Digital systems and design

What is an operational amplifier

This post answers the question :”What is an operational amplifier?”. In digital electronics, operational amplifiers are used in a broad range of applications.

The ideal op amplifier is a differential amplifier with infinite open- loop gain, infinite input impedance, zero output impedance, infinite bandwidth and zero noise.

It has two terminals and one is a negatively marked terminal and it’s called an inverting terminal (${V}_{1}$). And the other is a positively marked terminal called as non inverting terminal (${V}_{2}$) and there is one output there (${V}_{0}$).

So above we discussed what is an ideal operational amplifier. Here below we will cover characteristics of op amplifier. Ideal operational amplifier has the following characteristics:

Infinite Open Loop Gain

• It is gained without feedback
•  ${A}_{V}=\frac{{V}_{0}}{{V}_{d}}$
• It should be infinite to get maximum voltage of $+{V}_{cc}$ or $–{V}_{cc}$
• Gain is high because it’s designed to have high gain using multistage differential amplifiers

Infinite Input Impedance

• Due to infinite input impedance
• No current flows in the input terminals
• The operational amplifier can be driven by almost any source

Zero Output Impedance

• Due to zero output impedance there will not any change in output voltage due to change in load current
• That means it can drive any number of loads or devices ideally

Infinite Bandwidth

• Bandwidth is infinite means it can amplify all frequency signals from zero to infinite
• So the gain of amplifier remains constant from zero to infinite
• Therefore operational amplifier can amplify dc as well as ac signals

Zero Offset Voltage

• Presence of small output voltage through input voltage is zero is called as offset voltage
• Ideally offset voltage is zero means zero output voltage for zero input voltage i.e = = 0

Infinite Common Mode Rejection Ratio (CMRR)

• The ratio of differential gain to common mode gain as defined as Common Mode Rejection Ratio (CMRR)
• Infinite CMRR ensures zero common mode gain
• So the common mode noise output is zero for ideal operational amplifier
• $CMRR=\frac{{A}_{d}}{{A}_{c}}$

Infinite Slew Rate

• Infinite slew rate ensures that the changes in output voltage occur simultaneously with the changes in the output voltage.

Zero Power Supply Rejection Ratio

• Power supply rejection ratio is the degree of dependence of output voltage on changes in power supply.
• PSRR = 0 means the output voltage does not change with fluctuations in supply voltage.