Wednesday, February 26, 2014

Colpitts Oscillator

Good evening everyone, today I am going to write about Colpitts Oscialltor, those folks who are studying Electrical & Electronic Engineering or Communication or maybe Applied Physics might find this post helpful.

This oscillator was invented by an American engineer called Edwin H. Colpitts in 1918. This oscillator uses both capacitor and inductor to produce an oscillation at a certain frequency. The distinguishing feature of the Colpitts oscillator is that the feedback for the active device is taken from a voltage divider made of two capacitors in series across the inductor. So Colpitts oscillator is basically a LC oscillator. 

We know how LC oscillator works, in a LC oscillator there is a capacitor which becomes charged and discharge through a inductor, as a result capacitor gets depleted, then the inductor charges the capacitor again and again and magnetic field around it depletes, then again capacitor charges the inductor and this process goes on and on. Thus creating an oscillation. But in real life situation capacitor and inductor have resistance which draws power so the oscillation stops at a certain point unless power is given to the circuit properly.

Lets take a look at the circuit diagram first. 

From the diagram we can see that the feedback circuit is made of two tapped capacitors with an common inductor. So the whole tank circuit is made up of C1, C2 and L. This circuit uses one single transistor to amplify. The RF choke simply decouples any ac signal in the power lines from affecting the output signal.

How it Works :

When the circuit is powered, the capacitors C1 and C2 are charged. Capacitors discharges through the inductor L, setting up oscillations of frequency determined by its frequency expression. As shown on the figure, it is clear that the output voltage of the amplifier appears across capacitor,C1 and feedback voltage developed across C2. The voltage across C2 is 180degree out of phase with the voltage across C1 or Vout. Voltage feedback to the transistor provides positive feedback. A phase shift of 180degree is produced by the transistor and a further phase shift of 180degree is produced by the C1-C2 voltage divider. This way the feedback is properly phased to produce continuous undamped oscillation.

In this oscillator there is a LC circuit in the feedback loop to provide the necessary phase shift and
to act as a resonant filter that passes only the desired frequency of oscillation.

Frequency of Colpitts oscillator 

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