Colpitts Oscillator

A Colpitts oscillator looks just like the Hartley oscillator but the inductors and capacitors are replaced with each other in the tank circuit. The constructional details and operation of a colpitts oscillator are as discussed below.

Construction

Let us first take a look at the circuit diagram of a Colpitts oscillator.

The resistors R₁, R₂ and R_e provide necessary bias condition for the circuit. The capacitor C_e provides a.c. ground thereby providing any signal degeneration. This also provides temperature stabilization.

The capacitors C_c and C_b are employed to block d.c. and to provide an a.c. path. The radio frequency choke (R.F.C) offers very high impedance to high frequency currents which means it shorts for d.c. and opens for a.c. Hence it provides d.c. load for collector and keeps a.c. currents out of d.c. supply source.

Tank Circuit

The frequency determining network is a parallel resonant circuit which consists of variable capacitors C₁ and C₂ along with an inductor L. The junction of C₁ and C₂ are earthed. The capacitor C₁ has its one end connected to base via C_c and the other to emitter via C_e. the voltage developed across C₁ provides the regenerative feedback required for the sustained oscillations.

Operation

When the collector supply is given, a transient current is produced in the oscillatory or tank circuit. The oscillatory current in the tank circuit produces a.c. voltage across C₁ which are applied to the base emitter junction and appear in the amplified form in the collector circuit and supply losses to the tank circuit.

If terminal 1 is at positive potential with respect to terminal 3 at any instant, then terminal 2 will be at negative potential with respect to 3 at that instant because terminal 3 is grounded. Therefore, points 1 and 2 are out of phase by 180^o.

As the CE configured transistor provides 180^o phase shift, it makes 360^o phase shift between the input and output voltages. Hence, feedback is properly phased to produce continuous Undamped oscillations. When the loop gain |βA| of the amplifier is greater than one, oscillations are sustained in the circuit.

Frequency

The equation for frequency of Colpitts oscillator is given as

$$f = \frac{1}{2 \pi \sqrt{LC_T}}$$

C_T is the total capacitance of C₁ and C₂ connected in series.

$$\frac{1}{C_T} = \frac{1}{C_1} + \frac{1}{C_2}$$

$$C_T = \frac{C_1 \times C_2}{C_1 + C_2}$$

Advantages

The advantages of Colpitts oscillator are as follows −

• Colpitts oscillator can generate sinusoidal signals of very high frequencies.
• It can withstand high and low temperatures.
• The frequency stability is high.
• Frequency can be varied by using both the variable capacitors.
• Less number of components are sufficient.
• The amplitude of the output remains constant over a fixed frequency range.

The Colpitts oscillator is designed to eliminate the disadvantages of Hartley oscillator and is known to have no specific disadvantages. Hence there are many applications of a colpitts oscillator.

Applications

The applications of Colpitts oscillator are as follows −

• Colpitts oscillator can be used as High frequency sinewave generator.
• This can be used as a temperature sensor with some associated circuitry.
• Mostly used as a local oscillator in radio receivers.
• It is also used as R.F. Oscillator.
• It is also used in Mobile applications.
• It has got many other commercial applications.