In this post we are going to understand how LC oscillator circuits functions and we will be constructing one of the popular LC based oscillator - Colpitts oscillator.

By Girish Radhakrishnan

What are Oscillators

Electronic oscillators are used in most of our daily used electronic gadgets ranging from digital clock to high end core i7 processor. Oscillators are heart of all digital circuits but, not only digital circuit employee oscillators but also analogue circuits uses oscillatory circuits.

For instant AM, FM radio, where the high frequency oscillation is used as carrier signal to transport message signal.

There are many different kinds of oscillators such as RC, LC, crystal etc. Each one of them has their own advantages and disadvantages. So there is nothing called best or ideal oscillator, we have to analyse the circumstance of our circuit and choose the best one which suit, that’s why we find wide range of oscillators in every day used gadgets.

LC Oscillators

Let’s dive into the explanation of LC oscillator.

The LC oscillator consists of an inductor and a capacitor as shown in figure below.

The value of the capacitor and resistor determines the output oscillation. So how do they generate oscillation?

Well, we need to apply external energy between L and C i.e. voltage. When we apply the voltage, the capacitor gets charged-up. When the supply is cut-off, the stored energy from capacitor flows to inductor and inductor starts building magnetic field around it until the capacitor completely gets discharged.

When the capacitor is fully discharged, the magnetic field around inductor collapse and induces voltage and charge-up the capacitor with opposite polarity and the cycle repeats.

The charge and discharge between L and C produces oscillation and this oscillation is called resonance frequency. However the frequency generation won’t last forever due to parasitic resistance which dissipates the energy in the oscillatory circuit in the form of heat.

To maintain the oscillation and use the oscillation with reasonable output strength, we need an amplifier with zero degree phase shift and feedback.

The feedback feed small amount of output from amplifier back to LC network to compensate the loss due to parasitic resistance and maintain the oscillation. Thus we can generate steady sine wave output.

Application circuit:

Here is a colpitts oscillator circuit which can generate around 30Mhz signal.

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