value of resistance in opamp ?

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vicky29

Joined: 05 Feb 2005
Posts: 75

03 Sep 2005 11:55 value of resistance in opamp ?

dear friends

how should i calculate values of resistors Rg2 and Rf for a particular gain in the following diagram .for example i want to change the gain to 83.334 what will be the new values of the above resistors. conditions are 0<Vin<0.05 & Vref is ground.

thanks

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Hughes

Joined: 10 Jun 2003
Posts: 718
Helped: 87

03 Sep 2005 15:42 value of resistance in opamp ?

RG2=RG//RF

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vicky29

Joined: 05 Feb 2005
Posts: 75

03 Sep 2005 16:09 value of resistance in opamp ?

thanks "hughes" for replying but according to your method Rg2 should be = 990k/10k = 99k but it's 9.9k.....

SkyHigh

Joined: 13 Jan 2005
Posts: 376
Helped: 51

03 Sep 2005 16:33 Re: value of resistance in opamp ?

The above circuit is a practical non-inverting amplifier.
Series conductor resistance ±Rs remains unchanged.
RG2 should be the same value as RG in order to matched both input terminals so as to minimise common-mode voltage.
Since non-inverting circuit voltage gain = 1 + RF/RG = 83.334,
you should use RF = 990kΩ and RG=RG2=12kΩ that will give you the gain of 83.5, thus close to 83.334 that you want to achieve.

vicky29

Joined: 05 Feb 2005
Posts: 75

03 Sep 2005 16:51 value of resistance in opamp ?

Thanks a lot Skyhigh for replying. Actually i saw this calculator http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/opamp/opamp.html?opamp=OP1177%20+/-5V on analog.com. what ever gain value i give it shows RG <> RG2...why is it so althogh both of them are of nearby value but still why is that difference???? and moreover it always keep value of RG = 10k for example like shown in diagram

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SkyHigh

Joined: 13 Jan 2005
Posts: 376
Helped: 51

03 Sep 2005 17:17 Re: value of resistance in opamp ?

What do you actually want to achieve?

What is the maximum swing of your input signal and DC level?
What is the rail-to-rail voltage for your non-inverting amplifier to operate?