Editing Common base (section)

=== Voltage amplifier ===
[[Image:Common-base small signal.svg|thumbnail|300px|Figure 2: Small-signal model for calculating various parameters; Thévenin voltage source as signal]]

For the case when the common-base circuit is used as a voltage amplifier, the circuit is shown in Figure 2.

The output resistance is large, at least ''R''<sub>C</sub> || ''r''<sub>O</sub>, the value which arises with low source impedance (''R''<sub>S</sub> ≪ ''r''<sub>E</sub>). A large output resistance is undesirable in a voltage amplifier, as it leads to poor [[voltage division]] at the output. Nonetheless, the voltage gain is appreciable even for small loads: according to the table, with ''R''<sub>S</sub> = ''r''<sub>E</sub> the gain is ''A''<sub>v</sub> = ''g''<sub>m</sub> ''R''<sub>L</sub> / 2. For larger source impedances, the gain is determined by the resistor ratio ''R''<sub>L</sub> / ''R''<sub>S</sub>, and not by the transistor properties, which can be an advantage where insensitivity to temperature or transistor variations is important.

An alternative to the use of the hybrid-pi model for these calculations is a general technique based upon [[two-port network]]s. For example, in an application like this one where voltage is the output, a g-equivalent two-port could be selected for simplicity, as it uses a voltage amplifier in the output port.

For ''R''<sub>S</sub> values in the vicinity of ''r''<sub>E</sub> the amplifier is transitional between voltage amplifier and current buffer. For ''R''<sub>S</sub> ≫ ''r''<sub>E</sub> the driver representation as a [[Thévenin's theorem|Thévenin source]] should be replaced by representation with a [[Norton's theorem|Norton source]]. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower, as discussed next.