🔌 Op-Amp Resistor Calculator

Operational amplifiers are the workhorse of analog design, and two resistor values set the gain of the two most common configurations. For the non-inverting amplifier the gain is G = 1 + Rₘ/R₁, which is always greater than one. For the inverting amplifier the gain is G = −Rₘ/R₁, which can attenuate or amplify and flips the phase. Choosing resistor values is a constant juggling act: too small and you load the source and burn power; too large and noise and bias-current errors dominate.

Practical guidance: for gains around 1–10, use resistors in the 1–100 kΩ range. At higher gains, the feedback resistor multiplied by the input capacitance forms a pole that limits bandwidth, and large-value resistors generate thermal noise. The calculator handles the two-direction problem — gain from resistors, or the resistor that delivers a target gain — and reports the resulting gain in both linear and dB form.

In the inverting configuration, the virtual ground at the − input means the current through R₁ equals the current through Rₘ, giving G = −Rₘ/R₁. In the non-inverting configuration, the feedback forces the − input to follow V+, giving G = 1 + Rₘ/R₁. The calculator applies these directly and, in solve mode, rearranges them for the missing resistor.

The derived required feedback resistor (in solve mode) and the gain in dB are the outputs engineers copy straight into their design notes. Bandwidth limits come from the op-amp's gain-bandwidth product: the closed-loop bandwidth is GBW/G, so a 10 MHz op-amp configured for a gain of 10 gives roughly 1 MHz of bandwidth. Pair resistor choices with the resistor color code calculator to decode the parts you have on hand.

1. Choose the configuration: inverting or non-inverting.
2. Choose the solve direction: gain from resistors, or resistor from gain.
3. Enter the two resistor values (or gain plus one resistor).
4. Click Calculate to see the gain (linear and dB) or the missing resistor.
5. Pick a standard E-series value close to the result.

Why does the inverting gain formula have a minus sign?

Because the signal enters the inverting input, the output is 180° out of phase with the input. For AC signals this only matters for polarity; for DC circuits it means the output swings in the opposite direction from the input.

What resistor range should I use?

Aim for 1–100 kΩ. Below that, source impedance and op-amp output drive become limiting; above that, thermal noise (which scales with √R) and input bias current errors grow. For precision DC, keep Rₘ ≤ 1 MΩ.

How do I handle a non-inverting gain of exactly 1?

Set Rₘ = 0 (short) and remove R₁, or use the classic voltage-follower wiring. The calculator will return a feedback resistor of 0 for that case; you do not need a resistor to ground at the inverting input.

What limits the bandwidth at high gain?

The op-amp's gain-bandwidth product (GBW). Closed-loop bandwidth ≈ GBW / gain. A 10 MHz op-amp at gain 100 gives only 100 kHz of useful bandwidth. For high gain and high frequency, cascade stages or choose a faster amplifier.

Can I build an attenuator with an inverting stage?

Yes. With Rₘ < R₁ the inverting stage attenuates, and the input impedance is simply R₁. This is a common way to scale a large sensor signal down to ADC range while buffering the source.

Does the calculator consider offset voltage and bias current?

No — it assumes an ideal op-amp. Real parts have input offset voltage (microvolts to millivolts) and bias currents (pA to nA) that add small errors, worst in high-impedance or high-gain designs. Use low-offset or chopper amplifiers for precision DC.

Keep both resistors in the 1–100 kΩ range for most designs. Below 1 kΩ the op-amp output and source must drive low impedances; above 1 MΩ noise and bias currents creep in. Use standard E-series values and don't chase exact gains — 1% resistors give 2% gain accuracy. For the non-inverting input, add a resistor equal to the parallel combination R₁||Rₘ to balance input bias currents on bipolar op-amps. Remember the gain-bandwidth trade-off: at high gain the bandwidth falls, so use a cascade of two stages rather than one huge-gain stage. Decouple the supply pins with 100 nF capacitors close to the chip.