Op-Amps Explained: Fundamentals, Configurations, and Applications
Operational amplifiers, or op-amps, are the workhorses of analog electronics. They are used in amplifiers, filters, data converters, and countless circuits. If you are preparing for analog design interviews or just starting out, mastering op-amps is essential.
1. What is an Op-Amp?
An operational amplifier is a high-gain differential amplifier with two inputs (inverting and non-inverting) and one output. In simple terms, it amplifies the voltage difference between its inputs.
Symbol
An op-amp is represented as a triangle with a “+” (non-inverting input), a “–” (inverting input), and the output on the opposite side.
2. Ideal vs Practical Op-Amps
In theory, op-amps are ideal devices. In practice, they have limitations.
| Characteristic | Ideal Op-Amp | Practical Op-Amp |
|---|---|---|
| Open-loop gain | ∞ | ~105–106 |
| Input resistance | ∞ | ~106–1012 Ω |
| Output resistance | 0 | ~10–100 Ω |
| Bandwidth | ∞ | Limited by GBW product |
3. Golden Rules of Op-Amps
- The input terminals draw negligible current.
- The op-amp output adjusts to keep inputs at nearly equal voltages (when feedback is present).
4. Basic Op-Amp Configurations
a) Inverting Amplifier
Gain = –Rf / Rin
b) Non-Inverting Amplifier
Gain = 1 + (Rf / Rin)
Gain = 1, but provides high input and low output impedance.
d) Summing Amplifier
Adds multiple inputs into one output.
e) Difference Amplifier
Amplifies the difference between two signals.
f) Integrator
Output ∝ integral of input signal.
g) Differentiator
Output ∝ derivative of input signal.
5. Frequency Response & Stability
Op-amp bandwidth is governed by the gain-bandwidth product (GBW). If GBW = 1 MHz, then at gain of 10, bandwidth = 100 kHz.
Phase margin determines stability. Around 60° phase margin ensures a good trade-off between speed and stability.
6. Real-World Applications
- Audio amplifiers
- Filters (low-pass, high-pass, band-pass)
- Data converters (sample & hold, comparators)
- Instrumentation amplifiers
- Oscillators
7. Common Interview Questions on Op-Amps
- What is CMRR and why is it important?
- What is slew rate and how does it limit performance?
- Why do we use Miller compensation?
- Explain the difference between open-loop and closed-loop gain.
8. Practical Insights
Real op-amps have trade-offs: higher speed means higher power, higher gain means reduced bandwidth. Choosing the right op-amp is about balancing these constraints for the application.
Conclusion
Op-amps are at the heart of analog design. Understanding their behavior, configurations, and limitations is essential not only for interviews but also for real-world circuit design.
👉 Next Step: Want to go deeper? Download the Free Analog Interview Toolkit with 30+ questions and formulas for op-amps and other topics.