MM0704, Lesson 2
spondingly large inductive reactance to high frequencies, offers high resistance to the high frequency current that gets
by L1 and C1. Consequently, the T-section is much more effective, at any given frequency, than the L-section. The
total of the two inductors' inductive reactance is equal to the single inductor used in the L-section. (This assumes that
both circuits are to pass the same frequencies.)
section. In the T-section and L-section filters, the input component is an inductor, and the circuit is called an
inductive input filter. In all three, there are two principal characteristics: the inductance is in series with the line, and
the capacitance is across the line. The -section uses a capacitive input (figure 2-3). Although it is important that the
outputs from the inductive and capacitive input filters differ in that the voltage output is higher with a capacitive input,
frequency variations are the main consideration in this lesson. The capacitor C1 shorts most high frequency currents to
ground, while C2 provides a path to ground for those currents that manage to get through L1. Thus, the total capacitive
reactance should be equivalent to the single capacitance of the L-section and T-section filters.
Figure 2-3. -Section, Low-Pass Filter.
Transmission Characteristics. The output responses illustrated in this lesson are representative of specific filter
circuits, but do not think of them as the only outputs from the filters discussed. This is particularly evident in figure
2-4B where it is indicated that additional sections, as well as different values of components, will very definitely alter
the output response. Also affected by circuit changes would be the cutoff frequency, the bandwidth, and the
attenuation factor of the filter network.
High-Pass Filters
High-pass filters are used to pass high frequencies and reject low frequencies. A common use is to couple AC signals
while rejecting the DC voltage in amplifier sections.
L-section. The action of the L-section, high-pass filter (figure 2-5) is as follows: at low frequencies, the capacitance in
series offers large opposition to the flow of current. Any low-frequency current from the input that gets
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