--- title: "Feedback concept and square wave oscillator" subject: "Basic Electronics Engineering" chapter: 4 key: "semester_2/basic_electronics_engineering:feedback-theory-square-wave" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/feedback-theory-square-wave site: BCT Engineering --- # Feedback concept and square wave oscillator Basic Electronics Engineering · Chapter 4: The Operational Amplifier and Oscillator - Asked in: 2071 Magh, 2+4 marks - Video: [How Oscillator Works ? The Working Principle of the Oscillator Explained](https://www.youtube.com/watch?v=XVS8Puf4tiw) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/feedback-theory-square-wave - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-4 ## Question Explain the concept of feedback theory. Describe the working principle of square wave oscillator circuit using op-amp. ## Short answer Feedback returns part of output to input; negative feedback (opposite phase) reduces gain and improves stability via Af=A/(1+Aβ), while positive feedback (in phase) increases gain and causes oscillation when Aβ=1. The square wave oscillator uses an op-amp comparator with positive feedback and RC charging; capacitor voltage ramps between ±βVsat, output toggles, generating a square wave with f≈1/(2.2RC) for R1=R2. ## Answer #### Feedback theory Feedback means returning a part of the output signal to the input. Let the amplifier gain be A and the feedback fraction be β. - Feedback signal = β Vo, and the input to the amplifier is Vi' = Vs ± β Vo. - **Negative feedback** (feedback opposes the input): Af = A/(1 + Aβ). The gain falls, but stability improves, distortion and noise are reduced, and the bandwidth increases. - **Positive feedback** (feedback adds to the input): Af = A/(1 − Aβ). The gain increases; when Aβ = 1, Af = ∞ and the circuit gives an output without any input, i.e. it oscillates. Oscillators use positive feedback and need |Aβ| = 1 with a 0° or 360° loop phase shift (Barkhausen criteria). #### Square wave oscillator using op-amp ```text +----------[ R ]-----------+ | | | |\ | +---------|- \ | | | >------------+----o Vo === C +--|+ / | | | |/ | GND +-------[ R2 ]------+ | [ R1 ] | GND ``` The op-amp works as a comparator. The resistors R2 and R1 supply positive feedback, with β = R1/(R1 + R2). R and C set the timing. 1. Output at +Vsat: the non-inverting input is at +βVsat, and C charges through R. 2. When Vc > +βVsat, the output switches to −Vsat and the threshold changes to −βVsat. 3. C now discharges and charges negatively. When Vc < −βVsat, the output switches back to +Vsat. 4. This repeats, giving a symmetrical square wave. T = 2RC ln[(1 + β)/(1 − β)]. With R1 = R2, T ≈ 2.2 RC. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the gain with negative feedback? A. Af = A/(1 − Aβ) B. Af = A/(1 + Aβ) C. Af = A(1 + Aβ) D. Af = Aβ Answer: B. Negative feedback subtracts from the input, so gain falls to A/(1+Aβ). (Key point: Negative feedback gain) 2. What happens in positive feedback when Aβ = 1? A. Gain becomes zero B. Gain becomes stable at A C. Output is clipped to zero D. Af becomes infinite and the circuit oscillates without input Answer: D. Af = A/(1 − Aβ) tends to infinity, i.e. output without input. (Key point: Positive feedback) 3. In the square wave oscillator, what happens when Vc < −βVsat while output is −Vsat? A. The output stays at −Vsat B. The output switches to +Vsat C. The circuit turns off D. The output becomes βVsat Answer: B. The inverting input falls below the −βVsat reference, so the comparator flips. (Key point: Threshold) 4. What is the period T, and its value when R1 = R2? A. T = RC ln 2 ≈ 0.69RC B. T = 2πRC C. T = 2RC ln 3 ≈ 2.2RC only if R1 = 2R2 D. T = 2RC ln[(1+β)/(1−β)] ≈ 2.2RC Answer: D. With β = 0.5, (1.5/0.5) = 3, so T = 2RC ln 3 ≈ 2.2RC. 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