Q. Draw a neat circuit of Colpitt's oscillator using an n-p-n transistor. Give its equivalent circuit obtain expressions for (i) frequency of oscillation and(ii) minimum gain for sustained oscillations.

The Colpitt's oscillator circuit is a superb circuit and is widely used in commercial signal generators upto 100 MHz. The basic circuit of a Colpitt's oscillator is shown in fig.It basically consists of a single stage inverting amplifier and an L-C phase shift net,vork, as obvious from the circuit diagram shown. The two series capacitors C1 and C2 form the potential divider used for providing the feedback voltage-the voltage developed across capacitor C2 provides the regenerative feedback required for sustained oscillations. Parallel combination of RE and CE along with resistors Rl andR2 provides the stabilized self bias. The collector supply voltage V cc is applied to the collector through a radio.frequency choke (RFC) which permits an easy flow of direct current but at the same time it offers + Vcc very high impedance to the high frequency currents. The presence of coupling capacitor Cc in the circuit , which blocks dc but provides path for ac.

Transistor itself produces a phase shift of 180 and another phase shift of 180 is provided by the capacitive feedback. Thus a total phase shift of 360 is obtained which is an essential condition for developing oscillations. The output voltage is derived from a secondary winding L' coupled to Basic Circuit For Colpitts Oscillator. the inductance L. The frequency is determined by Fig. the tank circuit and is varied by gang-tuning the two capacitors CI and Cz. It is to be noted that capacitors C1 and Cz are ganged. As the tuning is varied, values of both capacitors vary simultaneously, the ratio of the two capacitances remaining the same.