Feedback in Amplifiers: Positive and Negative Types
Feedback in an amplifier means taking a part of the output and returning it to the input. If the returned signal adds to the input, it is positive feedback. If it opposes the input, it is negative feedback. Even a small feedback fraction can greatly change gain, distortion and stability.
Feedback is one of the most important ideas in electronics. Negative feedback makes amplifiers accurate and stable, while positive feedback is used to make oscillators that generate signals. Audio systems, radios and control systems all rely on choosing the right type and amount of feedback.
Key Concepts
- Feedback fraction βf: Part of output fed back, βf = Vf / Vout. It is dimensionless. Note: this beta is different from transistor current gain.
- Negative feedback: Feedback signal subtracts from input. Gain decreases but becomes more stable.
- Positive feedback: Feedback signal adds to input. Gain increases and the circuit may oscillate.
- Gain with feedback: Af = A / (1 + A βf) for negative feedback, where A is gain without feedback.
- Bandwidth: Negative feedback increases bandwidth while reducing gain.
- Distortion and noise: Negative feedback reduces distortion produced inside the amplifier.
How It Works
- The amplifier produces output from the difference between input and feedback signal in negative feedback.
- If output tries to rise too much, feedback also rises and reduces the effective input, correcting the change.
- This self-correction makes gain depend mainly on the feedback network, often just two resistors.
- In positive feedback, a rise in output increases input further, pushing the circuit to saturation or oscillation.
- When loop gain reaches the condition for sustained oscillation, the amplifier becomes a signal generator.
Worked Example
An amplifier without feedback has gain A = 1000 and feedback fraction βf = 0.009 with negative feedback. Find gain with feedback. Af = A / (1 + A βf) = 1000 / (1 + 1000 × 0.009) = 1000 / (1 + 9) = 1000 / 10 = 100. Gain falls from 1000 to 100, but it becomes far more stable against transistor variations.
Common Misconceptions
- Negative feedback is always bad because gain falls: The loss of gain buys stability, wider bandwidth and lower distortion.
- Positive feedback is a fault only: It is intentionally used in oscillators and Schmitt triggers.
- Feedback removes all distortion: It reduces distortion generated inside the loop, not distortion already present in the input.
Real-World Applications
- Op-amp circuits where gain is set precisely by resistors.
- Audio amplifiers with low distortion and flat response.
- Oscillators in radios, clocks and function generators.
- Automatic control systems in stabilised power supplies.
Negative feedback trades excess gain for accuracy, stability and wider bandwidth, while positive feedback creates oscillation. Controlling the returned signal is the key to reliable amplifier design.