Master the Basics: What is Feedback Circuits
In many operational amplifier (op-amp) circuits, you'll find a configuration known as a closed loop, where part of the output signal is fed back into one of the input terminals. This feedback loop helps the op-amp monitor its own output and adjust accordingly—preventing overreaction to changes in the input signal. It’s how op-amps maintain a stable, accurate output.
Feedback in Electronics — and Everyday Life
Think of feedback as something you're already familiar with. Let’s say you're riding a bicycle. One input is your phone giving directions, and the other is your eyes watching the road ahead. Your output is how you steer the handlebars.
If your phone says, “Go straight,” you set your course accordingly—but you still watch the road to stay aligned. That’s closed-loop feedback: using real-time information (your vision) to constantly correct your path.
If your phone says, “Turn right,” you make the turn, observe your direction, and adjust until you're going the right way.
Sometimes you might over- or under-turn. But thanks to feedback, you make small corrections to stay on course.
Now imagine riding a bike without looking—relying only on your phone's voice directions. That’s open-loop control. Without feedback, you keep turning based on one signal and hope for the best. You might drift off course or keep spinning in circles.
This analogy works perfectly to explain the behavior of op-amps.
What Is Feedback in an Op-Amp?
In electronics, feedback means feeding a portion of the output signal back to one of the op-amp's inputs.
Negative Feedback (most common): Output is routed back to the inverting (–) terminal. This causes the op-amp to correct its output so the difference between its two inputs becomes virtually zero. It’s like watching the road and adjusting your steering accordingly.
Positive Feedback: Output is routed back to the non-inverting (+) terminal. This causes the op-amp to swing to one extreme or another (fully on or off). Positive feedback is used less often but is useful for things like comparators and oscillators.
The Op-Amp's Golden Rules
When using feedback, op-amps follow two key principles:
- No current flows into the input terminals (ideal infinite input impedance).
- The op-amp adjusts its output to keep the voltage difference between the inputs at zero (with negative feedback).
So, in a closed-loop configuration, the op-amp continually adjusts its output to keep its inputs in perfect balance—just like you adjusting your steering to stay on the road.
Open-Loop: No Feedback, No Control
In open-loop mode, the op-amp acts based on input alone. With its extremely high gain (often over 100,000), even a small input difference results in full output swing. It’s like turning the handlebars sharply based on a whisper of a suggestion—with no idea where you're going. Without feedback, the output saturates quickly and loses all precision.
Recap: Feedback Keeps You Balanced
To sum it up:
- Closed-loop (negative feedback) = stability, control, accuracy
- Open-loop = overreaction, instability, saturation
- Just like a cyclist needs vision (feedback) to ride straight, an op-amp needs feedback to produce meaningful, controlled outputs.
As you dive deeper into op-amp circuits—like amplifiers, buffers, filters, and more—you’ll see negative feedback used again and again. Understanding how it works is key to mastering electronics.
