The Ideal Op-amp

The IC Op-amp comes so close to ideal performance that it is useful to state the characteristics of an ideal amplifier without regard to what is inside the package.

  1. Infinite voltage gain
  2. Infinite input impedance
  3. Zero output impedance
  4. Infinite bandwidth
  5. Zero input offset voltage (i.e., exactly zero out if zero in).
Compare to real op-amps
Practical departures from ideal op-amp

These characteristics lead to the golden rules for op-amps. They allow you to logically deduce the operation of any op-amp circuit.

Index

Electronics concepts

Op-amp concepts
 
HyperPhysics*****Electricity and magnetismR Nave
Go Back





The Op-amp Golden Rules

From Horowitz & Hill:

For an op-amp with external feedback

I. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.

The Voltage Rule

II. The inputs draw no current.

The Current Rule

Comments on 1: The voltage gain of a real op-amp is so high that a fraction of a millivolt input will swing the output over its full range. Practically, that amounts to rule 1.

Comments on 2: The input current is so low (0.08 microamps for the 741, picoamps for an FET-input op-amp) that rule 2 is practically correct.

The ideal op-amp
Index

Electronics concepts

Op-amp concepts

Reference
Horowitz and Hill
 
HyperPhysics*****Electricity and magnetismR Nave
Go Back










The 741 Op-amp Circuit

The standard 741 Op-amp circuit contains 20 transistors and 11 resistors. It starts with a differential input stage with a current mirror load. This is followed by an npn voltage amplification stage with an active output. A pnp emitter follower drives a push-pull emitter follower output stage. The output stage includes current limiting circuitry.

Index

Electronics concepts

Op-amp concepts
 
HyperPhysics*****Electricity and magnetismR Nave
Go Back