Purpose of the resistor

Resistors can play many different roles in electrical and electronic equipment. Here are some more common ways of how resistors are used.

Voltage  division

You have already learned a little about how voltage dividers can be designed using resistors. The resistors dissipate some power while doing this job, but the resulting voltages are required for the proper biasing of electronic transistors or vacuum tubes. This ensures that an amplifier will perform its job in the most efficient, reliable possible manner.

Current limiting

Resistors interfere with flow of electrons in the circuit. This is essential to prevent damage to the circuit. A resistor can keep the transistor from using lot of power by getting hot. Without resistors to limit or control the current, the transistor might be overstressed carrying direct current which does not contribute to the signal. An improperly designed amplifier may need to have its transistor replaced frequently, because a resistor was not included in the design where it was needed, or because the resistor isn't the right size. Figure given below shows a current limiting resistor connected in series with the transistor. The emitter circuit as shown in this figure, but it can be in the collector circuit.

Impedance matching

A more sophisticated use for resistors is in coupling in the chain of amplifiers, or in the input and output circuits of amplifiers. In order to generate the greatest possible amplification, the impedances should agree between the output of the given amplifier and the input of next. The same is true between the source of signal and the input of an amplifier. This applies between the output of last amplifier in the chain, and load, whether that load is a headset, a speaker, a FAX machine, or anything else. Impedance is the alternating-current cousin of resistance in direct-current circuits.