In an AC circuit including an inductor & a capacitor, the current and the phase of the voltage are usually different. If you change the circuit components or the power frequency, then the whole circuit will appear simply resistive. Once the LC circuit reaches this condition, then it is known as resonance. In this condition, the circuit reaches the whole impedance or high value. The main essence of resonance is that the energy of the magnetic field within the inductance & the energy of the electric field within the capacitor can be changed into each other. The LC oscillator circuit is mainly used to produce signals with a specific frequency. It is applicable for tuners, oscillation circuits, mixers & filter circuits, etc. The LC oscillator circuit ignores the dissipation energy caused through resistance.
What is LC Oscillator?
LC oscillator definition is an oscillator that is used to generate high-frequency signals which are also known as RF oscillator. This circuit is also called an LC-tuned LC resonant or tank circuit. By using the practical values of capacitors and inductors, it is possible to generate high range frequencies like > 500 MHz. LC oscillators are used in heating with high-frequency, RF generators, radios, TV receivers, etc. These types of oscillators use tank circuits including the components like a capacitor (C) and an inductor (L).
These circuits are used for either producing signals at a fixed frequency otherwise selecting a signal at a fixed frequency using a more complex signal which is known as a bandpass filter. The main purpose of an LC circuit is to fluctuate with minimum damping so that the resistance can be made less.
LC Oscillator Circuit
The construction of an LC oscillator circuit can be done by connecting an inductor and a capacitor in parallel. This circuit generates the electrical oscillation for any preferred frequency. The components which are used in this circuit are capable of energy storage. Once the potential difference across a capacitor exists, then the energy can be stored in its electric field.
Likewise, once the current flow throughout an inductor, then energy can be stored in its magnetic field. The LC oscillator circuit is shown below where the capacitor & inductor are connected in parallel through a switch and a voltage source. At first, the switch ‘S’ is arranged in position 1, then the capacitor will gets charged toward the voltage source.
Similarly, this switch is moved to the 2nd position, then the capacitor will discharge slowly using an inductor L. So, the flow of current in the inductor will increase and the voltage across the capacitor will decrease. When the current flow increases, then an electromagnetic field can be formed around the inductor coil. Similarly, when the capacitor is discharged completely then this energy can be transferred completely into the coil-like electromagnetic field.
So, there is no more energy within the capacitor to maintain the flow of current in the coil, the field in the region of the coil will start to reduce & the flow of current throughout the coil will reduce. So, the conductor will generate back emf because of electromagnetic induction to resist the change within the current. So again this back emf will start charging the capacitor.
Once the capacitor is charged completely, then the stored energy within the inductor like an electromagnetic field will be shifted to the capacitor like an electrostatic field. After that again the capacitor will start discharging. In the oscillator circuit, the transfer of cyclic energy among the capacitor & inductor is the reason behind the production of oscillations in the tank circuit.
If an ideal capacitor & inductor are used, this oscillation will maintain until the end of time. However, in a practical case, the inductor will have some ohmic resistance and the capacitor will have some amount of leakage. These imperfections will waste some amount of energy among the cycles which result in the amplitude loss gradually and finally, the oscillations will disappear.
This slow decay within amplitude which tends to the loss of an oscillation is known as damping. The oscillations which are generated within a damped oscillator circuit will look like the following waveforms.
Different Types
The types of LC oscillators are discussed below.
Tuned Collector Oscillator
The tuned collector oscillator is a type of transistor-based LC oscillator. This circuit can be built with a capacitor & a transformer. The connection of these components can be done to the transistor’s collector terminal. This is the simple type of LC oscillator circuit that performs like a simple resistive load on resonance to decide the frequency of the oscillator. The applications of tuned collector oscillator circuits include RF oscillator circuits, signal generators, mixers, frequency demodulators, etc.
Tuned Base Oscillator
This is a type of LC oscillator where this circuit can be arranged in between the two terminals of the transistor-like base & ground. The tuned circuit can be formed by connecting a capacitor and the transformer’s primary coil. The transformer’s secondary coil can be used as feedback.
