The Operational Amplifier (Op-Amp): A Pillar of Electronics

Op-Amp Definition

The operational amplifier, often abbreviated as Op-Amp, is a versatile and fundamental electronic component designed to amplify a voltage difference between its two inputs. Ideally, it has infinite open-loop gain, infinite input impedance, zero output impedance, and infinite bandwidth. In practice, these values are very large, allowing the Op-Amp to perform a multitude of functions in analog circuits. It is generally powered by a dual voltage source (positive and negative), although single-supply versions exist.

Op-Amp Symbolic Diagram

The Op-Amp symbol is a triangle with two inputs and one output:

• V+ (Non-Inverting Input): The voltage applied to this terminal is amplified and has the same phase as the output.
• V- (Inverting Input): The voltage applied to this terminal is amplified and has an opposite phase to the output.
• Vout (Output): The output voltage, proportional to the difference between V+ and V-.
• +Vs / -Vs (Power Supply): The positive and negative power supply terminals.

General Operation

The operating principle of the Op-Amp is based on amplifying the voltage difference between its inverting (V-) and non-inverting (V+) inputs. The fundamental relationship is given by:

Vout = Aol x (V+ - V-)
Where Aol is the open-loop gain, a very high value (typically from 100,000 to several million). In practice, the Op-Amp is almost always used with negative feedback (a portion of the output signal is fed back to the inverting input). This feedback helps stabilize the circuit's gain, make it predictable, and make it less dependent on the Op-Amp's internal characteristics. It is this negative feedback that allows for the various configurations we will explore.

Types of Op-Amp Configurations

1. Inverting Amplifier

In this configuration, the input signal is applied to the inverting input via a resistor, while the non-inverting input is grounded. A feedback resistor connects the output to the inverting input. The output signal is amplified and inverted with respect to the input signal.

The voltage gain (Av) is given by:

Av = -Rf / Rin
Where Rf is the feedback resistor(R2) and Rin is the input resistor(R1).

2. Non-Inverting Amplifier

Here, the input signal is applied directly to the non-inverting input. A portion of the output is fed back to the inverting input via a voltage divider, creating negative feedback. The output signal is amplified and in phase with the input signal.

The voltage gain (Av) is given by:

Av = 1 + (Rf / R2)
Where Rf is the feedback resistor and R2 is the resistor connected from the inverting input to ground.

3. Comparator