MM0704, Lesson 2
Figure 2-5. L-Section, High-Pass Filter
From the formula for inductive reactance, XL= 2 fL, you can see that for high frequencies XL is large, and for low
frequencies, small. Since voltages are directly proportional to impedances, it follows that much voltage is developed
for the output at high frequencies, and very little at low frequencies. That means this filter will pass high frequencies.
T-section. The T-section (figure 2-6) uses the principles that capacitors in series act like resistances in parallel. The
total capacitance of C1 and C2 should be equal to C1 of the half section, to provide a low opposition path to the output
for high frequencies. C2 further insures that any AC variations across L1 will be delivered to the load.
Figure 2-6. T-Section, High-Pass Filter.
-section. The inductors L1 and L2 act much the same as the two capacitors in the -section low-pass filter (figure
2-7); however, the low-frequency currents are provided two paths to ground in order to eliminate all but the higher-
frequency currents. Capacitor C1 presents a high impedance path to the output for low frequencies as well as a low-
impedance path for the desired frequency.
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