22 The transistor as an amplifier
In the common-emitter arrangement, a small signal (for example from a microphone) applied to the base circuit produces a base-current change ΔIB, which causes a change in the collector current ΔIC = β ΔIB, β times larger. If a load resistor RC is placed in the collector circuit, the corresponding voltage change across RC is much larger than that applied at the input. So the transistor amplifies current, voltage and power; the extra power comes from the battery in the collector circuit. This effect is called transistor action.