LC Oscillator: Basic, Working States, Advantages, and Applications

The LC oscillator circuit, also known as the resonant circuit, uses the electromagnetic coupling between inductors and capacitors to realize the mutual conversion of electrical energy and magnetic energy, thereby generating an oscillating current. It is used both to generate signals of specific frequencies and to separate signals of specific frequencies from more complex signals. As a key part of many electronic devices, especially radio equipment, they are used in oscillator, filter, tuner and mixer circuits. In addition, it is necessary to note that the LC circuit is an ideal model that ignores energy dissipation due to resistance.

LC Circuit Structure

In the basic LC oscillator circuit, L is the inductor and C is the capacitor. The so-called "resonance", in terms of energy relationship, means that the energy stored in the loop is unchanged and is only converted between the inductor and the capacitor, while the external electromotive force only provides the energy consumed by the loop resistance to maintain the same amplitude of the loop oscillation.

The composition conditions of the oscillator circuit:

1) A set of oscillator circuits, including two (or more than two) energy storage elements. For them, when one releases energy, the other receives it. This move can go back and forth, and its frequency depends on the value of the component.

2) An energy source to supplement the energy loss caused by the resistance of the oscillation circuit. In a transistor oscillator, this energy source is DC power.

3) A control device that enables the power supply to supplement the energy loss of the circuit at the correct moment to maintain constant amplitude oscillation, which is accomplished by active devices and positive feedback circuitry.

Oscillation Process

The reason why the LC oscillator circuit oscillates is that the circuit uses the energy storage devices of capacitors and inductors to alternately convert electromagnetic energy. In short, because electrical energy and magnetic energy have maximum and minimum values, so there is oscillation. However, in reality, all electronic components will have some losses, and there will also be losses when energy is converted between capacitors and inductors, resulting in a continuous decrease in energy. Therefore, the LC oscillation circuit must have an amplifying component, which can be a transistor, integrated operational amplifier or other components. Let’s briefly talk about the oscillation process in the ideal state.

If the capacitor is fully charged, the upper plate stores positive charge, the lower plate stores negative charge, and the capacitor charges the inductor:

At this time, the inductor current gradually increases, flowing in from the top and flowing out from the bottom. The voltage across the inductor gradually decreases until the capacitor releases all energy. When the electrical energy stored in the capacitor is completely converted into magnetic energy in the inductor, the voltage across the inductor is zero and the current flowing through the inductor is maximum in the forward direction.

The magnetic energy stored in the inductor is converted into the electrical energy of the capacitor, that is, the capacitor is charged. And the direction of the inductor current remains unchanged. When the inductor is charged reversely, the voltage across it slowly increases until reaches the maximum negative value, and the current flowing through it gradually becomes zero.

At this time, the energy of the inductor and capacitor is as shown in the figure:

Then the capacitor discharges in the reverse direction through the inductor. The inductor current gradually increases, flowing in from below and flowing out from above. The voltage across the inductor gradually decreases until the capacitor releases all energy. When the electrical energy stored in the capacitor is completely converted into magnetic energy in the inductor, the voltage across it is zero, and the current flowing through it is maximum negative.

At this time, the direction of the inductor current remains unchanged, flowing in from the bottom and flowing out from the top. Positive charges flow to the positive plate and negative charges flow to the negative plate until the inductor voltage is positive and the inductor current is zero. By analogy, the repeated charging and discharging between the inductor and the capacitor forms an oscillation wave pattern with sinusoidal changes.

Working States

1. Charge

When power is turned on in a circuit, current passes through the inductive element, creating a magnetic field, which is gradually increasing because the current is gradually increasing. When both ends of the capacitive element are charged to a certain voltage, the amount of charge in the capacitive element will gradually increase. At this stage, electrical energy is converted into magnetic energy and electrical charge energy.

