What is a Colpitts Oscillator | How to Construct a Colpitts Oscillator

Colpitts oscillator is a linear oscillator and it generates radio frequency signals in the range of 20 kHz to 300 MHz. The Colpitts oscillator was invented in 1918 by an American engineer called Edwin H Colpitts. Colpitts oscillator consists of an amplifier and a positive feedback circuit for its functioning. The amplifier uses an active device such as a bipolar transistor, field effect transistor or vacuum tube as an amplifier and uses a positive feedback from its output connected to the input by means of a parallel LC tuned circuit or tank circuit, which determines the frequency of oscillation.

Operation of a Colpitts Oscillator
In an oscillator, the amplification of the active device should be larger than the losses due to the attenuation at the capacitive voltage divider network for the oscillator to work constantly.

 

In an oscillator, the thermal noise in the amplifier gets amplified, which contains a number of different frequency components. The positive feedback circuit or the LC tank circuit selects a particular frequency from this range of frequencies and feeds it back to the input of the amplifier. In a Colpitts oscillator, the LC circuit consists of a series combination of two capacitors in parallel with the inductor. The frequency of the oscillator is approximately the resonant frequency of this LC tank circuit.

 

Working of Colpitts Oscillator
The voltage gain of the amplifier is denoted as A. The portion of the output signal of the amplifier that is fed back to the input of the amplifier is called the feedback factor which is denoted as beta. The total loop gain calculated as the product of A and beta is called as the A beta. The feedback circuit provides 180 degree phase shift and the amplifier provides another 180 degree of phase shift. So, there will be either a 0 degree or 360 degree phase shift happening in the signal during this process. The oscillator works only when the A beta becomes equal to 1 and there is 0 degree or 360 degrees phase shift in the signal. This is called the the Barkhausen criteria.

Barkhausen criteria is satisfied by the formula

A beta = 1
Beta = C1/C2
A > C2/C1

Frequency of the Colpitts Oscillator
The resonant frequency of the Colpitts oscillator is determined by the formula:

Fr = 1/2π√LC

where C= C1C2/C1+C2

In this oscillator, the frequency can be changed by varying the inductance using a variable inductor or by means of adding a variable capacitor connected parallel to the inductor. There are two types of Colpitts oscillator, the common base Colpitts oscillator and the common collector Colpitts oscillator.

Applications of Colpitts Oscillator

1. It is used as an oscillator in radio receivers.
2. It is used as a beat frequency oscillator in single side-band receivers.
3. It is used as a local oscillator in transceivers.
4. It is used as a carrier oscillator in transmitters.
5. It is used in signal generators.
6. It is used in radio direction finders and sensors.

Watch the Video: https://youtu.be/8xA2s3ibPfM