Purpose of the Resistor

Resistors play diverse roles in electrical and electronic equipment. Here are a few of the more common ways they are used.

Voltage Division

You’ve learned how voltage dividers can be designed using resistors. The resistors dissipate some power in doing this job, but the resulting voltages can provide the proper biasing of electronic circuits. This ensures, for example, that an amplifier or oscillator will function in the most efficient, reliable way possible.

Bias

The term bias means, in the case of a bipolar transistor, a field-effect transistor, or a vacuum tube, that the control electrode—the base, gate, or grid—is provided with a certain voltage, or made to carry a certain current, relative to the emitter, source, or cathode. Networks of resistors can accomplish this. A radio transmitting amplifier is biased differently than an oscillator or a low-level receiving amplifier. Sometimes voltage division is required for biasing. Other times it isn’t necessary. following Figure shows a bipolar transistor whose base is biased using a pair of resistors in a voltage divider configuration.

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A pair of resistors can act as a voltage divider to bias the base of a transistor.

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 getting hot. Without resistors to limit or control the current, the transistor can be overstressed carrying direct current that doesn’t contribute to the signal. Below Figure shows a current-limiting resistor between the emitter of a bipolar transistor and electrical ground.

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A resistor can limit the current that passes through the emitter of a transistor.

Power Dissipation

The dissipation of power in the form of heat is not always a bad thing. Sometimes a resistor can be used as a dummy component, so a circuit sees the resistor as if it were something more complicated. When testing a radio transmitter, for example, a resistor can be used to take the place of an antenna. This keeps the transmitter from interfering with communications on the airwaves. The transmitter output heats the resistor without radiating any signal. But as far as the transmitter knows, it’s connected to a real antenna below figure and a perfect one, too, if the resistor has just the right ohmic value!

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At A, a radio transmitter is connected to a real antenna. At B, the same transmitter is connected to a resistive dummy antenna.

Another situation in which power dissipation is useful is at the input of a power amplifier, such as the sort used in high-fidelity audio equipment. Sometimes the circuit driving the amplifier (supplying its input signal) has too much power. A resistor, or network of resistors, can dissipate this excess so that the amplifier doesn’t get too much drive. In any type of amplifier, overdrive (an excessively strong input signal) can cause distortion, inefficiency, and other problems.

Bleeding Off Charge

In a high-voltage, 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 can be electrocuted by this voltage. Bleeder resistors, connected across the filter capacitors, drain their stored charge so that servicing the supply is not dangerous. In following figure, the bleeder resistor, R, should have a value high enough so that it doesn’t interfere with the operation of the power supply, but low enough so it will discharge the capacitor, C, in a short time after the power supply has been shut down.

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A bleeder resistor (R) is connected across the filter capacitor (C) in a power supply

Impedance Matching

A more sophisticated application 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 a source of signal and the input of an amplifier. Also, this applies between the output of an amplifier and a load, whether that load is a speaker, a headset, or whatever. Impedance is the ac “big brother” of dc resistance.