Op Amps: The Practical Model

This post covers part two of The Guide to Practical Operational Amplifiers. The topic for this post is a model covering how op amps operate in the real world and how that differs from the ideal op amp model.

Click here for the full Guide to Practical Operational Amplifiers.

The ideal op amp follows some basic “golden rules”.  Ideal op amps have infinite open-loop gain.  No current flows into the inputs, and the positive input terminal has the same voltage as the negative input terminal.

The ideal operational amplifier does not exist!

Practical op amps are real world devices primarily based on transistors but also have other internal circuitry.  For example, the below figure shows a diagram of what is inside each op amp in the MC3317X series.

MC3317X Op Amp Series Internal Schematic

These non-ideal op amps have input currents and offsets, input and output impedance, limited gain, and other non-ideal properties.  The next figure and the resulting equations describes the practical op amp model taking these effects into account.

Simple Op Amp Model

This table compares these parameters with their ideal value.  There will be upcoming posts covering non-ideal effects such as temperature, noise, slew rate, and stability.

Parameter NameSymbolIdeal Value
Input CurrentIIN0 Amps
Input Offset CurrentIIO0 Amps
Input Offset VoltageVIO0 Volts
Input ImpedanceZIN∞ Ω
Output ImpedanceZOUT0 Ω
BandwidthBW∞ Hz
Summary of Op Amp Parameters

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