Commonbase

Commonbase refers to a transistor amplifier configuration in which the base terminal is the common node to both input and output, typically biased at a fixed potential and considered AC ground. The signal is applied at the emitter and taken from the collector. This arrangement can use either NPN or PNP transistors.

Key characteristics include a very low input impedance at the emitter, approximately re ≈ 26 mV / IE, typically a few ohms to a few tens of ohms. The output impedance at the collector is relatively high, on the order of RC. The current gain from input to output is close to unity, with IC ≈ αIE and α typically near 1. The voltage gain can be substantial, roughly Av ≈ α (RC / re), especially when RC is large.

Frequency response and usage: Common-base stages offer wide bandwidth and good high-frequency performance, partly because the base is at AC ground and Miller effects are minimized. They are less common for standalone, high-gain amplification but are valued for fast, wideband operation. A drawback is the very low input impedance, which requires a low-impedance signal source and careful biasing.

Applications: The configuration is commonly used in radio frequency (RF) preamplifiers and detector front-ends, where wide bandwidth and impedance matching are important. It also appears as a component of cascode amplifiers, where the common-base stage helps improve frequency response and gain stability.

Overall, commonbase is one of the standard transistor configurations, offering low input impedance, high output impedance, near-unity current gain, and wide bandwidth suitable for high-frequency applications.