Operational Amplifier 

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The fundamental component of any analogue computer is the operational amplifier, or op amp. An operational amplifier (often called an op-amp,) is a high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output.

Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost significantly more.

Operation

Basic Op Amp CircuitThe amplifier's differential inputs consist of a V+ input and a V input, and ideally the op amp amplifies only the difference in [[voltage]] between the two (called the differential input voltage). The output voltage of the op-amp is given by the equation:

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where V+ is the voltage at the non-inverting terminal, V- is the voltage at the inverting terminal and AOL is the open-loop gain of the amplifier.

The magnitude of AOL is typically very large (seldom less than a million) and even a quite small differential input voltage will result in amplifier saturation.

If linear operation is desired, negative feedback must be used and is usually achieved by applying a portion of the output voltage to the inverting input. The feedback enables the output of the amplifier to keep the inputs at or near the same voltage so that saturation does not occur.

Idea Op Amp

An ideal op-amp is would have the following properties. In practice, none of these ideals can be realized, and various shortcomings and compromises have to be accepted.

  • Infinite open-loop gain
  • Infinite voltage range available at the output
  • Infinite bandwidth
  • Infinite input impedance
  • Zero input current
  • Zero input offset voltage
  • Infinite slew rate
  • Zero output impedance
  • Zero noise
  • Infinite common-mode rejection ratio (CMRR)
  • Infinite power supply rejection ratio

Applications

Op-amps are often used in electronic circuit design. The use of op-amps as circuit blocks is generally much easier and clearer than specifying all their individual circuit elements ([[transistors]],[[resistor| resistors]], etc.).

Basic Op 'Amp' Circuits

 

Circuit Transfer Function
Inverting amplifier
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Non-inverting amplifier
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Differential input amplifier

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Adding, Subtracting & Scaling
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Voltage follower
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Current amplifier

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Integrating amplifier
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Differentiating amplifier

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Voltage Gain of Op-amps

Op Amp Gain
Op-amp Gain

Real op-amp such as the commonly available uA741, do not have infinite gain or bandwidth but have a typical "Open Loop Gain". This is defined as the amplifiers output amplification without any external feedback signals connected to it and for a typical operational amplifier is 105 to 106 at DC (zero Hz). This output gain decreases linearly with frequency down to "Unity Gain" or 1, at about 1MHz.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

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  1. on ap's avatar on ap says:
    10/26/2013 4:59 PM

    I like this page so much. All imformation here help for my work


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