1-32
The use of resistance by itself in filter circuits does not provide any filtering action, because it
opposes the flow of any current regardless of its frequency. What it does, when connected in series or
parallel with an inductor or capacitor, is to decrease the "sharpness," or selectivity, of the filter. Hence, in
some particular application, resistance might be used in conjunction with inductance or capacitance to
provide filtering action over a wider band of frequencies.
Filter circuits may be divided into four general types: LOW-PASS, HIGH-PASS, BANDPASS,
AND BAND-REJECT filters.
Electronic circuits often have currents of different frequencies. The reason is that a source produces
current with the same frequency as the applied voltage. As an example, the a.c. signal input to an audio
amplifier can have high- and low-audio frequencies; the input to an rf amplifier can have a wide range of
radio frequencies.
In such applications where the current has different frequency components, it is usually necessary for
the filter either to accept or reject one frequency or a group of frequencies. The electronic filter that can
pass on the higher-frequency components to a load or to the next circuit is known as a HIGH-PASS filter.
A LOW-PASS filter can be used to pass on lower-frequency components.
Before discussing filters further, we will review and apply some basic principles of the frequency-
response characteristics of the capacitor and the inductor. Recall the basic formula for capacitive
reactance and inductive reactance:
Assume any given value of L and C. If we increase the applied frequency, XC decreases and XL
increases. If we increase the frequency enough, the capacitor acts as a short and the inductor acts as an
open. Of course, the opposite is also true. Decreasing frequency causes XC to increase and XL to decrease.
Here again, if we make a large enough change, XC acts as an open and XL acts as a short. Figure 1-13
gives a pictorial representation of these two basic components and how they respond to low and high
frequencies.
Figure 1-13.—Effect of frequency on capacitive and inductive reactance.
