1.1. What is an Operational Amplifier?

An Operational Amplifier is a direct-coupled, high-gain amplifier that can work with both AC and DC signals. Its fundamental structure consists of two input terminals and one output terminal.

• The Inverting Terminal (-): An input signal applied here will produce an output signal that is 180 degrees out of phase (inverted).
• The Non-Inverting Terminal (+): An input signal applied here will produce an output signal that is in phase with the input.

The names of these terminals are based directly on the phase relationship between their respective input signals and the resulting output signal.

1.2. The “Virtual Short”: The Most Important Op-Amp Rule

To understand how op-amp circuits work, you must first grasp the concept of the virtual short. This is the single most important rule for analyzing op-amp behavior in most configurations.

The principle is simple: when an op-amp is configured with a negative feedback path, its high internal gain works to keep the voltage at the inverting input terminal equal to the voltage at the non-inverting input terminal. Although the terminals are not physically connected, they are ‘virtually’ at the same voltage. This ‘virtual short’ is the key to analyzing nearly every circuit we will discuss.

1.3. The Ideal vs. The Practical Op-Amp

To simplify circuit analysis, we often start by using a theoretical “ideal” model of an op-amp. In the real world, “practical” op-amps have characteristics that deviate from this ideal, but the ideal model provides an excellent foundation for understanding their function. The table below compares the key characteristics of both models.

With these core concepts established, we can now explore the op-amp’s most fundamental application: building amplifiers.