An operational amplifier, or op-amp, is a high-gain electronic building block used to compare, amplify, filter, and process voltage signals. Its triangle symbol has two inputs, the non-inverting input labeled + and the inverting input labeled -, plus one output and power supply pins. Op-amps matter because they let engineers build precise analog circuits using only a few resistors, capacitors, and feedback paths.
They appear in audio systems, sensors, control circuits, medical instruments, and data acquisition hardware.
By itself, an ideal op-amp has extremely large open-loop gain, so even a tiny input voltage difference can drive the output toward a supply rail. Most useful op-amp circuits use negative feedback, where part of the output is fed back to the inverting input to control the gain and make the circuit stable. The two golden rules for an ideal op-amp with negative feedback are that the input currents are zero and the input voltages are equal.
Feedback resistor ratios set common gains, such as the inverting amplifier gain Av = -Rf/Rin and the non-inverting amplifier gain Av = 1 + Rf/Rg.
Understanding Engineering: The Operational Amplifier
Negative feedback does more than set a chosen gain. It makes the circuit correct its own output. If the output moves too far in one direction, the returned signal reduces the difference between the two input pins.
The op-amp then drives its output in the direction that reduces the error. This correction happens continuously and very quickly. It is why a circuit can give a predictable result even when the op-amp chip itself has an uncertain internal gain.
The input pins are often described as being at a virtual short. Their voltages are nearly equal during normal feedback operation, but they are not physically connected.
A virtual ground is a related idea. An inverting input held near zero volts by feedback behaves like ground for voltage, while it is not actually connected to the ground wire.
Real op-amps have limits that ideal circuit diagrams can hide. The output needs a small amount of voltage room from each power supply rail in many devices. A so-called rail to rail op-amp can work closer to the rails, though even it has limits.
The input voltages must stay within an allowed common mode range. If they do not, the chip may no longer control its output properly. Output current is limited too.
An op-amp can provide a signal voltage, but it cannot directly power a motor, large speaker, or lamp. Students should check the data sheet for supply voltage range, input common mode range, output swing, and maximum output current before choosing a part.
Speed matters whenever a signal changes. Every op-amp has a gain bandwidth limit. At higher frequencies, the available closed loop gain becomes smaller.
A circuit designed to amplify slow temperature changes may work poorly for audio or radio signals. The slew rate gives another limit. It states how fast the output voltage can move.
A large, fast signal may become distorted because the output cannot change quickly enough. Capacitors used in feedback paths make useful filters and integrators, but they add phase shift. Too much phase shift can turn negative feedback into positive feedback at some frequency.
The result can be ringing or unwanted oscillation. Good designs leave enough stability margin.
Op-amps are common wherever a weak sensor signal needs preparation before a digital system reads it. A thermistor, light sensor, strain gauge, or microphone may produce a small voltage mixed with electrical noise. An op-amp stage can scale that voltage, remove unwanted frequency ranges, or shift it to a range an analog to digital converter accepts.
In a comparator circuit, the output is intended to switch sharply when one input passes a reference voltage. This is different from a linear amplifier, where feedback keeps the output proportional to the input.
When building circuits, keep wires short around sensitive inputs, use a shared ground plan, and place small bypass capacitors close to the power pins. These practical details often decide whether a circuit behaves as expected.
Key Facts
- Ideal op-amp input current is zero: I+ = I- = 0.
- With negative feedback, the inputs are at nearly the same voltage: V+ ≈ V-.
- Open-loop output relation: Vout = Aol(V+ - V-), where Aol is very large.
- Inverting amplifier gain: Av = Vout/Vin = -Rf/Rin.
- Non-inverting amplifier gain: Av = Vout/Vin = 1 + Rf/Rg.
- The output voltage cannot exceed the supply rails, so -Vsupply < Vout < +Vsupply in practical circuits.
Vocabulary
- Operational amplifier
- An operational amplifier is a high-gain voltage amplifier with differential inputs and a single output.
- Inverting input
- The inverting input is the terminal labeled - where an increase in voltage tends to make the output decrease.
- Non-inverting input
- The non-inverting input is the terminal labeled + where an increase in voltage tends to make the output increase.
- Negative feedback
- Negative feedback sends part of the output back to the inverting input to stabilize the circuit and set a predictable gain.
- Saturation
- Saturation occurs when the op-amp output reaches a limit near one of its supply voltages and can no longer increase or decrease linearly.
Common Mistakes to Avoid
- Assuming the output can be any voltage is wrong because a real op-amp output is limited by its positive and negative supply rails.
- Forgetting the minus sign in an inverting amplifier is wrong because the output is 180 degrees out of phase with the input, so Av = -Rf/Rin.
- Treating the op-amp inputs as drawing large current is wrong for ideal analysis because the input currents are approximately zero in most basic op-amp models.
- Using the golden rules without negative feedback is wrong because V+ ≈ V- only applies when the op-amp is operating linearly with negative feedback.
Practice Questions
- 1 An inverting amplifier has Rin = 2.0 kΩ and Rf = 10.0 kΩ. If Vin = 0.40 V, calculate the voltage gain and Vout.
- 2 A non-inverting amplifier uses Rg = 1.5 kΩ and Rf = 6.0 kΩ. Find the voltage gain, then find Vout when Vin = 0.80 V.
- 3 An op-amp comparator has no negative feedback and is powered by +12 V and -12 V supplies. Explain why the golden rule V+ ≈ V- should not be used for this circuit.