Hartley Oscillator
The type of LC oscillator like the Hartley oscillator is also called the RF oscillator. This circuit includes one capacitor and two inductors. The arrangement of the inductor can be done in series whereas the capacitor is in parallel to the series combination. The Hartley oscillator’s typical operating frequency ranges from 20 KHz -20 MHz. A Hartley oscillator’s oscillation frequency can be decided through an LC oscillator. These oscillators are normally tuned to generate signals in the RF band.
Colpitts Oscillator
A Colpitts Oscillator is also one kind of LC oscillator. This oscillator is invented in the year 1918 by an American engineer namely Edwin H. Colpitts. Similar to the LC oscillator, this kind of oscillator uses a combination of a capacitor & an inductor to generate an oscillation at a fixed frequency. The main feature of this oscillator is that the feedback of the active device can be taken from a voltage divider that can be made with two capacitors that are connected in series across the inductor.
Clapp Oscillator
The Colpitts oscillator is an alteration of the Colpitts oscillator. This oscillator can be designed by adding an extra capacitor that can be connected in series with the inductor in the oscillator circuit. This extra capacitor can be made changeable in changeable frequency applications. This additional capacitor separates the other two capacitors from the transistor parameters effects such as junction capacitance & enhances the stability of frequency.
Difference between LC and RC Oscillator
The main difference between LC & RC oscillators includes the following.
RC Oscillator | LC Oscillator |
The frequency of oscillations of this oscillator depends on R & C values. | The frequency of oscillations of this oscillator depends on L & C values. |
These oscillators are applicable at medium and low frequencies | These oscillators are applicable at high frequencies |
The best examples of RC oscillator are wein bridge and phase shift | The best examples of LC oscillators are Colpitts, clap, hartley, etc. |
The stability of frequency is poor | As compared to RC, the stability of frequency is poor, except for the clap oscillator, |
These oscillators are used as medium & low-frequency signal generators. | These oscillators are used in frequency synthesizers, high-frequency sources in TV & in radios. |
Frequency
The frequency generated by the LC oscillator circuit completely depends on the capacitor and inductor values as well as their resonance condition. The LC oscillator frequency can be expressed as
XL = 2π f L
XC = 1/ 2π f C
XL = XC at resonance
2π f L = 1/ 2π f C
f2 = 1/ ((2π)2 LC)
f = 1/ (2π √ (LC))
Where:
‘L’ is the Inductance measured in Henries
‘C’ is the Capacitance, measured in Farads
‘ƒr’ is the Output Frequency, measured in Hertz
From the above frequency equation, we can conclude that if either capacitor or inductor is reduced, then the frequency will increase. This output frequency can be normally stated as ‘ƒr’ to identify it as the resonant frequency.
To maintain the oscillations in an LC circuit, we have to change all the energy which is lost in every oscillation at a stable level. So, the energy changed should be equivalent to the energy lost throughout every cycle. The simplest method to replace this lost energy is to get the output from the LC oscillator circuit, amplify it, and then feed it back again into the LC circuit.
This process can be achieved through a voltage amplifier with a FET, bipolar transistor, or operational amplifier like its active device. But, if the feedback amplifier’s loop gain is very small, then the preferred oscillation decomposes to zero. If the loop gain is large, then the waveform will be distorted.
To produce a stable oscillation, the energy level can be fed back to the LC oscillator should be controlled precisely. So, there should be some type of gain control or automatic amplitude once the amplitude attempts to change from a reference voltage. To maintain a stable oscillation, the overall circuit gain should be equivalent to unity.
Please refer to this link to know more about Oscillators MCQs
Applications
The applications of the LC oscillator include the following.
- In mobile phones
- For broadcasting of TV & radio programs
- In HV transmission systems & radios for carrier line communication
- Civil & Defense Communication
- Filter circuits with combinations of LC
- Surge controls in power capacitors & APFC panels
- Frequency mixers, RF signal generators, Sine Wave Generators, Tuners, and RF Modulators.
- Transformers with stable voltage
Thus, this is all about an overview of LC oscillator circuits, types, working & their applications. Here is a question for you, what are the different types of oscillators?