Op-Amp Gain Calculator
Gain, dB, output voltage, input impedance and bandwidth of inverting, non-inverting and difference op-amp stages, or standard resistors for a target gain.
Resistors accept 4.7k, 4k7, 4,7k or 100 kΩ; GBW accepts 1M or 1 MHz.
Ideal op-amp model: infinite open-loop gain, no offset, no input current. Bandwidth assumes a single-pole voltage-feedback part.
How to use
- Pick Inverting, Non-inverting, Difference or Design.
- Enter the resistors (4.7k, 4k7 and 100 kΩ all work), the input voltage, the supply rail and the op-amp's gain-bandwidth product.
- Read the gain, output voltage, noise gain, input impedance and bandwidth; in Design mode, enter a target gain and a series to get the nearest standard resistor pair.
About this tool
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Works out what an op-amp stage actually does from its resistors. For an inverting amplifier the gain is −Rf/Rin; for a non-inverting one it is 1 + Rf/Rg. Next to the gain the tool shows it in decibels, the output voltage for your input, the noise gain, the input impedance and the closed-loop bandwidth, and it warns when the output would have to swing past the supply rails.
The bandwidth figure is the one datasheets make easy to get wrong. A voltage-feedback op-amp trades gain for bandwidth through its gain-bandwidth product (GBW), but the gain that sets the bandwidth is the noise gain, 1 + Rf/Rin, not the signal gain. An inverting stage with a gain of −1 therefore has a noise gain of 2 and only half the GBW as bandwidth, the same as a non-inverting stage with a gain of +2. The estimate assumes a single dominant pole, which suits general-purpose parts; it does not apply to current-feedback amplifiers, and slew rate can limit large signals long before this bandwidth is reached.
The difference-amplifier mode takes all four resistors separately, because that is where real circuits go wrong. With R2/R1 equal to R4/R3 the output is (R2/R1)·(V2 − V1) and a voltage common to both inputs cancels. Any mismatch leaves a common-mode gain, and the tool reports it together with the resulting common-mode rejection ratio: with R1 = R3 = 10 kΩ and R2 = 100 kΩ, making R4 101 kΩ instead of 100 kΩ — one resistor 1 % off — already limits CMRR to about 61 dB. Precision difference amplifiers use laser-trimmed resistor networks for exactly this reason.
The design mode works backwards: give a target gain and a resistor series (E12, E24 or E96) and it returns the standard-value pair whose ratio comes closest, with the gain those parts really give and the error. The input resistor is kept between 1 kΩ and 10 kΩ, where offset from bias current and loading of the source both stay small; the feedback resistor may be anything from 100 Ω to 10 MΩ. A non-inverting stage cannot have a gain below 1, and a gain of exactly 1 needs no resistors at all — use a voltage follower.
Everything is an ideal-op-amp model: infinite open-loop gain, no offset, no input current. Resistor values accept 4.7k, 4k7, 4,7k and 100 kΩ, and nothing you type leaves the browser.
Frequently asked questions
Why is the bandwidth of a gain −1 inverting stage only half the GBW?
Bandwidth follows the noise gain, 1 + Rf/Rin, not the signal gain. With Rf = Rin the signal gain is −1 but the noise gain is 2, so a 1 MHz GBW op-amp gives about 500 kHz. A non-inverting stage with a gain of +2 has the same noise gain and the same bandwidth.
What does the clipping warning mean?
It appears when the ideal output would exceed the supply rail you entered. A real op-amp stops before that: many only reach within 1–2 V of the rails, and even rail-to-rail parts lose tens of millivolts under load. Check the output swing in your part's datasheet.
Why does a 1 % resistor mismatch ruin a difference amplifier?
The common-mode voltage only cancels when R2/R1 equals R4/R3 exactly. With R1 = R3 = 10 kΩ and R2 = 100 kΩ, an R4 of 101 kΩ leaves a common-mode gain of about 0.009 against a differential gain of about 10, which is a CMRR of roughly 61 dB. Use matched networks or an integrated difference amplifier when the common-mode voltage is large.
Why can't I design a non-inverting gain below 1?
Its gain is 1 + Rf/Rg, which is at least 1 for any positive resistors. For attenuation, divide the signal first or use an inverting stage; for a gain of exactly 1, use a voltage follower with the output tied to the inverting input and no resistors.
Which input resistor does the design mode choose?
It keeps the input (or ground-leg) resistor in the 1 kΩ–10 kΩ decade and searches the feedback resistor from 100 Ω to 10 MΩ in the chosen series. Lower values load the source and waste current; much higher values make bias-current offset and noise worse. Scale both resistors by the same factor if you need a different impedance — the ratio, and so the gain, stays the same.
Is the input impedance of the non-inverting stage really infinite?
No, but it is the op-amp's own input impedance, typically megohms for bipolar parts and teraohms for CMOS and JFET inputs, so the resistors do not set it. The inverting stage is different: its input impedance is simply Rin, because the inverting input is held at virtual ground.
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