Input impedances for both inverting and non-inverting amplifiers infinity?

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Discussion Overview

The discussion revolves around the input impedances of inverting and non-inverting amplifiers, specifically questioning whether these impedances can be considered infinite. The scope includes theoretical considerations as well as practical implications based on specific op-amp models.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the input impedances for both inverting and non-inverting amplifiers are infinite.
  • Another participant argues that input impedances are not infinite and refers to the equivalent circuit diagrams in op-amp datasheets, noting that input impedances vary based on the type of input transistors used.
  • A participant mentions using a 411 amplifier and clarifies that the transistors discussed are within the op-amp itself.
  • One participant seeks a theoretical perspective on whether input impedances could be considered infinite for both amplifier types.
  • Another participant provides specific information about the LF411 op-amp, stating its DC input resistance is very high but not infinite, and discusses the impact of input capacitance on impedance.
  • A participant highlights that the input resistances differ significantly between inverting and non-inverting configurations, with feedback affecting the input resistance in the inverting configuration.

Areas of Agreement / Disagreement

Participants generally agree that the input impedances are not infinite, but there is a theoretical inquiry into whether they could be considered infinite under certain conditions. The discussion reflects multiple viewpoints on the practical and theoretical aspects of input impedance.

Contextual Notes

Participants reference specific op-amp models and their datasheets, noting that input impedance can vary based on the type of transistors used and the configuration of the amplifier. There is mention of limitations regarding the measurement of very high input resistances and the influence of input capacitance at higher frequencies.

Who May Find This Useful

This discussion may be useful for individuals interested in operational amplifier theory, electrical engineering students, and practitioners looking to understand the nuances of input impedance in amplifier configurations.

math&science
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are the input impedances for both inverting and non-inverting amplifiers infinity?
 
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Nope. Just look at the equivalent circuit diagram in the datasheet for whatever opamp you are using. It will either have bipolar or FET input transistors. The datasheet will list the input impedances. The Zin will be much higher for a MOSFET input opamp, but the Zin will not be infinite. Also keep in mind that the input capacitance will limit Zin at higher frequencies.
 
but i was using a 411 amplifier and not transistors
 
math&science said:
but i was using a 411 amplifier and not transistors
What's the full part number of the opamp? The transistors that I'm referring to are inside the opamp -- that's how an opamp is made. Take a look at the datasheet for your opamp, or the one for a simple LM741, for example. In addition to listing the input characteristics of the opamp, they will usually show you a simplified equivalent circuit. That will show you approximately how they have built the opamp out of transistors, resistors and capacitors on the monolithic opamp IC.

http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=14&M=LM741CM
 
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Oh, I see what you're saying. I used an LF411. I guess my question was more of a theoretical one. Would it be infinity theoretically for both types?
 
math&science said:
Oh, I see what you're saying. I used an LF411. I guess my question was more of a theoretical one. Would it be infinity theoretically for both types?
Well, the LF411 has JFET input transistors, and its DC input resistance is listed as 10^12 Ohms typical. That's pretty high, but not infinite. The datasheet doesn't list input capacitance, unfortunately, but guess it's around 10pF and that gives about -j160kOhms of impedance, definitely not infinite.
 
oh thank you so much berkeman! i guess it's not infinite but too large to measure here. thank you!
 
It would be best to refer to a text on basic op-amp theory for the explanation, but the input resistances of the inverting and non-inverting configurations differ vastly.
The effect of feedback on the inverting configuration is to reduce the input resistance; perhaps a few ohms, while the non-inverting configuration increases it beyond the intrinsic input resistance of the op-amp itself.
An splendid text is Thomas Frederiksens's "Intuitive Operational Amplifiers"
ISBN 0-07-0211966-4
 

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