What is the difference between positive feedback and negative feedback
Positive feedback and negative feedback are two fundamental concepts in electronic circuits and systems. They describe how a system's output is fed back into its input to influence the behavior of the system. Here’s a detailed comparison:
Negative Feedback:
Definition:
- Negative feedback occurs when a portion of the output signal is inverted and fed back to the input, effectively reducing the input signal. This process helps stabilize the system and control its gain.
Characteristics:
- Stability: Negative feedback improves the stability of the system, making it less sensitive to component variations and external disturbances.
- Reduced Gain: It reduces the overall gain of the system, as part of the output is subtracted from the input.
- Improved Bandwidth: Negative feedback typically increases the bandwidth of the system, allowing it to respond to a wider range of frequencies.
- Distortion Reduction: It helps in reducing distortion and non-linearities in the system, resulting in a more accurate output.
- Impedance Effects: It increases the input impedance and decreases the output impedance, improving the interaction with other circuit stages.
Applications:
- Negative feedback is widely used in amplifiers, control systems, and operational amplifier circuits to achieve stable and predictable performance.
Example:
- An operational amplifier configured as a voltage follower (buffer) uses negative feedback to ensure the output voltage follows the input voltage exactly, providing unity gain with high input impedance and low output impedance.
Positive Feedback:
Definition:
- Positive feedback occurs when a portion of the output signal is fed back to the input in phase with the input signal, effectively increasing the input signal. This process can lead to amplification and potential instability.
Characteristics:
- Instability: Positive feedback tends to make the system less stable and can lead to oscillations if not properly controlled.
- Increased Gain: It increases the overall gain of the system, as part of the output is added to the input.
- Oscillations: In many cases, positive feedback is deliberately used to create oscillations, as in oscillators and signal generators.
- Reduced Bandwidth: Positive feedback can narrow the bandwidth of the system because it emphasizes certain frequencies over others.
- Sensitivity: It makes the system more sensitive to component variations and external disturbances.
Applications:
- Positive feedback is used in oscillators, bistable multivibrators (flip-flops), and certain types of amplifiers where intentional oscillation or hysteresis is required.
Example:
- A Schmitt trigger circuit uses positive feedback to create hysteresis, providing a sharp transition between high and low output states, which is useful for signal conditioning and noise immunity in digital circuits.
Key Differences:
| Aspect | Negative Feedback | Positive Feedback |
|---|---|---|
| Stability | Increases stability | Decreases stability |
| Gain | Reduces overall gain | Increases overall gain |
| Oscillations | Tends to dampen oscillations | Can cause or sustain oscillations |
| Bandwidth | Typically increases bandwidth | Typically reduces bandwidth |
| Distortion | Reduces distortion | Can increase distortion |
| Input Impedance | Increases input impedance | Varies (often decreases) |
| Output Impedance | Decreases output impedance | Varies (often increases) |
| Applications | Amplifiers, control systems, voltage regulators | Oscillators, Schmitt triggers, multivibrators |
In summary, negative feedback is generally used to stabilize and linearize systems, reducing distortion and broadening bandwidth, while positive feedback is used to create oscillations or hysteresis, which can be useful in specific applications like oscillators and digital circuits. Understanding the differences between these two types of feedback is crucial for designing and analyzing electronic systems.
