Which type of feedback is used in case of an oscillator circuit?
Understanding Feedback in Oscillator Circuits
Oscillator circuits are electronic circuits that produce a periodic oscillating electronic signal, such as sine wave or square wave. Unlike amplifiers which increase the amplitude of an input signal, oscillators generate their own output signal without any external input signal.
What is Feedback?
Feedback is a process where a portion of the output signal of a system is fed back to the input. Feedback can be broadly classified into two types:
- Positive Feedback: A portion of the output signal is fed back to the input in phase with the input signal. This increases the overall input signal.
- Negative Feedback: A portion of the output signal is fed back to the input out of phase (typically 180 degrees) with the input signal. This reduces the overall input signal.
Feedback Type for Oscillators
Oscillator circuits require a specific type of feedback to sustain continuous oscillations. Let's look at why different types of feedback are used in electronic circuits:
- Negative Feedback: This is widely used in amplifier circuits. It helps to stabilize the gain, reduce distortion, increase bandwidth, and improve input/output impedance characteristics. Negative feedback opposes the input signal.
- Positive Feedback: In contrast, positive feedback reinforces the input signal. When used in an oscillator, a small noise signal at the input is amplified, and a portion of this output is fed back in phase to the input. If the loop gain (product of amplifier gain and feedback network gain) is sufficiently high and the phase shift around the loop is \(0^\circ\) or \(360^\circ\), this process continues, and the signal grows until limited by the circuit's characteristics, resulting in sustained oscillations.
Therefore, for an oscillator circuit to generate and sustain oscillations, it requires positive feedback.
Comparing Feedback Types
| Feature | Positive Feedback | Negative Feedback |
|---|---|---|
| Effect on Signal | Reinforces | Opposes |
| Phase Shift (Feedback loop) | \(0^\circ\) or \(360^\circ\) | \(180^\circ\) |
| Typical Application | Oscillators | Amplifiers |
| Effect on Gain | Increases (can lead to instability) | Decreases (improves stability) |
Based on this, it is clear that oscillator circuits utilize positive feedback to create the conditions necessary for self-sustaining oscillation.
Revision Table: Oscillator Feedback
| Concept | Description |
|---|---|
| Oscillator Purpose | Generate periodic signals without external input. |
| Feedback Role | Portion of output fed back to input. |
| Required Feedback Type | Positive Feedback. |
| Why Positive Feedback? | Reinforces signal to sustain oscillations. |
| Contrast with Negative Feedback | Negative feedback used in amplifiers for stability/gain control. |
Additional Information: Barkhausen Criterion
For sustained oscillations to occur in a linear circuit using positive feedback, two conditions, known as the Barkhausen criterion, must be met:
- The loop gain (\(\text{A}\beta\)) must be equal to unity in magnitude, i.e., \(|\text{A}\beta| = 1\). Here, A is the gain of the amplifier and \(\beta\) is the gain of the feedback network.
- The total phase shift around the loop (amplifier and feedback network) must be zero or an integer multiple of \(360^\circ\), i.e., \(\angle \text{A}\beta = 0^\circ\) or \(360^\circ\).
These conditions ensure that the signal fed back is in phase with the input and has sufficient amplitude to sustain the oscillation.
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