Resistors can play numerous different roles in electrical and electronic equipment.
Here are a few of the more common purposes resistors are used:
1. Voltage division
Voltage dividers can be designed using resistors. 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.
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. Networks 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 oscillator or a low-level receiving amplifier. Sometimes voltage division is required for biasing. Other times it isn’t necessary. The figure shows a transistor whose base is biased using a pair of resistors in a voltage-dividing configuration.
3. 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 a receiving amplifier. A resistor can keep the transistor from using up a lot of power just to get hot. Without resistors to limit or control the current, the transistor might be overstressed carrying a direct 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. The figure 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.
4. 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 antenna. A transmitter can then be tested in such a way that it doesn’t interfere with signals 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.
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 excess so that the power amplifier doesn’t get too much drive.
5. Bleeding off charge
In a high-voltage, direct-current (dc) power supply, capacitors are used to smooth out the fluctuations in the output. These capacitors acquire an electric charge, and they store it for a while. In some power supplies, these filter capacitors hold the full output voltage of the supply, say something like 750 V, even after the supply has been turned off, and even after it is unplugged from the wall outlet. If you attempt to repair such a power supply, you might get clobbered by this voltage. Bleeder resistors, connected across the filter capacitors, drain their stored charge so that servicing the supply is not dangerous.
It’s always a good idea to short out all filter capacitors, using a screwdriver with an insulated handle, before working on a high-voltage dc power supply.
6. Impedance matching
A more subtle, more sophisticated use for resistors is in the coupling in a chain of amplifiers, or in the input and output circuits of amplifiers. In order to produce the greatest possible amplification, the impedances must agree between the output of a given amplifier and the input of the next. The same is true between the source of the signal and the input of an amplifier. Also, this applies between the output of the last amplifier in a chain, and the load, whether that load is a speaker, a headset, a FAX machine, or whatever.