Figure 1 shows the circuit diagram for colpitt’s oscillator. It consist of tank circuit which is made up of two capacitors connected in series and an inductor is connected parallel to this series combination. The frequency of oscillator is determined by the value of capacitor and inductor.

RFC (Radio Frequency Choke) is used to prevent high frequency oscillations passing through power supply.

In this circuit resistor R1 and R2 provides voltage divider biasing to the transistor Q1.Cin is the input DC decoupling capacitor while Cout is the output decoupling capacitor. Re is the emitter resistor which is bypassed by capacitor Ce. If bypass capacitor is not used then AC signal will drop across$R_E$ , hence it will alter DC bias condition of transistor and reduces gain.

Feedback is taken from junction of capacitor $C_2$ and inductor L & fed back to the base of transistor Q1.

When power supply is switched ON, noise in the circuit due power supply and resistors gets amplified and capacitor C1 and C2 starts charging. When they are fully charged they starts discharging through the inductor L.

When the capacitors are fully discharged, the electrostatic energy stored in the capacitors gets transferred to the inductor as magnetic flux. The inductor starts discharging and capacitors gets charged again.

This transfer of energy back and forth between capacitors and inductor is the basis of oscillation. Voltage across$ C_2$ is in opposite phase to that of the voltage across the $C_1 $and the voltage across $C_2$ that is fed back to the transistor.

The feedback signal at the base of transistor appears in the amplified form across the collector and emitter of the transistor.

The energy lost in the tank circuit is compensated by the transistor and the oscillations are sustained.

The tank circuit produces 180° phase shift and the transistor itself produces another 180° phase shift. That means the input and output are in phase and it is a necessary condition of positive feedback for maintaining sustained oscillations.

The frequency of oscillations of the colpitt’s oscillator is given by,

$F=\frac{1}{2 \pi \sqrt {LC}}$

Where,

L= Inductance of inductor in tank circuit

C= Effective capacitance of capacitors of tank circuit = C1C2/C1+C2

Typically used in RF application and operating range is 20KHz to 300MHz.

Advantages:

Improved performance in high frequency region.

Simple Construction.

Disadvantages:

Because of inductor L circuit becomes bulky and cost of circuit is more.

Poor frequency stability.

Applications:

RF application.

Microwave application.

High frequency generator.