Uses of transistors in circuits
In amplifier circuits in radios, record players and similar devices, the ‘old-fashioned’ thermionic diode and triode have been superseded by semiconductor diodes and transistors. These components are much smaller and lighter. They operate from low
voltages of only a few volts, compared with tens or hundreds of volts for thermionic devices. For this reason, among others, semiconductor circuits are safer than their predecessors. They are also more economical. A thermionic device, as its name suggests, operates using heat energy to emit electrons from the cathode. Semiconductors operate cold, and use much less energy than thermionic circuits.
The transistor has two basic uses. Both can be explained in terms of the voltage graph in Fig. 45.15 (b). Because the graph line is very steep, the transistor is an amplifier. A small change in the input voltage produces a large change in the output voltage. A small a.c. input signal gives a similar, larger, inverted output signal, if the rest of the circuit is properly designed. This is a use of transistor in an analogue circuit, where gradual voltage changes occur. The fact that the voltage change on the graph is so sudden makes the transistor useful as a switch. There are two basic states: low input/high output (off) and high input/low output (on). This is a digital switch, like a light switch (on/off), compared with a dimmer or variable resistor (an analogue control). This switch can be operated by sensors as shown in Figs. 45.17 and 45.18. It can also be used as the basis for many logic circuits of the kind used in calculators or computers. In both analogue and digital applications, several (perhaps very many) transistors are often fitted into one microchip package.