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 an inductor and a capacitive voltage divider network. This feedback is essential for sustaining oscillations. The Colpitts oscillator is a type of electronic oscillator that typically uses a combination of inductance (L) and capacitance (C) to generate a periodic output signal, such as a sine wave.
How Positive Feedback Works in a Colpitts Oscillator:
The Colpitts oscillator typically consists of the following components:
- An LC tank circuit (comprising an inductor and a pair of capacitors)
- A transistor (typically a bipolar junction transistor or FET)
- A biasing network to ensure proper operation of the transistor
The capacitive voltage divider is formed by two capacitors, ( C_1 ) and ( C_2 ), connected in series between the transistor’s collector (or drain in the case of a FET) and the emitter (or source for FET). The junction between these two capacitors provides the feedback signal. The inductor, ( L ), is typically placed in series with the collector to help form the resonance condition necessary for oscillations.
Process of Positive Feedback:
- Initial Oscillation: The transistor is biased to operate in an active region. Initially, a small noise or disturbance is needed to kickstart the oscillations. This disturbance gets amplified by the transistor.
- Voltage Divider Effect: The capacitors ( C_1 ) and ( C_2 ) form a voltage divider. The voltage at the junction of these capacitors provides the necessary feedback to the transistor’s base (or gate), with the correct phase shift to sustain oscillations.
- Sustaining Oscillations: The feedback signal must be in phase with the input signal at the base (or gate) of the transistor. The capacitive divider ensures that this condition is met, providing the positive feedback required for continuous oscillations.
The value of ( C_1 ), ( C_2 ), and ( L ) determines the frequency of the oscillation. When the Colpitts oscillator is in steady-state, the loop gain (product of the amplifier gain and the feedback signal) equals or exceeds one, ensuring sustained oscillations without any external input signal.
In summary, the positive feedback in a Colpitts oscillator is supplied by the capacitive voltage divider, which ensures that the phase of the feedback matches the requirement for oscillation, maintaining the ongoing signal generation in the circuit.