Why is the output voltage inverted?

In an inverting amplifier the input signal is applied to the inverting (−) terminal of the op-amp through Rin, while the non-inverting (+) terminal is tied to ground. This topology causes a 180-degree phase shift, so a positive input produces a negative output. The gain formula Gain = -(Rf / Rin) captures that sign inversion.

What happens when Rf equals Rin?

When Rf = Rin the closed-loop gain is exactly −1, so the circuit acts as a unity-gain inverter (sign changer). The output magnitude equals the input magnitude but with opposite polarity.

Does the op-amp supply voltage affect the calculation?

The formula Vout = -(Rf/Rin) × Vin assumes an ideal op-amp with unlimited output swing. In practice the output is clipped a volt or two below the supply rails. If your computed Vout exceeds the supply headroom, the op-amp will saturate.

What is the input impedance of this configuration?

The input impedance seen by the source is approximately equal to Rin, because the virtual ground at the inverting terminal holds that node near 0 V. Choose Rin large enough not to load your source, but not so large that bias current causes significant offset.

Can I use this for AC signals?

Yes. The gain formula applies at any frequency where the op-amp open-loop gain is much larger than the closed-loop gain. At higher frequencies the gain-bandwidth product of the specific op-amp limits performance, but this calculator gives the ideal mid-band result.

How do I pick resistor values for a desired gain?

Choose a target gain magnitude, then select a convenient Rin value (10 kΩ is a common starting point) and compute Rf = |Gain| × Rin. Prefer standard E24 or E96 resistor values and keep both resistors in the 1 kΩ – 1 MΩ range to balance noise, bias current error, and bandwidth.

Inverting Amplifier Calculator

Name: Inverting Amplifier Calculator
Availability: InStock
Author: Nham Vu

Enter the input voltage, input resistor, and feedback resistor to instantly calculate the closed-loop gain and output voltage of an op-amp inverting amplifier.

Op-Amp Inverting Amplifier

Input Voltage — V_in

Input Resistor — R_in

Feedback Resistor — R_f

Enter values on the left and click Calculate to see the results.

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Summary

Enter the input voltage, input resistor, and feedback resistor to instantly calculate the closed-loop gain and output voltage of an op-amp inverting amplifier.

How it works

Enter the input voltage (Vin) in volts.
Enter the input resistor value (Rin) in ohms, kilohms, or megaohms.
Enter the feedback resistor value (Rf) in ohms, kilohms, or megaohms.
The calculator applies Gain = -(Rf / Rin) to find the closed-loop gain.
Output voltage is computed as Vout = Gain × Vin.
Results update instantly as you type.

Use cases

Design signal conditioning circuits with a specific gain target.
Verify resistor selections for audio amplifier stages.
Quickly check gain and output swing before building a prototype.
Teach op-amp inverting amplifier theory with live feedback.
Size feedback resistors for instrumentation amplifier front ends.
Confirm polarity inversion and magnitude for control system loops.
Estimate output voltage headroom against supply rails.
Convert sensor output levels to ADC input ranges.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu