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Unit 9: Resonance and Passive Filters


Resonant circuits and filters are two types of circuits that rely on the fact that capacitive and inductive reactances change as frequency changes.

Filters are circuits that pass signals of certain frequencies but block signals of other frequencies. Filters have many uses: one of the most obvious is in a radio tuner, which must select radio signals of one frequency out of the many frequencies present in the air around us. A series RC circuit can serve as a simple filter; its operation relies on the fact that capacitors have high reactance at low frequencies, and low reactance at high frequencies. A series RL circuit can also serve as a simple filter; its operation relies on the fact that inductors have low reactance at low frequencies, and high reactance at high frequencies.

Resonant circuits make use of the fact that, in a circuit containing capacitors and inductors, there must be a frequency at which the circuit’s capacitive reactance is equal to its inductive reactance. At this frequency, called the circuit’s resonant frequency, the circuit’s currents and voltages typically reach extreme values (maximums or minimums). This phenomoneon is useful in building a variety of circuits, including certain types of filters.

Unit 8 Review

Impedance of Series RLC Circuits
The Effect of Changing Frequency
Series Resonance
Formula for Resonant Frequency
Current at Series Resonance
Resistor Voltage at Series Resonance
Capacitor and Inductor Voltages at Series Resonance

Impedance of Parallel LC Circuits
The Effect of Changing Frequency
Parallel Resonance
Current at Parallel Resonance
The Non-Ideal Case

Frequency-response Curve
Bode Plots
Low-pass Filters
High-pass Filters
Band-pass Filters
Band-stop Filters
RC Filters
RL Filters
Series-Resonant RLC Filters
Parallel-Resonant RLC Filters

Unit 9 Review

Congratulations! You’ve completed the e-Lesson for this unit.