LC circuits with resonance effect

What’s an LC Circuit?

LC circuit refers to an electric circuit consisting of an inductance L and a capacitor C to form a frequency selection network, which is used to generate high-frequency sine wave signals. In many cases, the LC circuit is also called an oscillator circuit, resonant circuit, tank circuit or tuning circuit. Common LC sine wave oscillating circuits include transformer feedback type LC oscillating circuit and inductor three-point LC oscillating circuit and capacitor three-point LC oscillator circuit. The radiation power of the LC oscillator circuit is proportional to the fourth power of the oscillation frequency. To allow an oscillating LC circuit to radiate sufficiently strong electromagnetic waves, the oscillation frequency must be increased and the circuit has an open form.

How does an LC circuit work?

The LC circuit uses the energy storage characteristics of capacitors and inductors to alternately convert electromagnetic energy. That is to say, electric energy and magnetic energy will have a maximum and minimum values, and there will be oscillation. However, this is only an ideal situation. Virtually all electronic components will lose energy. The energy is either lost or leaked to the outside in the process of mutual conversion between the capacitor and the inductance. The energy will continue to decrease, so the actual LC oscillator circuit is an amplifying element is needed, either a triode, or an integrated op amp or other digital LC. Using this amplifying element, through various signal feedback methods, the constantly consumed oscillating signal is fed back and amplified, thereby finally outputting an amplitude and frequency relatively stable signal. The LC circuit frequency calculation formula is , and f is the frequency, the unit is Hertz (Hz); L is the inductance, the unit is Henry (H); C is the capacitance, the unit is Farah (F).

The LC electromagnetic oscillation process involves many physical quantities, and the changes of various physical quantities are also more complicated. In the actual analysis process, if you notice the asynchronous changes of the electric field (electric field energy, voltage, electric field intensity) and the magnetic field (magnetic energy, current intensity, magnetic induction intensity), and the synchronous changes of the electric field and magnetic field, make full use of the electric field. Conservation of energy including energy and magnetic field energy, and other physical changes radiated by energy changes, can quickly understand the changes of various physical quantities and determine the state of the circuit.

LC circuit model conditions

The resistance R of the entire circuit (including coils and wires) is zero. From the energy point of view, no other forms of energy are converted into internal energy, that is, there is no heat loss.

The inductor L contains the inductance of the entire circuit, and the capacitor C contains the capacitance of the entire circuit, and there is no latent inductance and latent capacitance.

The LC oscillation circuit doesn’t radiate electromagnetic waves to the external space when electromagnetic oscillation occurs. Strictly speaking, it is a closed circuit. Only the mutual conversion between the coil magnetic field energy and the capacitor electric field energy occurs inside the LC circuit, even if it is the change generated in the capacitor. The electric field and the changing magnetic field generated in the coil don’t excite the corresponding magnetic field and electric field according to Maxwell’s electromagnetic field theory, and radiate electromagnetic waves to the surrounding space.

Applications of LC circuits

The resonance effect of the LC circuit has many important applications in communication systems and signal processing.

The parallel resonance circuit generates current amplification.

The series resonance circuit generates voltage amplification.

The LC circuit is used to select or generate a specific frequency signal.

The application of LC circuits is reflected in many electronic devices, especially radio devices, such as transmitters, radio receivers and television receivers, amplifiers, oscillators, filters, tuners and frequency mixers.

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September 2020
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