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and useful to designers. Technicians should have some knowledge of the factor because it affects so many
things. The factor is known as Q. Some say it stands for quality (or merit). The higher the Q, the better the
circuit; the lower the losses (I2R), the closer the circuit is to being perfect.
Having studied the first part of this chapter, you should not be surprised to learn that resistance (R)
has a great effect on this figure of merit or quality.
Q Is a Ratio
Q is really very simple to understand if you think back to the tuned-circuit principles just covered.
Inductance and capacitance are in all tuners. Resistance is an impurity that causes losses. Therefore,
components that provide the reactance with a minimum of resistance are "purer" (more perfect) than those
with higher resistance. The actual measure of this purity, merit, or quality must include the two basic
quantities, X and R.
The ratio
does the job for us. Let's take a look at it and see just why it measures quality.
First, if a perfect circuit has zero resistance, then our ratio should give a very high value of Q to
reflect the high quality of the circuit. Does it?
Assume any value for X and a zero value for R.
Then:
Remember, any value divided by zero equals infinity. Thus, our ratio is infinitely high for a
theoretically perfect circuit.
With components of higher resistance, the Q is reduced. Dividing by a larger number always yields a
smaller quantity. Thus, lower quality components produce a lower Q. Q, then, is a direct and accurate
measure of the quality of an LC circuit.
Q is just a ratio. It is always just a number — no units. The higher the number, the "better" the
circuit. Later as you get into more practical circuits, you may find that low Q may be desirable to provide
certain characteristics. For now, consider that higher is better.
Because capacitors have much, much less resistance in them than inductors, the Q of a circuit is very
often expressed as the Q of the coil or:
The answer you get from using this formula is very near correct for most purposes. Basically, the Q
of a capacitor is so high that it does not limit the Q of the circuit in any practical way. For that reason, the
technician may ignore it.
