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Unknown voltage of a Op amp

LvW

Apr 12, 2014
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May I ask you: What is your level of education? Are you a student or an interested amateur or...?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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25,510
Dig some more.

Why do you think V+ is 0V?

You've already said no current flows through the resistor R1. In that case, what can you say about the voltage at each end? Look at post 18. That's telling you the voltage across the resistor, not the voltage at any particular point.
 

Offshore again

Jun 24, 2016
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Steve,
Obviously getting myself confused and being honest your questions had thrown my train of thought (even though the intention was to make me see what was going on).

The bias current compensating resistor (R1) is placed in series with the noninverting (+) input to compensate for bias current voltage drops in the divider network: And should be the same as the parallel sum of R2 and R3.

So the voltage drop across R1 would be I (Bias current) x R (value required for compensating resistor.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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true, and in this case the bias current is zero, and you have correctly calculated the voltage drop across it must be zero.

So if the voltage (with respect to ground) at one end of the resistor is 100mV, what is the voltage at the other end (I.e. At the noninverting input of the op amp)?

And therefore, what must be the voltage the feedback loop will have the op-amp try to maintain at the inverting input
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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oh, it would be worth answering LvW's question as im sure he can provide a lot more assistance suited to your existing level of knowledge.
 

LvW

Apr 12, 2014
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The bias current compensating resistor (R1) is placed in series with the noninverting (+) input to compensate for bias current voltage drops in the divider network: And should be the same as the parallel sum of R2 and R3..

I think, you have found the above explanation somewhere.
OK - that`s "legal", no problem. However, do you really understand the task of this resistor?
For example, what could happen if R1=0 ?
Would the opamp still work?
 

Offshore again

Jun 24, 2016
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true, and in this case the bias current is zero, and you have correctly calculated the voltage drop across it must be zero.

So if the voltage (with respect to ground) at one end of the resistor is 100mV, what is the voltage at the other end (I.e. At the noninverting input of the op amp)?

And therefore, what must be the voltage the feedback loop will have the op-amp try to maintain at the inverting input

Steve voltage on Non Inverting will be 100mV.
The voltage on the other input will also be 100mV ( V−=V+ because the op-amp is ideal )
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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My course notes show that:
V−=V+ because the op-amp is ideal and V+=Vin

So is this as simple as as saying V- =100mV ???

So that was your question

Steve voltage on Non Inverting will be 100mV.
The voltage on the other input will also be 100mV ( V−=V+ because the op-amp is ideal )

And you've shown that the answer is yes.
 
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