A feedback resistor is the resistor that routes part of an op-amp’s output back to its input, setting gain and stability. In Electrical Circuits and Systems I, it shows up most often in summing and difference amplifiers.
A feedback resistor is the resistor that sends a controlled portion of an amplifier’s output back to its input, usually in an op-amp circuit. In Electrical Circuits and Systems I, you use it to set closed-loop gain instead of letting the op-amp run open-loop at an impractically large gain.
The basic idea is simple: the output is sampled through the feedback path, and that sample is compared with the input. If the resistor is chosen well, the circuit settles into a predictable linear response. If the feedback resistor changes, the amount of feedback changes too, so the gain and behavior of the amplifier change with it.
In an inverting amplifier, the feedback resistor works with the input resistor to set gain. A common relationship is that the gain depends on the ratio between the feedback resistor and the input resistor, which is why resistor values matter so much in design problems. A larger feedback resistor usually means a larger magnitude of gain, while a smaller one reduces the gain.
That same resistor also affects real-world performance, not just the textbook gain number. More feedback can improve linearity and make the output follow the input more faithfully, but circuit bandwidth and stability can shift as you change values. If the feedback path is poorly chosen, the amplifier can become less predictable or more sensitive to noise and component tolerances.
You will also see feedback resistors in summing and difference amplifiers. In a summing amplifier, the feedback resistor helps turn several input currents into one weighted output voltage. In a difference amplifier, carefully matched feedback and input resistors help the circuit reject signals that appear on both inputs while preserving the difference between them.
The feedback resistor is one of the fastest ways to turn op-amp theory into an actual working circuit. Instead of treating the op-amp like a mystery part with huge gain, you use the resistor to control what the circuit does. That makes it central to amplifier design, especially when you need a predictable output for sensor signals, audio mixing, or lab measurements.
This term also ties together several core ideas in Electrical Circuits and Systems I. You have to use Ohm’s law, current flow at a summing node, and the ideal op-amp rules to solve these circuits correctly. Once you know how the feedback resistor fits into the loop, summing amplifiers and difference amplifiers stop feeling like separate tricks and start looking like variations on the same feedback pattern.
It also shows up in design tradeoffs. Changing the feedback resistor can improve gain, but it can also affect linearity, bandwidth, and how well the circuit handles mismatched components. That makes it a good term for homework problems where you are asked to explain not just what the output is, but why a certain resistor choice makes sense.
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Visual cheatsheet
view galleryOperational Amplifier
The feedback resistor only matters because it sits in an op-amp feedback loop. The op-amp tries to drive its inputs toward the same voltage, and the resistor controls how much of the output is sent back to help that happen. If you understand the op-amp’s behavior, the resistor’s effect on gain becomes much easier to see.
Inverting Amplifier
This is the classic circuit where the feedback resistor shows up in a clean gain formula. The input resistor brings signal into the summing node, and the feedback resistor sends output back to that same node. Many circuit problems ask you to read the resistor ratio and find the output voltage from it.
Gain
The feedback resistor is one of the main parts that sets closed-loop gain in op-amp circuits. In a summing or inverting setup, changing its value changes how strongly the output responds to the input. That makes it a design knob, not just a passive part on the schematic.
common-mode rejection ratio (CMRR)
In a difference amplifier, feedback resistor matching affects how well common-mode signals are rejected. If the resistor ratios are off, unwanted shared noise can leak into the output. That is why this term comes up when you discuss precision and error in differential circuits.
A quiz or problem-set question will usually give you a resistor network and ask you to find the output voltage, identify the gain, or explain why the circuit is stable. You may need to use the feedback resistor together with the input resistor ratio in an inverting or summing amplifier, then check the sign of the output and whether the circuit is linear. In a difference amplifier problem, the resistor values may be chosen so you can test whether the common-mode signal is rejected. If the values are mismatched, you should be ready to explain why the output is no longer a perfect difference of the two inputs. On a lab report, you might compare measured gain to the theoretical value and comment on how resistor tolerance changes the result.
A feedback resistor sends part of an op-amp output back to the input so the circuit has controlled, predictable gain.
In an inverting amplifier, the gain depends on the ratio between the feedback resistor and the input resistor.
Changing the feedback resistor changes more than gain, it can also affect linearity, bandwidth, and stability.
Summing amplifiers use the feedback resistor to combine multiple inputs into one weighted output.
Difference amplifiers rely on resistor matching, including the feedback resistor, to reject common-mode signals.
It is the resistor that feeds part of an op-amp’s output back to its input. That feedback sets the closed-loop gain and helps the circuit behave predictably instead of saturating at huge open-loop gain.
In many op-amp circuits, the gain is set by the ratio of the feedback resistor to the input resistor. A larger feedback resistor usually gives a larger magnitude of gain in an inverting setup, while a smaller one lowers it.
No. The input resistor brings the signal into the summing node, while the feedback resistor returns part of the output to the input side. They work together, but they do different jobs in the circuit.
Matching resistor ratios helps the circuit reject signals that are common to both inputs. If the feedback resistor does not match the rest of the network, common-mode noise can leak into the output and the subtraction is less accurate.