--- title: "Op-amp properties; square wave generator" subject: "Basic Electronics Engineering" chapter: 4 key: "semester_2/basic_electronics_engineering:opamp-five-props-square-wave" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/opamp-five-props-square-wave site: BCT Engineering --- # Op-amp properties; square wave generator Basic Electronics Engineering · Chapter 4: The Operational Amplifier and Oscillator - Asked in: 2072 Magh, 2+4 marks - Video: [Astable Multivibrator (using op-amp) Explained](https://www.youtube.com/watch?v=T7T3At9N9dk) — 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/opamp-five-props-square-wave - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-4 ## Question Mention any five properties of ideal Op-Amp. Draw the circuit diagram and explain the operation of square wave generator using Op-Amp. ## Short answer Ideal op-amp properties: infinite open-loop gain, infinite input impedance, zero output impedance, infinite bandwidth, infinite CMRR. The square wave generator is an astable multivibrator with a comparator and RC network; the capacitor charges/discharges between ±βVsat, toggling the output to produce a square wave of period T≈2.2RC when R1=R2. ## Answer #### Properties of an ideal op-amp 1. Infinite open-loop voltage gain (A = ∞). 2. Infinite input impedance (Zin = ∞), so the input current is zero. 3. Zero output impedance (Zout = 0). 4. Infinite bandwidth (response from 0 Hz to ∞). 5. Infinite common-mode rejection ratio (CMRR = ∞), so common-mode noise is rejected. 6. Zero offset voltage: the output is zero when both inputs are zero. #### Square wave generator (astable multivibrator) A square wave generator is an op-amp used as a comparator with positive feedback, together with an RC charging network. It needs no input signal. ```text +----------[ R ]-----------+ | | | |\ | +---------|- \ | | | >------------+----o Vo === C +--|+ / | | | |/ | GND +-------[ R2 ]------+ | [ R1 ] | GND ``` Circuit: the output is connected through R to the inverting terminal, where capacitor C connects to ground. The non-inverting terminal receives a fraction of the output through the divider formed by R2 (from the output) and R1 (to ground). Feedback fraction: β = R1/(R1 + R2). **Operation** 1. Suppose the output is at +Vsat. The reference at the non-inverting input is +βVsat. C charges towards +Vsat through R. 2. When the capacitor voltage reaches +βVsat, the inverting input exceeds the non-inverting input and the output switches to −Vsat. 3. The reference now becomes −βVsat. C discharges and then charges towards −Vsat. 4. When the capacitor voltage reaches −βVsat, the output switches back to +Vsat. 5. This repeats, giving a square wave between +Vsat and −Vsat. The capacitor voltage rises and falls exponentially between ±βVsat. **Time period:** T = 2RC ln[(1 + β)/(1 − β)] If R1 = R2 (β = 0.5), T = 2RC ln 3 ≈ 2.2 RC, so f = 1/(2.2 RC). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is NOT a property of an ideal op-amp? A. Infinite CMRR B. Infinite bandwidth C. Finite output resistance D. Zero offset voltage Answer: C. Ideal output resistance is zero. (Key point: Ideal op-amp properties) 2. When does the output switch from +Vsat to -Vsat? A. When the capacitor voltage reaches +βVsat B. When the capacitor voltage reaches zero C. When the capacitor voltage reaches +Vsat D. When the input signal is applied Answer: A. The non-inverting input is at +βVsat; once Vc exceeds it, the comparator flips. (Key point: Switching thresholds) 3. What kind of circuit is the op-amp square wave generator? A. An integrator with negative feedback only B. A differential amplifier with no feedback C. A comparator with positive feedback and an RC timing network D. A unity-gain buffer Answer: C. The R1-R2 divider gives positive feedback and R, C set the timing; the output switches between +Vsat and -Vsat. (Key point: Comparator with positive feedback) 4. What is the period T, and its value when R1 = R2? A. T = 2RC ln[(1+β)/(1−β)] ≈ 2.2RC B. T = RC ln 2 ≈ 0.69RC C. T = 2πRC D. T = 2RC ln 3 ≈ 2.2RC only if R1 = 2R2 Answer: A. With β = 0.5, (1.5/0.5) = 3, so T = 2RC ln 3 ≈ 2.2RC. 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