A transistor can be in different states, in which the behavior corresponds to that of a switch, or to that of a current source. In a sense, also a diode behaves like a switch, but there is an important difference: the state of the transistor can be externally controlled. The state of a diode depends completely on the surrounding components in the circuit. In a rectifier e.g. it is the polarity of the alternating voltage source that determines whether or not a diode is conducting. There is no external control. A transistor on the other hand allows an external circuit to control its state.
Note that an ideal switch never dissipates any power. Either the switch is closed, and then the voltage is zero, or the switch is open, and then there is no current. In practice there will be a small power loss in a stationary state (due to leakage currents or non-zero resistance of the used materials), but it is during the switching process that a lot of energy is wasted (see further).
Figure 3.1 A transistor is often used as a controllable switch. In either case – open or closed – there is no power loss under ideal conditions.
A typical field of application is the switching of relatively high powers. A load is to be switched on or off, and during the on-state a high current is demanded. The switching is controlled by an intelligent circuit, e.g. an integrated circuit (IC), that is not capable of delivering a high current. The latter comes from an external energy source (without intelligence). The switch S is closed when the IC voltage is high (Vi = high), and is open when this voltage is low (Vi = 0 V).
Figure 3.2 Smart devices can often not deliver large currents. Therefore, the smart device controls the state of a switch, which closes the loop containing the load and an external power supply.
Another typical field of application is digital electronics, where transistors are also used as switches, e.g. to make digital gates. The most elementary gate is the inverter. If the input voltage is low (Vi = 0 V), the switch S is open, and the pull-up resistor R pulls the output voltage up (Vo = 5 V). If the input voltage is high (Vi = 5 V), the switch S is closed, and the output voltage will be low (Vo = 0 V). A low Vi results in a high Vo and vice versa: exactly what you can expect from an inverter.
Figure 3.3 Inverter with controllable switch and pull-up resistor. For a low input voltage, the switch is open, and the output voltage is high. For a high input voltage, the switch is closed, and the output voltage is low.
In the preceding examples the transistor was used as a switch. When it operates as a current source, it is said to operate in the active area. In that case there is a non-zero voltage across the transistor and a non-zero current through the transistor. The transistor is using power, and will convert this energy into heat. A typical field of application are amplifiers, that e.g. convert a very small voltage to a sufficiently high one. This can be done with operational amplifiers (see next chapter), but these circuits do not always succeed in delivering high powers, like for loudspeakers.