LC Oscillators

LC Oscillators

• The oscillators which use the elements L and C to produce the oscillations are called LC oscillators. The circuit using elements L and C is called tank circuit or oscillatory circuit, which is an important part of LC oscillators. This circuit is also referred as resonating circuit, or tuned circuit. These oscillators are used for high frequency range from 200 kHz upto few GHz. Due to high frequency range, these oscillators are often used for sources of RF (Radio Frequency) energy. Let us study the basic action of LC tank circuit first.

1. Operation of LC Tank Circuit

• The LC tank circuit consists of elements L and C connected in parallel as shown in the Fig. 10.7.1.

• Let the capacitor is initially charged from a d.c. source with the polarities as shown in the Fig. 10.7.2.

• When the capacitor gets charged, the energy gets stored in a capacitor called electrostatic energy. When such a charged capacitor is connected across inductor L in a tank circuit, the capacitor starts discharging through L, as shown in the Fig. 10.7.3.

• The arrow indicates direction of flow of conventional current. Due to such current flow, the magnetic field gets set up around the inductor L. Thus inductor starts storing the energy. When capacitor is fully discharged, maximum current flows through the circuit. At this instant all the electrostatic energy get stored as a magnetic energy in the inductor L. This is shown in the Fig. 10.7.4.

• Now the magnetic field around L starts collapsing. As per Lenz's law, this starts charging the capacitor with opposite polarity making lower plate positive and upper plate negative, as shown in the Fig. 10.7.5. 

• After some time, capacitor gets fully charged with opposite polarities, as compared to its initial polarities. This is shown in the Fig. 10.7.6. The entire magnetic energy gets converted back to electrostatic energy in capacitor.

• Now capacitor again starts discharging through inductor L. But the direction of current through circuit is now opposite to the direction of current earlier in the circuit. This is shown in the Fig. 10.7.7. Again electrostatic energy is converted to magnetic energy. When capacitor is fully discharged, the magnetic field starts collapsing, charging the capacitor again in opposite direction.

 Key Point : Thus capacitor charges with alternate polarities and discharges producing alternating current in the tank circuit.

• This is nothing but oscillatory current. But every time when energy is transferred from C to L and L to C, the losses occur due to which amplitude of oscillating current keeps on decreasing everytime when energy transfer takes place. Hence actually we get exponentially decaying oscillations called damped oscillations. These are shown in the Fig. 10.7.8. Such oscillations stop after sometime.

Key Point : In LC oscillator, the transistor amplifier supplies this loss of energy at the proper times.

• The care of proper polarity is taken by the feedback network. Thus LC tank circuit alongwith transistor amplifier can be used to obtain oscillators called LC oscillators. Due to supply of energy which is lost, the oscillations get maintained hence called sustained oscillations or undamped oscillations.

• The frequency of oscillations generated by LC tank circuit depends on the values L and C and is given by,

f =  1/ 2π √LC  Hz

where L is in henries and C is in farads.