A signal can be amplified by using a device which allows a small current or voltage to control the flow of a much larger current from a dc power source. Vacuum tubes and transistors are the basic devices capable of providing control of this kind.
There are many varieties of transistors. Two general types are the bipolar junction and field-effect transistor. With bipolar transistors, a small input current controls a large current flow through the device. With field-effect transistors, an small input voltage controls a large current flow through the device.