In one line: An op amp is a very high-gain amplifier that, with negative feedback, does exactly what its surrounding resistors and capacitors tell it to. It is the workhorse for amplifying, combining and filtering analog signals.
Core idea
An operational amplifier amplifies the difference between its two inputs by an enormous factor, typically 100,000 or more. On its own that is nearly useless. With negative feedback, part of the output returned to the inverting input, the circuit’s behavior is set almost entirely by the external components, not by the op amp’s imprecise internal gain.
Two rules describe an ideal op amp with negative feedback:
- No current flows into the inputs.
- The output does whatever is needed to keep the two inputs at the same voltage. This is called a “virtual short.”
How it works
Standard configurations:
| Circuit | Output | Typical use |
|---|---|---|
| Inverting amplifier | gain with inversion | |
| Non-inverting amplifier | gain, high input impedance | |
| Voltage follower | buffering a weak source | |
| Summing amplifier | weighted sum of inputs | mixing, digital-to-analog |
| Difference amplifier | gain × | measuring between two points |
| Integrator and differentiator | integral or derivative of the input | filters, wave shaping, control |
| Comparator (no feedback) | high or low | threshold detection |
Example: an inverting amplifier with kΩ and kΩ has a gain of −10.
The instrumentation amplifier combines three op amps to measure a tiny difference between two inputs that both sit on a large shared voltage. It has very high input impedance and rejects the shared part. This is how a heart monitor extracts millivolts from a body picking up interference.
Real-world limits:
- Gain-bandwidth product. Gain falls with frequency. An op amp with a 1 MHz gain-bandwidth set to a gain of 100 has only about 10 kHz of bandwidth.
- Common-mode rejection ratio (CMRR). How well the amplifier ignores voltage common to both inputs. It worsens at higher frequencies.
- Other limits: offset voltage, input bias current, slew rate (the maximum rate of change of the output), and the output swing allowed by the supply voltage.
In practice
Nearly every sensor passes through an op amp stage before being digitized by a microcontroller (see Embedded Microcontrollers). A practical habit is reading the datasheet’s typical and worst-case specifications for gain, bandwidth and CMRR. Many field problems come from designs that work at the typical value and fail at the worst case.
Questions to ask the experts
- Is there enough gain-bandwidth for both the gain and the signal frequency we need?
- What common-mode voltage and interference must the input reject?
- Were the worst-case datasheet values used, not just the typical ones?
Connections
- The discipline: Electrical and Computer Engineering. Filters built with them: RLC Circuits.
- What’s inside: Semiconductor Devices. Where the signal goes next: Embedded Microcontrollers.
- Capabilities exercised: Inquiry, Resourcefulness, Connecting Across Disciplines.
Sources
- Course lecture on operational amplifiers; no public source.