2. Discharge

As the voltage across the capacitive element increases, the magnetic field energy in the inductive element also gradually increases. When the voltage across the capacitor reaches a certain value, the magnetic field energy in the inductor and the charge energy in the capacitor will reach a balanced state. At this time, the number of charges in the capacitive element reaches its maximum value, and the magnetic field energy in the inductive element also reaches its maximum value. Next, the capacitive element begins to discharge. In this process, the number of charges in the capacitive element gradually decreases, and the magnetic field energy in the inductive element is gradually converted into current energy. And the current will flow in the opposite direction, so the direction of the magnetic field of the current will also be reversed.

3. Reverse charging

When the number of charges in the capacitive element is reduced to a certain level, the magnetic field energy in the inductive element also begins to gradually weaken. At this time, the voltage across the capacitive element will gradually decrease, and the current in the inductive element will also gradually decrease. In this process, electrical energy is converted into magnetic energy and charge energy again.

4. Reverse discharge

When the voltage across the capacitive element drops to a certain level, the magnetic field energy in the inductive element is already very weak. At this time, the capacitive element begins to discharge in the reverse direction, that is, the current direction is opposite to the previous discharge direction. During this process, the number of charges in the capacitive element will gradually decrease, and the magnetic field energy in the inductive element will gradually be converted into current energy.

5. Oscillation

Through the cycle of the above four stages, the LC oscillator circuit will generate a continuous oscillating current. The frequency of this oscillating current depends on the parameters of the inductor and capacitor components in the circuit, as well as conditions such as the voltage and current of the external power supply. During oscillation, the length of each cycle is determined by the values of the inductor and capacitor components. Normally, the smaller the resistance of the inductive component and the larger the self-inductance coefficient, the longer the oscillation period; while the larger the resistance and the larger the capacity of the capacitive component, the shorter the oscillation period.

6. Energy conversion

In an LC oscillator circuit, electrical and magnetic energy are continuously converted between inductive and capacitive components. During the charging stage and reverse charging stage, electrical energy is converted into magnetic energy and charge energy; while during the discharge stage and reverse discharge stage, magnetic energy and charge energy are converted into electrical energy. This energy conversion process can realize the transmission and reception of electromagnetic waves, so it has wide application value in wireless communications, radar and other application fields.

In addition, an LC oscillator can be made to operate at audio frequencies if the inductance is high enough, for example, audio transformers, power supply transformers, and DC power supply filter chokes could work in such a circuit.

Advantages

1) High frequency stability: When the values of the inductor and capacitor are determined, the LC oscillation circuit can provide a very stable oscillation frequency.

2) Simple and reliable: The LC oscillator circuit has a simple structure, is easy to implement and adjust, and has good reliability.

3) Low cost: Compared with other types of oscillation circuits, the manufacturing cost of LC oscillation circuits is low.

Disadvantages

1) High sensitivity: LC oscillator circuit is relatively sensitive to changes in component parameters, and slight changes in component parameters may cause frequency deviation.

2) Energy loss: Due to the resistance of circuit components and wires, as well as other energy loss mechanisms, the signal of the LC oscillator circuit will gradually attenuate.

Applications

1) Wireless communication

As a basic oscillation source, LC oscillator circuit is widely used in wireless communication systems, for example, it is used to generate high frequency signals such as the transmission and reception of radio frequency (RF) signals.

2) Clock source

LC oscillator circuits can be used as stable clock sources in computers and digital systems. By adjusting the values of the inductor and capacitor, the desired oscillation frequency can be achieved.

3) Sensor

LC oscillator circuits are also used in sensor applications. For example, pressure sensors and temperature sensors can detect changes in environmental parameters by measuring changes in capacitance or inductance.

4) Signal generator

LC oscillator circuits can be used to make simple signal generators for use in test and measurement fields. By adjusting the values of the inductor and capacitor, a signal of a specific frequency can be generated.

5) Oscillator

LC oscillator circuits are also often used as basic oscillation devices to provide stable oscillation signals, for example, they are widely used in various electronic devices and systems, including audio equipment, radio frequency equipment, etc.

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