Colpitts Oscillator

Colpitts Oscillators

In some ways the Colpitts Oscillator is the opposite to the Hartley Oscillator we looked at in the previous tutorial. Like the Hartley oscillator, the tuned tank circuit consists of an LC resonance circuit connected between the Collector and Base of the transistor amplifier. The basic configuration of the Colpitts Oscillator resembles that of the Hartley Oscillator but the difference being is that the centre tapping is now made from a "Capacitive Voltage Divider" network instead of a tapped autotransformer type inductor as shown below.

Basic Colpitts Oscillator Circuit

The transistor amplifiers Emitter is connected to the junction of capacitors, C1 and C2 which are connected in series with the required external phase shift is obtained in a similar manner to that in the Hartley Oscillator. The amount of feedback is determined by the ratio of C1 and C2 which are generally "ganged" together to provide a constant amount of feedback. The frequency of oscillations for a Colpitts Oscillator is determined by the resonant frequency of the LC tank circuit and is given as:

where C_T is the capacitance of C1 and C2 connected in series and is given as:.

The configuration of the transistor amplifier is of a Common Emitter Amplifier with the output signal 180^o out of phase with regards to the input signal. The additional 180^o phase shift require for oscillation is achieved by the fact that the two capacitors are connected together in series but in parallel with the inductive coil resulting in overall phase shift of the circuit being zero or 360^o. Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in the normal manner while the capacitor acts as a DC-blocking capacitors.

Example No1

A Colpitts Oscillator circuit having two capacitors of 10pF and 100pF respectively are connected in parallel with an inductor of 10mH. Determine the frequency of oscillations of the circuit.

The frequency of oscillations for a Colpitts Oscillator is given as:

The circuit consists of two capacitors in series, so the total capacitance is given as:

The inductor is of 10mH then the frequency of oscillation is:

Then the frequency of oscillations for the Oscillator is 527.8kHz