--- title: "Properties of ideal op-amp" subject: "Basic Electronics Engineering" chapter: 4 key: "semester_2/basic_electronics_engineering:ideal-opamp-properties" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/ideal-opamp-properties site: BCT Engineering --- # Properties of ideal op-amp Basic Electronics Engineering · Chapter 4: The Operational Amplifier and Oscillator - Asked in: 2070 Bhadra, 2+4 marks; 2067 Chaitra, 3 marks (asked 2 times) - Video: [Introduction to Operational Amplifier: Characteristics of Ideal Op-Amp](https://www.youtube.com/watch?v=kiiA6WTCQn0) — 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/ideal-opamp-properties - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-4 ## Question State any (four/six) important properties of an ideal operational amplifier. ## Short answer An ideal op‑amp has infinite open‑loop gain, infinite input resistance, zero output resistance, infinite bandwidth, infinite CMRR, and zero offset. In negative feedback, input currents are zero and the two input terminals are virtually shorted (equal voltage). ## Answer An **operational amplifier (op-amp)** is a high-gain, direct-coupled differential amplifier with a high input impedance and a low output impedance. An **ideal op-amp** has the following properties: ```text +Vcc |\ | V- o-| \ | | >--o Vo V+ o-| / |/ | -Vcc ``` 1. **Infinite open-loop voltage gain (AOL = ∞):** a tiny input difference gives a large output, so the differential input voltage is practically zero in linear operation (virtual short). 2. **Infinite input resistance (Ri = ∞):** no current flows into either input terminal. 3. **Zero output resistance (Ro = 0):** the output behaves as an ideal voltage source and can drive any load without a voltage drop. 4. **Infinite bandwidth:** the gain is constant at all frequencies from dc to infinity. 5. **Infinite common-mode rejection ratio (CMRR = ∞):** a common-mode signal produces no output. 6. **Zero offset voltage:** the output is exactly zero when both inputs are zero. 7. **Infinite slew rate:** the output can change instantly. 8. **Zero drift:** the characteristics do not change with temperature. | Property | Ideal | Typical 741 | |---|---|---| | Gain | ∞ | 2 × 10⁵ | | Ri | ∞ | 2 MΩ | | Ro | 0 | 75 Ω | | Bandwidth | ∞ | 1 MHz (unity gain) | | CMRR | ∞ | 90 dB | | Slew rate | ∞ | 0.5 V/µs | Two golden rules follow for circuits with negative feedback: (i) no current enters the input terminals, and (ii) V+ = V− (virtual short). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the consequence of infinite input resistance of an ideal op-amp? A. No current flows into either input terminal B. Output current is zero C. Gain is zero D. Output resistance is infinite Answer: A. Infinite Ri means the input terminals draw no current. (Key point: Input resistance) 2. What does zero output resistance imply? A. The output acts as an ideal voltage source that can drive any load without drop B. The output cannot supply current C. The gain is zero D. The bandwidth is zero Answer: A. With Ro = 0 the output voltage does not fall when load current is drawn. (Key point: Output resistance) 3. Why is the differential input voltage practically zero in linear operation? A. The open-loop gain is infinite so a tiny input gives a large output B. Input resistance is zero C. CMRR is zero D. Offset is large Answer: A. Because AOL → ∞, the input difference needed is almost zero (virtual short, V+ = V-). (Key point: Virtual short) 4. Which set lists ideal op-amp values correctly? A. Bandwidth ∞, CMRR ∞, slew rate ∞, offset 0 B. Bandwidth 1 MHz, CMRR 90 dB, slew rate 0.5, offset 1 V C. Bandwidth 0, CMRR 0, slew rate 0, offset ∞ D. Bandwidth ∞, CMRR 0, slew rate 0, offset ∞ Answer: A. Ideal values are infinite bandwidth, CMRR and slew rate, and zero offset; the 741 figures are typical real values. 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