common_base.svg.png)
As the current sinks from the emitter, this provides a potential difference that causes the transistor to conduct. The current conducted through the collector is proportional to the voltage across the base-emitter junction, taking into account the polarization, as with other configurations. Therefore, if there is no current in the emitter, the transistor does not conduct.
This arrangement is not very common in low frequency discrete circuits, where it is generally used for amplifiers that require an unusually low input impedance, for example to act as a preamplifier for mobile coil microphones. However, it is popular in integrated circuits and 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 due to the relative high isolation between the entrance and the exit. This high insulation means that there is little feedback from the exit to the entrance, which leads to great stability.
This configuration is also useful as a current buffer, since it has a current gain of approximately one unit (see formulas below). Often, a common base is used in this way, preceded by a common emitter stage. The combination of these two forms the cascode configuration, which possesses several of the benefits of each configuration, such as high input and isolation impedance.