--- title: "Ideal op-amp properties; non-inverting gain" subject: "Basic Electronics Engineering" chapter: 4 key: "semester_2/basic_electronics_engineering:opamp-props-noninverting-gain" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/opamp-props-noninverting-gain site: BCT Engineering --- # Ideal op-amp properties; non-inverting gain Basic Electronics Engineering · Chapter 4: The Operational Amplifier and Oscillator - Asked in: 2074 Bhadra, 2+4 marks; 2072 Magh, 2+4 marks (asked 2 times) - Video: [Operational Amplifiers - Inverting & Non Inverting Op-Amps](https://www.youtube.com/watch?v=idJEMYhrIfs) — The Organic Chemistry Tutor - 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-props-noninverting-gain - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-4 ## Question Mention any four properties of ideal Op-amp. Derive the expression of voltage gain of non-inverting amplifier using Op-amp. ## Short answer Ideal op‑amp properties: infinite open‑loop gain, infinite input resistance, zero output resistance, infinite bandwidth. For the non‑inverting amplifier, virtual short gives V– = Vin = Vo R1/(R1+Rf), so voltage gain Av = Vo/Vin = 1 + Rf/R1. ## Answer #### Four properties of an ideal op-amp 1. Infinite open-loop voltage gain. 2. Infinite input resistance, so no current flows into the input terminals. 3. Zero output resistance. 4. Infinite bandwidth and infinite CMRR (also zero offset voltage). ```text +Vcc |\ | V- o-| \ | | >--o Vo V+ o-| / |/ | -Vcc ``` #### Voltage gain of the non-inverting amplifier ```text |\ Vin o--------|+ \ | >------+------o Vo +----|- / | | |/ [Rf] | | +---------------+ | [R1] | GND ``` R1 is connected from the inverting input to ground and Rf from the output to the inverting input (negative feedback). The input Vin is applied directly to the non-inverting terminal. **Derivation** (ideal op-amp) - No current enters the op-amp inputs. - Virtual short: V− = V+ = Vin. - The voltage at the inverting node is set by the divider R1, Rf: V− = Vo × R1/(R1 + Rf). Equating V− = Vin: Vin = Vo R1/(R1 + Rf) **Av = Vo/Vin = 1 + Rf/R1** The gain is positive (the output is in phase with the input) and always at least 1. For Rf = 0 or R1 = ∞ (open), the circuit becomes a unity gain buffer (voltage follower), Av = 1. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is true of an ideal op-amp? A. Infinite input resistance and zero output resistance B. Zero input resistance and infinite output resistance C. Zero gain D. Zero bandwidth Answer: A. Ideal op-amps draw no input current and have zero output resistance, with infinite gain, bandwidth and CMRR. (Key point: Ideal properties) 2. Which assumption is used to derive the non-inverting gain? A. V- = V+ = Vin (virtual short) and no input current B. V- = 0 C. Vo = Vin D. R1 = Rf Answer: A. The input is at V+, so the inverting node is also at Vin, and it is set by the Rf, R1 divider. (Key point: Key assumption) 3. What is the voltage gain of the non-inverting amplifier? A. -Rf/R1 B. 1 + Rf/R1 C. Rf/R1 D. 1 - Rf/R1 Answer: B. Vin = Vo R1/(R1 + Rf) gives Av = 1 + Rf/R1, positive and at least 1. (Key point: Result) 4. What happens when Rf = 0 (or R1 = ∞)? A. Gain is infinite B. Unity gain buffer (voltage follower), Av = 1 C. Gain is -1 D. Output is zero Answer: B. Av = 1 + 0 = 1, so the circuit follows the input. (Key point: Special case) ## Close variants set in other exams - [Ideal op-amp properties; inverting gain](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-inverting-gain) — Basic Electronics Engineering, chapter 4, asked 3× ## Related in Basic Electronics Engineering - [Voltage gain of non-inverting op-amp](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-noninverting-gain) — Basic Electronics Engineering, chapter 4, asked 2× - [Op-amp properties and summing amplifier](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-summer-v1-2v2-3v3) — Basic Electronics Engineering, chapter 4 - [Properties of ideal op-amp](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aideal-opamp-properties) — Basic Electronics Engineering, chapter 4, asked 2× - [Op-amp properties and integrator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-integrator) — Basic Electronics Engineering, chapter 4 - [Op-amp properties; square wave generator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-five-props-square-wave) — Basic Electronics Engineering, chapter 4 - [Op-amp properties and differentiator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-differentiator) — Basic Electronics Engineering, chapter 4, asked 2× ## Related in other subjects - [Non-inverting op-amp voltage gain](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Anon-inverting-amplifier-gain) — Fundamental of Electrical and Electronics Engineering, chapter 6 - [Output voltage of two op-amp IA](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atwo-opamp-ia-output) — Advanced Electronics, chapter 4 - [Input impedance of inverting op-amp](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainverting-input-impedance) — Advanced Electronics, chapter 2 - [Output offset voltage of 741 inverting amplifier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoutput-offset-741-inverting) — Advanced Electronics, chapter 2