Side-by-side comparison
| Parameter | Ideal | Non-Ideal Op-Amp |
|---|---|---|
| Open-loop voltage gain (A_OL) | Infinite | LM741: ~200,000 (106 dB); TL071: ~200 V/μV |
| Input impedance | Infinite | LM741: 2 MΩ; TL071 (JFET): ~10^12 Ω |
| Output impedance | Zero | LM741: ~75 Ω; TL071: ~100 Ω |
| Bandwidth | Infinite | LM741 GBW: 1 MHz; TL071 GBW: 3 MHz |
| Slew Rate | Infinite (output follows input instantly) | LM741: 0.5 V/μs; TL071: 13 V/μs |
| Input offset voltage (V_OS) | Zero | LM741: ±5 mV typ; TL071: ±3 mV typ |
| Input bias current (I_B) | Zero | LM741: ~80 nA; TL071: ~65 pA |
| CMRR | Infinite | LM741: 90 dB typ; TL071: 86 dB typ |
| PSRR | Infinite | LM741: 96 dB; degrades at higher frequencies |
| Output voltage swing | ±V_supply (rail-to-rail assumed) | LM741 on ±15 V: output limited to ±13 V typ |
Key differences
The ideal model assumes infinite gain and bandwidth, so closed-loop gain is set purely by resistors. In reality, LM741 gain-bandwidth product is 1 MHz — close a loop for gain of 100, and usable bandwidth collapses to 10 kHz. Slew rate is the harsher limit: LM741 at 0.5 V/μs cannot swing a 10 V, 100 kHz sine wave (needs 6.28 V/μs). Input offset voltage of ±5 mV in a gain-1000 amplifier produces a ±5 V DC error at output — catastrophic for sensor circuits without trimming.
When to use Ideal
Use ideal op-amp assumptions when doing first-pass circuit analysis to find closed-loop gain, input/output impedance formulae, and filter pole frequencies — all standard university exam scenarios.
When to use Non-Ideal Op-Amp
Account for non-ideal parameters when designing a real circuit: for a precision strain-gauge amplifier using INA128, input offset voltage, bias current cancellation, and CMRR (minimum 90 dB) all appear in the error budget.
Recommendation
For exam analysis questions, start with the ideal model to derive gain expressions quickly. For design questions with precision or high-frequency requirements, choose a real op-amp like TL071 or OPA2134 and verify slew rate and GBW against your specs.
Exam tip: Examiners specifically test the gain-bandwidth product relationship — if GBW = 1 MHz and closed-loop gain = 50, the bandwidth is 20 kHz; expect a numerical question on this and on slew-rate-limited frequency.
Interview tip: Interviewers at analog design companies ask you to explain why a unity-gain buffer using LM741 distorts a 1 MHz sine wave — answer using slew rate: 0.5 V/μs × 10^6 = 0.5 V peak output, not the full supply swing.