LC Oscillator Circuit : Working and Its Applications

An oscillator is an electronic circuit used to change an input DC to an output AC. This can have an extensive range of waveforms with different frequencies based on the application. Oscillators are used in several applications like test equipment which generate any of these waveforms like a sinusoidal, sawtooth, square wave, triangular waveforms. LC Oscillator is usually used within RF circuits due to their high-quality phase noise characteristics as well as easy implementation. Basically, an oscillator is an amplifier that includes positive or negative feedback. In electronic circuit design, the main problem is to stop the amplifier from oscillating when trying to acquire oscillators to oscillate. This article discusses an overview of LC oscillator and circuit working.

What is LC Oscillator?

Basically, an oscillator uses positive feedback and generates an o/p frequency without using an input signal. Thus these are self-supporting circuits that generate a periodic o/p waveform at an exact frequency. LC oscillator is a kind of oscillator where a tank circuit (LC) is used to give the required positive feedback for maintaining the oscillations.

This circuit is also called as LC tuned or LC resonant circuit. These oscillators can understand with the help of FET, BJT, Op-Amp, MOSFET, etc. The applications of LC oscillators mainly include frequency mixers, RF signal generators, tuners, RF modulators, sine wave generators, etc. Please refer to this link to know more about Difference Between Capacitor and Inductor

LC Oscillator Circuit Diagram

An LC circuit is an electric circuit that can be built with an inductor and capacitor where the inductor is denoted with ‘L’ and the capacitor is denoted with ‘C’ both allied within a single circuit. The circuit works like an electrical resonator which stores energy to oscillate at the resonant frequency of the circuit.

These circuits are used either to select a signal at the particular frequency through the compound signal otherwise generating signals at a particular frequency. These circuits work like major components within a variety of electronic devices such as radio apparatus, circuits such as filters, tuners, and oscillators. This circuit is a perfect model that imagines that the dissipation of energy doesn’t happen because of resistance. The main function of this circuit is to oscillate through the least damping to make the resistance minimum possible.

LC Oscillator Derivation

When the oscillator circuit is energized with stable voltage using time changing frequency, after that the reactance of RL, as well as RC, is also changed. Therefore the frequency and amplitude of the o/p can be changed when contrasted with i/p signal.

The inductive reactance and the frequency can be directly proportional to each other while the frequency and the capacitive reactance can be inversely proportional to each other. So, at lesser frequencies, the inductor’s capacitive reactance of the inductor is extremely small performs like short circuit while the capacitive reactance is higher & performs like an as open circuit.

At higher frequencies, the reverse will happen i.e., capacitive reactance acts as short circuit whereas inductive reactance acts as an open circuit. The circuit at a specific combination of an inductor and capacitor will become tuned or resonant frequency at both the reactance’s of capacitive and inductive are the same & stop with each other.

Therefore there will be simply resistance is there within the circuit for opposing the current flow & thus the voltage cannot produce the LC phase shift oscillator current with the help of a resonant circuit. So the flow of current and voltage will be in phase with each other.

The continued oscillations can be attained by giving the voltage supply to the components like inductor and capacitor. As a result, LC oscillator uses the LC or tank circuit to generate the oscillations.

The oscillations frequency can be produced from the tank circuit which completely relies upon the inductor, capacitor values & their condition of resonance. So it can be stated by using the following formula.

XL = 2*π* f* L

XC = 1/ (2*π* f* C)

We know that, at resonance, XL is equal to XC. So the equation will become like the following.

2*π* f* L = 1/ (2*π* f* C)

Once the equation can be shortened then the equation of LC oscillator frequency includes the following.

f2 = 1/ ((2π) * 2 LC)

f = 1/ (2π √ (LC))

Types of LC Oscillators

LC oscillator is classified into different types which include the following.

  Tuned Collector Oscillator

This oscillator is a basic type of LC oscillator. This circuit can be built with a capacitor and a transformer by connecting in parallel across the oscillator’s collector circuit. The tank circuit can be formed by the capacitor and main of the transformer. The minor of the transformer feeds backside a portion of the oscillations generated within the tank circuit to the base of the transistor. Please refer to this link to know more about Tuned Collector Oscillator

  Tuned Base Oscillator

This is one kind of LC transistor oscillator wherever this circuit is located among the two terminals of transistor-like the ground and the base. The tuned circuit can be formed by using a capacitor & main coil of a transformer. The minor coil of the transformer is used as feedback.

  Hartley Oscillator

This is a kind of LC oscillator wherever the tank circuit includes one capacitor and two inductors. The capacitor is connected in parallel and inductors are connected in series to the combination of series. This oscillator was made-up by Ralph Hartley in the year 1915. He is an American scientist. Typical Hartley oscillator’s operating frequency ranges from 20 kHz-20MHz. It can be recognized by using FET, BJT, otherwise op-amps. Please refer to this link to know more about Hartley Oscillator

  Colpitts Oscillator

This is another kind of oscillator wherever the tank circuit can be built with one inductor & two capacitors. The connection of these capacitors can be done in series whereas the inductor can be connected in parallel toward the capacitor’s series combination.

This oscillator was made-up by scientists namely Edwin Colpitts in 1918. The operating frequency range of this oscillator ranges from 20 kHz – MHz. This oscillator includes superior frequency strength as contrasted to the Hartley oscillator. Please refer to this link to know more about Colpitts Oscillator

  Clapp Oscillator

This oscillator is an alteration of the Colpitts oscillator. In this oscillator, an extra capacitor can be connected in series toward the inductor within the tank circuit. This capacitor can be made uneven in the applications of variable frequency. This extra capacitor separates the remaining two capacitors from the transistor parameter effects such as junction capacitance as well as advances the frequency strength.

Applications

These oscillators are broadly used for producing high-frequency signals; therefore these are also named as RF oscillators. By using the practical values of capacitors & inductors, It is probable to generate a higher range of frequencies like > 500 MHz.

The applications of LC oscillators mainly include in radio, television, high-frequency heating, and RF generators, etc. This oscillator uses a tank circuit which includes a capacitor ‘C’ and an inductor ‘L’.

Difference between LC and RC Oscillator

We know that the RC network offers regenerative feedback & decides the operation of frequency within RC oscillators. Each oscillator that we have discusses above uses a resonant LC tank circuit. We know that how this tank circuit stores energy within the used components in the circuit like capacitor and inductor.

The main difference between LC and RC circuits is that the frequency deciding device within the RC oscillator is not an LC circuit. Consider, the operating of an LC oscillator can be done using biasing like class A otherwise class C due to the action of the oscillator in the resonant tank.  The RC oscillator should utilize class-A biasing as determining the RC frequency device doesn’t contain the ability of oscillation of a tank circuit.

Thus, this is all about what is LC Oscillation and deviation using the circuit. Here is a question for you, what are the advantages of LC Circuit?