Resonant circuits

Alessandro N. Vargas

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Resonant circuits. This page presents information about resonant circuits, which have been implemented over the last few years at UTFPR to help us advance the knowledge of electronic engineering.

Resonant circuits, also known as RLC circuits, are electronic circuits that consist of resistors (R), inductors (L), and capacitors (C). These components are interconnected so that they can store and exchange energy in the form of electrical and magnetic fields. A resonant circuit has the property of "resonate," which is the natural phenomenon where the circuit keeps shaking up and down, indefinitely, oscillating at a particular frequency with maximum amplitude.

One of the most common applications of resonant circuits is in filters. Resonant circuits can be designed to selectively pass or reject specific frequencies of an input signal. Resonant circuits are also used in oscillators, which are circuits that generate continuous sine-wave signals at a specific frequency. Applications:

1. Series Resonant Circuit: In a series resonant circuit, the inductor and capacitor are connected in series with the resistor. This type of resonant circuit exhibits a high impedance at the resonant frequency, allowing a narrow range of frequencies around the resonant frequency to pass through with minimum attenuation while attenuating frequencies outside that range. Series resonant circuits are used in band-pass filters, where a specific frequency range needs to be passed through.
2. Parallel Resonant Circuit: In a parallel resonant circuit, the inductor and capacitor are connected in parallel with the resistor. This type of resonant circuit exhibits a low impedance at the resonant frequency, allowing a narrow range of frequencies around the resonant frequency to pass through with minimum attenuation while attenuating frequencies outside that range. Parallel resonant circuits are used in band-stop filters, where a specific frequency range needs to be attenuated.
3. LC Oscillator: An LC oscillator is a type of resonant circuit that uses inductors and capacitors in combination with active components like transistors or operational amplifiers to generate sinusoidal signals. LC oscillators are used in signal generators, oscillators in radio transmitters and receivers, and frequency synthesis circuits.
4. Band-Pass Filter: A band-pass filter is a type of resonant circuit that allows a specific frequency range to pass through with minimum attenuation while attenuating frequencies outside that range. It typically consists of a series resonant circuit followed by a parallel resonant circuit or vice versa. Band-pass filters are used in radio communication systems, audio equipment, and signal processing.
5. Band-Stop Filter: A band-stop filter, also known as a notch filter, is a type of resonant circuit that attenuates a specific frequency range while allowing frequencies outside that range to pass through. It typically consists of a parallel resonant circuit in parallel with a series resonant circuit or vice versa. Band-stop filters are used in noise reduction, interference rejection, and audio equalization.
6. Resonant Transformer: A resonant transformer is a type of inductive device that uses resonant circuits to transfer energy between input and output coils at a specific frequency. It typically consists of one or more resonant circuits with inductors and capacitors. It is used in power electronics, wireless power transfer, and high-frequency amplifiers.
7. Crystal Oscillator: A crystal oscillator is a type of resonant circuit that uses a quartz crystal as the resonant element to generate precise and stable oscillations at a specific frequency. Crystal oscillators are designed to be oscillating at high-frequency, and they are used in microelectronics, communication systems, timing circuits, and frequency synthesis.