opamp calculator

What this op-amp calculator does

This calculator helps you quickly analyze three of the most common operational amplifier circuits: inverting, non-inverting, and differential amplifiers. It gives you the ideal closed-loop gain, the ideal output voltage, and a rail-limited output estimate so you can immediately see whether your signal will saturate.

It also includes a quick bandwidth estimate based on gain-bandwidth product (GBP): as closed-loop gain goes up, usable small-signal bandwidth goes down. This is useful for early-stage design checks before detailed simulation.

Supported formulas

1) Inverting amplifier

• Gain: A_v = -R_f / R_in
• Output: V_out = A_v × V_in
• Phase relationship: output is 180° out of phase with input

2) Non-inverting amplifier

• Gain: A_v = 1 + (R_f / R_g)
• Output: V_out = A_v × V_in
• Phase relationship: output is in phase with input

3) Differential amplifier (ideal matched network)

• Differential gain: A_d = R_f / R₁
• Output: V_out = A_d × (V2 - V1)
• Best accuracy requires resistor matching (typically 0.1% or better)

How to use it effectively

Start by selecting the topology you are actually building. Enter realistic resistor values in ohms and voltage inputs in volts. Then enter your supply rails because the rail-limited result often matters more than the ideal result. If you know the op-amp GBP from the datasheet, include it for a quick first-order bandwidth estimate.

If the calculated ideal output exceeds rails, the circuit will clip or saturate. That means your linear assumptions no longer hold, and distortion becomes significant.

Design tips for better real-world performance

• Check output swing limits: many op-amps cannot swing fully to rails, even if powered from those rails.
• Mind input common-mode range: especially for single-supply circuits near ground.
• Use practical resistor ranges: values from 1 kΩ to 200 kΩ are common tradeoffs for noise, bias current error, and power.
• Account for input bias current: high resistor values can create measurable DC offsets.
• Verify stability: capacitive loads and high closed-loop gain can reduce phase margin.

Example quick checks

Inverting example

With Vin = 1 V, Rin = 10 kΩ, and Rf = 47 kΩ, gain is -4.7, so ideal output is -4.7 V. On ±15 V rails, this is safely linear.

Non-inverting example

With Vin = 0.25 V, Rg = 10 kΩ, and Rf = 90 kΩ, gain is 10, so ideal output is 2.5 V. On a 0 V to 12 V supply, this is also linear for most rail-to-rail output parts.

Differential example

With V2 = 0.2 V, V1 = 0.05 V, and resistor ratio Rf/R1 = 10, output is 1.5 V. If resistor ratios are mismatched, common-mode rejection degrades and error increases.

Limitations of simple calculators

This tool uses ideal closed-loop equations with simple rail clipping. It does not model slew rate, offset voltage, finite open-loop gain, noise density, temperature drift, or large-signal settling behavior. For final verification, use SPICE simulation and check your exact datasheet conditions.