A resonant circuit stores and transfers energy between an inductor and capacitor, oscillating at a preferred frequency. The Q factor measures energy loss, with a higher Q factor producing more output for each input. An oscillator can replace energy lost by a non-ideal Q factor, and a radio tuner is a resonant circuit with a high Q factor.
A resonant circuit, also known as an LC circuit, tank circuit, or tuned circuit, is a circuit that stores energy and transfers it back and forth repeatedly, similar to a swinging pendulum. Energy passes between an inductor, a circuit component that stores energy in a magnetic field, and a capacitor, which stores energy in an electric field. When the two work at the same frequency, the circuit is said to be tuned. Such tuning circuits are used in tuners and amplifiers.
The inductor and the capacitor work together. The capacitor stores energy in the form of voltage and then releases it in the form of current. The inductor stores energy from the current in its magnetic field and then releases the energy to the capacitor. The two components of the circuit pass their stored energy back and forth, a phenomenon called oscillation. The number of times energy is transferred back and forth each second is considered the frequency of the resonant loop.
A resonant circuit is like a pendulum. A person pulls the pendulum to one side, thereby storing potential energy, because the weight of the pendulum is higher than before. When the pendulum is released, the potential energy is transformed into kinetic energy, the energy of motion. Kinetic energy causes the pendulum to pass through the neutral position to rise on the other side, again storing potential energy. The pendulum swings back and forth until it runs out of energy.
Like a pendulum, a resonant circuit works most efficiently when it swings at its preferred or resonant frequency. The rate at which the capacitor and inductor each absorb and release energy is a function of time. If you try to drive the circuit faster than its resonant frequency, neither the capacitor nor the inductor will be able to absorb and release the energy fast enough. The resonant frequency of the circuit is defined by equation 1 divided by the square root of L x C. L represents inductance in Henries and C represents capacitance in Farads.
Like a child on a seesaw, resonant circuits lose some energy as energy is passed back and forth, so new energy must be added to keep the circuit going. Wires have resistance. Capacitors do not release as much energy as they absorb. The loss in a resonant circuit is measured by the quality factor or Q factor. A higher Q factor means that less energy is lost with each oscillation.
The Q factor is calculated as the ratio of the amplitude, or strength, of the oscillations leaving the circuit, relative to what entered the circuit. A higher Q factor means that less energy is required to maintain the loop and more output is produced for each input. By analogy, on a child’s swing, this can be compared to how far the swing travels after the parent’s push, versus how far the parent’s hand traveled while pushing the child.
An oscillator is a special type of circuit that replaces energy lost by a non-ideal Q factor. When a child pumps a swing at the correct frequency, adding energy to the system at regular intervals to overcome the loss due to friction and wind resistance, the child can swing indefinitely. A radio tuner is a resonant circuit with a high Q factor. Turning the knob changes the capacity of a variable capacitor. When the resonant circuit is tuned to the same frequency as the radio station’s transmitter, the circuit produces a clear, high-amplitude audio transmission.
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