determine resistance in op amp circuit

[enas]

Dear all.I have some question about determining resistance value in designing op amp circuit. For example we want to design inverting amplifier with gain=-2. We can choose R combination (Rin=1k and Rf=2k) or (Rin=100k and Rf=200k). The gain is the same. What's difference between two R combination? Is there any effect of choosing small R or big R ?
thanks

 

[erikl]

We can choose R combination (Rin=1k and Rf=2k) or (Rin=100k and Rf=200k). The gain is the same. What's difference between two R combination? Is there any effect of choosing small R or big R ? thanks

Sure there is: The output must be able to drive Rf, and Rin is the input resistance for your input signal (to be inverted). Usually you want a high input impedance, so Rin should be relatively big (or perhaps as big as you want it). Then the gain decides on Rf.

And here exists another limit: The voltage drop on Rf due to the input current should be neglectable - or should be cancelled by an appropriate resistor in front of the non-inverting input, its value = (Rin || Rf). If this is possible, only the voltage drop over Rf due to the input offset current - which is several orders of magnitudes lower - must be neglectable. So this decides on the upper limit of Rf.

Another criterion is noise: If a low-noise input stage is considered (which usually has a large gain), these resistors add noise, hence they should be as low as practicable.

 

[rajanarender_suram]

Another criteria:

Rin+Rf>> RL(min) i.e. minimum load resistance supported by the Opamp so that the feedback does-not effect the driving capability of Opamp