What is an Operational Amplifier?
An operational amplifier (op-amp) is a high-gain integrated circuit that amplifies the voltage difference between its two inputs. With feedback resistors, it becomes a precise, configurable amplifier used throughout analog electronics.
An op-amp amplifies the difference between its inputs; with feedback its gain is set by external resistors, e.g. Vout = −(Rf/Rin)·Vin for the inverting configuration.
- •Input applied at the inverting (−) terminal
- •Output is 180° out of phase with input
- •Gain = −Rf/Rin
- •Virtual ground at the (+) input
- •Input applied at the non-inverting (+) terminal
- •Output is in phase with input
- •Gain = 1 + Rf/Rin
- •Very high input impedance
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Step-by-step worked examples
An inverting amplifier has Rf = 10 kΩ, Rin = 1 kΩ and Vin = 0.5 V. Find Vout.
Vout = −(Rf/Rin)×Vin = −(10/1)×0.5 = −5 V
A non-inverting amplifier has Rf = 20 kΩ, Rin = 5 kΩ and Vin = 1 V. Find Vout.
Gain = 1 + Rf/Rin = 1 + 20/5 = 5 Vout = 5 × 1 = 5 V
An op-amp has open-loop gain A = 100,000 and inputs V+ = 2.0001 V, V− = 2 V. Find Vout.
Vout = A×(V+ − V−) = 100000 × 0.0001 = 10 V (would saturate near the supply rail in practice)
Flashcards
Quick quiz
Q1.An ideal op-amp has input impedance:
Q2.The inverting amplifier gain formula is:
Q3.The non-inverting amplifier gain formula is:
Q4.In an inverting amplifier, the output is:
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Common mistakes
Assuming the op-amp output is just the input amplified without feedback. — Correct: Practical op-amp circuits use feedback resistors (Rf/Rin) to precisely set the gain.
Forgetting the negative sign in the inverting gain formula. — Correct: Av = −Rf/Rin — the output is inverted relative to the input.
Assuming the op-amp input draws significant current. — Correct: An ideal op-amp draws essentially zero input current.
Mixing up the inverting and non-inverting gain formulas. — Correct: Inverting gain has no +1 term; non-inverting gain always includes the +1.
FAQ
What is an operational amplifier?
An operational amplifier (op-amp) is a high-gain integrated circuit amplifier with two inputs and one output, used with feedback to set precise gain.
What is the op-amp gain formula?
Inverting configuration: Av = −Rf/Rin. Non-inverting configuration: Av = 1 + Rf/Rin.
What are examples of op-amp circuits?
Summing amplifiers, integrators, differentiators, comparators and active filters all use op-amps.
How do you calculate op-amp output voltage?
Multiply the gain (set by the feedback resistors) by the input voltage: Vout = Av × Vin.