Editing Common base (section)

=== Current follower ===
[[Image:Common base with Norton driver.svg|thumbnail|300px|Figure 3: Common base circuit with Norton driver; ''R''<sub>C</sub> is omitted because an [[active load]] is assumed with infinite small-signal output resistance]]

Figure 3 shows the common base amplifier used as a current follower. The circuit signal is provided by an AC [[Norton's theorem|Norton source]] (current ''I''<sub>S</sub>,  Norton resistance ''R''<sub>S</sub>) at the input, and the circuit has a resistor load ''R''<sub>L</sub> at the output.

As mentioned earlier, this amplifier is '''bilateral''' as a consequence of the output resistance ''r''<sub>O</sub>, which connects the output to the input. In this case the output resistance is large even in the worst case (it is at least ''r''<sub>O</sub> || ''R''<sub>C</sub> and can become (''β'' + 1) ''r''<sub>O</sub> || ''R''<sub>C</sub> for large ''R''<sub>S</sub>). Large output resistance is a desirable attribute of a current source because favorable [[current division]] sends most of the current to the load. The current gain is very nearly unity as long as ''R''<sub>S</sub> ≫ ''r''<sub>E</sub>.

An alternative analysis technique is based upon [[two-port network]]s. For example, in an application like this one where current is the output, an h-equivalent two-port is selected because it uses a current amplifier in the output port.