How is positive feedback supplied in a colpitts oscillator?
The Correct Answer and Explanation is :
In a Colpitts oscillator, positive feedback is supplied by the combination of inductive and capacitive elements in the feedback network. The basic structure of a Colpitts oscillator consists of a transistor (typically a BJT or FET), an inductor, and a pair of capacitors. The key to the oscillator’s operation lies in the capacitive voltage divider that provides the necessary positive feedback to sustain oscillations.
Positive Feedback in a Colpitts Oscillator:
In a Colpitts oscillator, the positive feedback is provided by the capacitive divider formed by two capacitors, ( C_1 ) and ( C_2 ), which are connected in series between the collector and ground. The junction between these two capacitors is connected to the base of the transistor, which provides the necessary input for the transistor’s amplification.
The inductor, typically placed in the emitter (in a common emitter configuration) or collector path, acts as the reactive component that helps define the frequency of oscillation. The inductor’s role is to store energy in the form of a magnetic field, which then contributes to the oscillation cycle when combined with the capacitors.
How the Feedback Works:
- Capacitive Voltage Divider: The two capacitors, ( C_1 ) and ( C_2 ), form a voltage divider between the output (collector) and the ground. The feedback voltage is taken from the junction between ( C_1 ) and ( C_2 ), which is fed to the base of the transistor.
- Transistor Amplification: The transistor amplifies the input signal received at its base. In a properly designed oscillator, the phase shift through the active device and the feedback network ensures that the feedback voltage is in phase with the input signal at the base. This phase relationship is crucial for sustaining oscillations.
- Sustained Oscillations: To achieve sustained oscillations, the loop gain (product of the transistor gain and the feedback network gain) must be greater than or equal to unity, and the phase shift must be 360° (or 0°). The feedback through the capacitors helps maintain the conditions necessary for continuous oscillations.
In summary, the Colpitts oscillator relies on the capacitive feedback network to supply the necessary positive feedback for oscillations, with the frequency of oscillation determined by the inductor and the two capacitors.