Difference between revisions of "Real electronics"

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Real electronics sometimes behave significantly different than the idealized models presented in lecture.
 
Real electronics sometimes behave significantly different than the idealized models presented in lecture.
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==Real power supplies==
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==Real voltage and current measurement devices==
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==Real electronic breadboards==
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==Real capacitors==
  
 
==Real op-amps==
 
==Real op-amps==
  
[[Image:Real_Electronics_Op-amp_Model.png|frame|center|250px|A model of an op-amp highlighting its behavior in practice.]]
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[[Image:Real_Electronics_Op-amp_Model.png|thumb|center|600px|A model of an op-amp highlighting its behavior in practice.]]
  
 
Amplifiers using op-amps unfortunately do not follow the golden rules to the letter. The open-loop gain is not infinite. Therefore the difference between the input voltages is not zero and the current into or out of the inputs is not zero. A realistic model of an op-amp must incorporate a specification for the input offset voltage ''V<sub>OS</sub>'', an internal voltage that adds to whatever voltage is externally applied to the input terminals V- and V+. The model also incorporates a bias current ''I<sub>B</sub>'' for each input. These are close in real op-amps so the average is specified on the datasheet. However they are not perfectly matched and the difference is specified as the input offset current ''I<sub>OS</sub>''. All of these non-idealities have the net effect of causing a non-zero voltage, sometimes large, at the output in a negative feedback implementation even when the input is zero. Luckily there are corrective actions available.
 
Amplifiers using op-amps unfortunately do not follow the golden rules to the letter. The open-loop gain is not infinite. Therefore the difference between the input voltages is not zero and the current into or out of the inputs is not zero. A realistic model of an op-amp must incorporate a specification for the input offset voltage ''V<sub>OS</sub>'', an internal voltage that adds to whatever voltage is externally applied to the input terminals V- and V+. The model also incorporates a bias current ''I<sub>B</sub>'' for each input. These are close in real op-amps so the average is specified on the datasheet. However they are not perfectly matched and the difference is specified as the input offset current ''I<sub>OS</sub>''. All of these non-idealities have the net effect of causing a non-zero voltage, sometimes large, at the output in a negative feedback implementation even when the input is zero. Luckily there are corrective actions available.
  
 
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{{Template:20.309 bottom}}

Revision as of 03:21, 23 August 2013

20.309: Biological Instrumentation and Measurement

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Real electronics sometimes behave significantly different than the idealized models presented in lecture.

Real power supplies

Real voltage and current measurement devices

Real electronic breadboards

Real capacitors

Real op-amps

A model of an op-amp highlighting its behavior in practice.

Amplifiers using op-amps unfortunately do not follow the golden rules to the letter. The open-loop gain is not infinite. Therefore the difference between the input voltages is not zero and the current into or out of the inputs is not zero. A realistic model of an op-amp must incorporate a specification for the input offset voltage VOS, an internal voltage that adds to whatever voltage is externally applied to the input terminals V- and V+. The model also incorporates a bias current IB for each input. These are close in real op-amps so the average is specified on the datasheet. However they are not perfectly matched and the difference is specified as the input offset current IOS. All of these non-idealities have the net effect of causing a non-zero voltage, sometimes large, at the output in a negative feedback implementation even when the input is zero. Luckily there are corrective actions available.