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.
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.