# Op-Amp Calculator - Inverting & Non-Inverting Gain, Active Filters, Schmitt Trigger, Bode Plot

> Op-amp calculator with schematics: inverting, non-inverting, summing, difference, instrumentation, Sallen–Key, Schmitt trigger, plus Bode plots and E24 design.

- URL: https://qilingtools.com/en/physics/op-amp
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

Pick one of 14 common op-amp circuits and type component values as 10k, 4.7u, 100n or 2k2. The tool draws a labelled schematic and reports the ideal gain, the actual gain and phase, input impedance, noise gain and closed-loop bandwidth. Switch between an ideal op-amp and models such as the LM741, TL072 or NE5532: the finite open-loop gain and gain-bandwidth product show up in the Bode plot. Feed in a sine, square or triangle wave and the output is clipped at the rails and slope-limited by the slew rate. A design helper suggests E24/E96 resistor pairs for a target gain or designs a Sallen–Key low-pass from f₀ and Q.

## How to use

1. Pick a circuit and edit the component values (engineering and RKM notation both work)
2. Choose an op-amp model and supply voltages; the difference between ideal and real op-amps shows in gain, bandwidth and waveforms
3. Set the input waveform, amplitude, frequency and DC offset, and for multi-input circuits choose which input carries the signal
4. Read the result cards, waveforms and Bode plot, and use the design helper to pick standard values for a target gain or cut-off and apply them in one click

## Features

- 14 circuits: inverting, non-inverting, follower, summing, difference, 3-op-amp instrumentation amp, integrator, differentiator, 1st-order and Sallen–Key 2nd-order active filters, transimpedance amp and Schmitt trigger
- Automatically drawn SVG schematics labelled with component values
- Ideal op-amp vs single-pole finite-gain model (A₀, GBW) with closed-loop −3 dB bandwidth and a Bode plot including open-loop gain
- Steady-state output for sine, square and triangle inputs, clipped at the rails and limited by slew rate
- Schmitt trigger thresholds, hysteresis and the hysteresis loop
- E12/E24/E48/E96 resistor pairs for a target gain, plus RC design for Sallen–Key and first-order filters

## FAQ

### How do inverting and non-inverting gains differ?

In an inverting amplifier the input reaches the inverting node through R1, Rf feeds back from the output and the non-inverting input is grounded. The virtual-short rule gives Vout = −(Rf/R1)·Vin, so the output is inverted and the input impedance equals R1. A non-inverting amplifier takes the signal on the non-inverting input with gain 1 + Rf/Rg, always at least 1, and a very high input impedance. Both have a noise gain of 1 + Rf/R1 (or 1 + Rf/Rg), and that is what sets the closed-loop bandwidth.

### How does the gain-bandwidth product set the bandwidth?

A typical voltage-feedback op-amp has an open-loop gain close to a single pole, A(s) = A₀/(1 + s/ω_p). Above that pole, gain times frequency stays constant at the GBW. In a closed loop, bandwidth is roughly GBW divided by the noise gain: an LM741 with GBW = 1 MHz wired as a ×10 non-inverting amplifier has a −3 dB bandwidth of about 100 kHz. In non-ideal mode the tool uses this model to compute the full transfer function, so you can see the phase change too.

### Why does the output get flattened or turn triangular?

An op-amp cannot swing beyond its supplies, and most need 1–2 V of headroom (rail-to-rail parts excepted), so when gain times input amplitude exceeds that range the peaks are clipped. The other limit is the slew rate SR: the output cannot change faster than SR, and a sine needs a peak slope of 2πf·V_pk, so beyond that the output becomes a triangle. The tool shows the full-power bandwidth SR/(2πV_pk) for reference.

### How are f₀ and Q of a Sallen–Key filter found, and how do I design one?

A Sallen–Key second-order filter has f₀ = 1/(2π√(R1R2C1C2)), and the low-pass Q = √(R1R2C1C2)/(C2(R1+R2) + R1C1(1−K)), where K is the non-inverting gain. At unity gain with R1 = R2, Q = ½√(C1/C2), so a Butterworth response (Q ≈ 0.707) needs C1 ≈ 2C2. The design helper takes a target f₀, Q and chosen C2, computes R and C1 and snaps them to standard values. Push K too high and Q goes negative, so the circuit oscillates.

### How do you find the thresholds of a Schmitt trigger?

Take the inverting type: the input goes to the inverting pin, and the non-inverting pin connects to Vref through R1 and to the output through R2, so V+ = (Vref·R2 + Vout·R1)/(R1+R2). With the output saturated high the threshold is higher, and with it low the threshold is lower. The difference, ΔV = (V_sat+ − V_sat−)·R1/(R1+R2), is the hysteresis. It stops the output from chattering when a noisy input hovers near the threshold, which is why Schmitt triggers are used to square up slow or noisy signals.

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