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OPAMP: General question about chosing resistors

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Juja

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Dear forum,

just a simple (?) beginner question:

What is the difference e.g. having a differential opamp circuit with v=10 and using

1) 10k-resistor and 100k-resistor (OPAMP_1.png)

OR

2) 1k-resistors and 10k-resistor (OPAMP_2.png)

Thanks so far!
 

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  • OPAMP_2.png
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same gain, difference is input impedance and output drive current

Iout for rail-rail OP Amps is small ( due to high RdsON of internal CMOS output stage) so 10k may not reach 100% both rails if reference is Vbat/2, so >> 10k feedback R is preferred for low Iout.

But if R values are too high, then input bias current offset may cause a Vin offset which may be significant, espec. if using high ratio gains >>10 like 1k~10k
 

Dear forum,
just a simple (?) beginner question:

An answer to your question was given above.
However, as you are a beginner: Are you aware that using only one single supply voltage (s shown in your diagrams) imposes some severe operational restrictions?
The classical opamp application requires dual supply (+/- voltages).
 

Another consideration may be noise. If you make the resister values 10 times higher, the Johnson noise will be 10dB higher. This only matters if you want very low noise obviously, and then you need to use a low-noise opamp otherwise the opamp noise will be higher than the resistor noise anyway.
 

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