Common Base
A common-base amplifier is the bipolar-transistor stage in which the base is held at AC ground and acts as the terminal common to input and output: the signal is applied to the emitter and the amplified result is taken from the collector. Of the three basic connections it is the least intuitive and the least used in everyday audio circuits, but it has a distinctive combination of properties — a current gain just under 1, a very low input impedance, a very high output impedance and excellent high-frequency behaviour — that makes it indispensable in radio-frequency work and as the upper half of the cascode pair.
The common-base stage amplifies voltage, not current, and its gain comes from the impedance mismatch between its two ports. The current gain is alpha, the ratio of collector current to emitter current, a number of 0.98 to 0.998. The emitter, however, looks into the small internal resistance re — about 26 millivolts divided by the emitter current, so roughly 26 ohms at 1 milliampere — while the collector behaves like a high-impedance current source feeding the collector load. A signal current driven into that low-resistance emitter appears in nearly the same size at the collector and develops a large voltage across a load of several kilohms. The voltage gain is about RC / re, comparable in size to a common-emitter stage but, unlike it, non-inverting: a rise at the emitter produces a rise at the collector, so input and output are in phase.
Wide bandwidth is the configuration's real strength, and it follows directly from where the capacitance sits. In a common-emitter stage the capacitance between collector and base is effectively multiplied by the stage gain — the Miller effect, named for John Milton Miller's 1919 analysis of triode capacitance — which pulls down the bandwidth. In the common-base stage the base is grounded, so that same collector-base capacitance is connected from output to ground and is not multiplied, and the input node, with its tiny impedance, forms a very high-frequency pole with the small input capacitance. The stage can therefore operate close to the transistor's transition frequency. The grounded-base connection therefore remains common in the front ends of VHF and UHF receivers and in microwave amplifier stages.
The cascode circuit combines a common-emitter stage with a common-base stage and keeps the good properties of each. The lower transistor, common-emitter, supplies transconductance and input impedance; the upper transistor, common-base, receives the collector current of the lower one at its low-impedance emitter, holds the lower collector voltage nearly constant, and delivers the signal to the load. Because the lower collector hardly moves, the Miller effect is removed and the output impedance is raised greatly. The idea originated in vacuum-tube circuits of the late 1930s, and its transistor and MOSFET versions appear today in RF amplifiers, high-speed operational amplifier front ends, current mirrors with raised output resistance, and analogue integrated circuits. The folded and telescopic cascode topologies used in CMOS analogue design are direct descendants.
The practical limitations explain why the common-base stage is a specialist. With an input resistance of a few tens of ohms it needs a low-impedance driver, such as a current source or a transformer or matching network; connected to a typical high-impedance voltage source it would waste almost all the signal. Its current gain below 1 means it cannot drive a heavy load without a following stage. A common pitfall is misreading the symbol: the input still goes to the emitter, and tying the base directly to ground gives no DC bias, so real circuits hold the base at a fixed voltage through a divider and a bypass capacitor. Another confusion arises in the datasheet itself, where the parameter hFB (the common-base current gain) is rarely quoted, since most data sheets give the common-emitter hFE instead.
Read the common-base stage as a current conveyor: it passes the emitter's current to the collector almost unchanged, trades current gain for a very low-impedance input and a fast, uncapacitive output, and thereby earns its place at the highest frequencies and at the top of every cascode.