Op-Amp
An op-amp takes two analog inputs and amplifies the difference between them by a huge amount. With external resistors and capacitors that gain becomes anything you want — buffer, amplifier, filter, integrator, comparator.
An op-amp amplifies the voltage difference between its inputs. In a stable negative-feedback circuit operating within its limits, the output moves so that this difference becomes very small. That useful approximation stops working when the output saturates, the input range is exceeded or the feedback arrangement is unsuitable.
In plain terms
A tireless seesaw that compares two ends and forces them to match. Wire it cleverly and you can build buffers, summers, integrators, and filters.
Also called: operational amplifier, opamp, instrumentation amplifier, in-amp, voltage follower, buffer amplifier.
Why designers use it
- Buffer a high-impedance sensor so the next stage doesn't load it.
- Amplify a small microphone or thermocouple signal up to ADC range.
- Build active low-pass, high-pass, or band-pass filters.
- Sum, subtract, or invert analog signals.
- Drive a precision DAC reference.
Best for
- Sensor front-ends
- Active filters
- Audio gain stages
- Precision instrumentation
Key specifications
- Open-loop gain: 10⁵ – 10⁷ V/V
- Gain-bandwidth: 100 kHz – 1 GHz
- Slew rate: 0.1 V/µs – 5,000 V/µs
- Input offset: 1 µV – 10 mV
- Supply: 1.8 V single – ±18 V dual
- Quiescent: 0.5 µA – 25 mA
When not to use it
- Where you only need clean digital output — a comparator is the right tool.
- Above its bandwidth — gain falls off quickly past a typical op-amp's gain-bandwidth product.
- Rail-to-rail-output assumed without checking — many op-amps can't reach within ~1 V of the rails.
Common mistakes
- Skipping the bypass cap on the supply — high-gain stages oscillate.
- Picking a part whose offset voltage swamps your sensor's actual signal.
- Forgetting input common-mode range and getting weird saturated output.
Where you will find it
- Analogue computers used feedback amplifiers to perform operations such as addition and integration on voltages.
- Modern op-amp circuits can amplify or filter signals, but gain, bandwidth, input range and output drive must fit the job.
A short history
Operational amplifiers grew out of feedback amplifiers used for analogue computation. John Ragazzini's 1947 paper named the operational amplifier for the mathematical operations it could perform. Later integrated designs included Bob Widlar's µA709 and Dave Fullagar's µA741, introduced in 1968 with internal frequency compensation.
Good to know
- The word 'operational' refers to mathematical operations on signals, not simply to an amplifier being switched on.
- The famous 741 was Dave Fullagar's design, not Bob Widlar's. Its internal compensation made it easier to use.