How op-amps work

To simplify analysis, operational amplifiers are often studied using an ideal model. This model assumes perfect performance characteristics, allowing engineers to focus on circuit behavior rather than device limitations.

An ideal operational amplifier is assumed to have the following properties:

• Infinite open-loop gain: A very small voltage difference between the inputs produces a large output.

• Infinite input impedance: No current flows into either input terminal.

• Zero output impedance: The output voltage is delivered without internal voltage drop.

• Infinite bandwidth: The amplifier responds equally to all input frequencies.

These assumptions closely approximate the behavior of real operational amplifiers, which typically exhibit extremely high gain and very high input impedance, making the ideal model highly effective for circuit analysis (Next Electronics).

When no feedback is applied, an operational amplifier operates in open-loop mode. In this configuration, the output voltage depends solely on the difference between the two input voltages.

Because the open-loop gain is extremely large (often exceeding 100,000 V/V), even a very small input difference can drive the output to its positive or negative saturation limit. This saturation limit is defined by the power supplies. As a result, open-loop operation is not suitable for amplifying signals linearly. However, it is useful in applications such as comparators, where the goal is to detect whether one input voltage is greater than another (Electronics Notes).

Most practical operational amplifier circuits use negative feedback, a configuration in which a portion of the output signal is returned to the inverting input of the amplifier.

This feedback mechanism stabilizes the output and allows the amplifier to operate in a predictable linear region. Instead of driving the output into saturation, the op-amp continuously adjusts its output so that the voltage difference between its input terminals becomes very small (All About Circuits).

In Figure 4, negative feedback is implemented by connecting a feedback path from the output terminal back to the inverting input, forming a closed-loop system. This feedback path, highlighted in red, contains a resistor, but it may include other components depending on design. Note that the power supplies are not included, which is common in op-amp circuit diagrams where they are known. This configuration is fundamental in practical op-amp circuit design because it allows controlled and stable operation.

Negative feedback improves circuit performance by:

• Increasing stability

• Reducing distortion

• Controlling gain

• Increasing effective bandwidth

These advantages make negative feedback essential in nearly all operational amplifier applications (Engineering Devotion).

A typical operational amplifier operating cycle follows these steps:

1. A voltage signal is applied to the input terminals.

2. The operational amplifier detects the voltage difference between the inverting and non-inverting inputs.

3. The output voltage changes proportionally in response to this difference, increasing or decreasing depending on the polarity of the input signal.

4. The system checks whether the output remains within the supply voltage limits. If the output reaches these limits, the amplifier enters saturation, where further changes in input no longer produce proportional changes in output. If not, linear amplification continues.

5. In most practical circuits, negative feedback returns a portion of the output signal to the inverting input. This reduces the input difference and prevents saturation, forcing the amplifier to operate in a stable, linear region.

6. The system continuously adjusts its output through this feedback process until it reaches a controlled and predictable operating point.

7. This continuous adjustment allows operational amplifiers to maintain stable amplification under varying conditions.

As shown in Figure 5, this sequence is represented as a flowchart in which each step corresponds to a stage in the behavior of an op-amp with negative feedback. The diagram illustrates the progression from input signal application to differential amplification, followed by the saturation condition, and finally the role of negative feedback in stabilizing operation. 

Operational amplifiers are characterized by several key electrical specifications that describe their real-world performance in circuit design. While ideal op-amp models assume perfect behavior, these parameters reflect practical limitations that affect how operational amplifiers actually operate in physical circuits, including constraints on gain, bandwidth, impedance, and supply voltage. These specifications are listed in a table in Figure 6 (Texas Instruments).