Input Resistance & Bias Currents: Explained Simply

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Discussion Overview

The discussion revolves around the relationship between input resistance, bias currents, and error voltage in operational amplifiers (op-amps), particularly in the context of inverting and non-inverting configurations. Participants explore how these factors interact and the implications for circuit design, including practical considerations for connecting inputs on a breadboard.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether input resistance affects the mismatching of bias currents or only increases error voltage, indicating a lack of clarity on the topic.
  • Another participant suggests connecting the non-inverting input to ground through a resistor equal to that seen by the inverting input to minimize input current imbalance.
  • A different view asserts that input resistance does not influence input bias or offset currents, emphasizing that these parameters are solely dependent on the op-amp itself.
  • One participant explains that the arrangement of negative feedback affects the total output offset voltage, which is influenced by both input bias current and input offset voltage.
  • There is a concern raised about the categorization of the discussion, with one participant questioning why it was moved to a different section, asserting its relevance to electronics.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between input resistance and bias currents, with no consensus reached. Some argue for a connection between these concepts, while others firmly state that input resistance does not affect bias or offset currents.

Contextual Notes

Participants acknowledge that the calculations and relationships discussed are complex and may require detailed analysis, indicating that assumptions about configurations and parameters are critical to understanding the topic.

Who May Find This Useful

This discussion may be of interest to students and practitioners in electronics, particularly those working with operational amplifiers and seeking to understand the implications of input resistance and bias currents in circuit design.

gothloli
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If it is in inverting or non-inverting configuration, will the input resistance increase the mismatching of the bias currents, or will it only increase the error voltage?
Please Explain like your talking to an idiot, I need to write an essay on this, and I don't know what to do!

Also how would you connect one of the inputs to ground, say for example if it's an inverting amp, how would you connect the non-inverting input to ground on a solderless breadboard?
 
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Hi.
It's usually recommended that you connect the non-inverting input to ground via a resistance of equal value to that seen by the other input. This reduces the imbalance in input currents to the bases of the input stage, i believe.
 


Your answer is "Does input resistance in negative feedback increase input offset current of op amp?"

Answer is yes.

Therefore we use and OM(offset minimizing) resistance at the non inverting(connected at ground). Normally OM is equal to the value of input resistance at inverting input(connected with supply). OM helps to minimizes input offset current which is caused by negative feedback.
 


The input resistance of the op-amp has nothing to do with the input bias or offset current...Period. Nothing at the input can affect the input bias or offset current nor the input offset voltage...Period.

The arrangement of the negative feedback WILL affect the total OUTPUT offset voltage. This might be what you meant by error voltage.

1) Output offset voltage cause by input bias current:

Input bias current has to got through the resistors that connect to the input. This will cause a voltage developed across the resistors. Then this voltage will be amplified by the closed loop gain of the op-amp and become the output offset error.

2) Output offset voltage cause by input offset voltage:

This will be amplified by the gain of the closed loop gain of the op-amp.

The total output offset voltage is the sum of the two above. This is a very simplistic view. You have to work out the detail calculation. But again, to be clear. Input offset current, input bias current and input offset voltage are ONLY op-amp dependent, they do not change no matter what configuration of the closed loop feedback.
 


How come this is moved here? This is every bit electronics. It's about op-amp input bias current!
 

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