Resistors can play any of numerous different roles in electrical and electronic equip- ment. Here are a few of the more common ways resistors are used.Voltage division You’ve already learned a little about how voltage dividers can be designed using resis- tors. The resistors dissipate some power in doing this job, but the resulting voltages are needed for the proper biasing of electronic transistors or vacuum tubes. This ensures that an amplifier or oscillator will do its job in the most efficient, reliable possible way.
Biasing In order to work efficiently, transistors or tubes need the right bias. This means that the control electrode—the base, gate, or grid—must have a certain voltage or current. Net- works of resistors accomplish this. Different bias levels are needed for different types of circuits. A radio transmitting amplifier would usually be biased differently than an os- cillator or a low-level receiving amplifier. Sometimes voltage division is required for bias- ing. Other times it isn’t necessary. Figure 6-1 shows a transistor whose base is biased using a pair of resistors in a voltage-dividing configuration.
Current limiting
Resistors interfere with the flow of electrons in a circuit. Sometimes this is essential to prevent damage to a component or circuit. A good example is in a receiving amplifier. A resistor can keep the transistor from using up a lot of power just getting hot. Without re- sistors to limit or control the current, the transistor might be overstressed carrying di- rect current that doesn’t contribute to the signal. An improperly designed amplifier might need to have its transistor replaced often, because a resistor wasn’t included in the design where it was needed, or because the resistor isn’t the right size. Figure 6-2 shows a current-limiting resistor connected in series with a transistor. Usually it is in the emitter circuit as shown in this diagram, but it can also be in the collector circuit.
Power dissipation
Dissipating power as heat is not always bad. Sometimes a resistor can be used as a “dummy” component, so that a circuit “sees” the resistor as if it were something more complicated. In radio, for example, a resistor can be used to take the place of an an- tenna. A transmitter can then be tested in such a way that it doesn’t interfere with sig- nals on the airwaves. The transmitter output heats the resistor, without radiating any signal. But as far as the transmitter “knows,” it’s hooked up to a real antenna (Fig. 6-3). Another case in which power dissipation is useful is at the input of a power amplifier. Sometimes the circuit driving the amplifier (supplying its input signal) has too much power for the amplifier input. A resistor, or network of resistors, can dissipate this ex- cess so that the power amplifier doesn’t get too much drive.
