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For my electronics lab course I need to find the values of this op amp's resistors :

enter image description here

I found & tried these equations :

enter image description here

My question is : How can I find R1,R2,R3 & R4 ; considering I want Vout = 0V when V1 = 5.6V and Vout = 7V when V1 = 5.3V and considering R1=R2 & R3=R4.

(V2 is always 9V, Vcc+ = 9V & Vcc- = 0V, wanted current 100mA, using DC power supply)

All this information was given by my teacher but I couldn't use the equations properly to find the resistors & that's why I am asking for help.

thank you for your help !

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    \$\begingroup\$ The equations you derived are wrong - R2 and R3 appear to be swapped. Maybe there are more mistakes? Yes there's another mistake. \$\endgroup\$
    – Andy aka
    May 4, 2018 at 16:17
  • \$\begingroup\$ you're right, the schematic i found online got swapped resistors numbering compared to mine, i corrected it, do you see another error ? \$\endgroup\$
    – Corn
    May 4, 2018 at 16:22
  • \$\begingroup\$ 0V = G ( 9V - 5.6V) .... Unless you have a gain of zero, this equation is unsolvable. \$\endgroup\$
    – mike65535
    May 4, 2018 at 18:08
  • \$\begingroup\$ that is one of the things i don't understand. The teacher gave it and it's everywhere on the web for that op amps' kind of use. Yet it doesn't seem mathematically logical \$\endgroup\$
    – Corn
    May 4, 2018 at 18:58
  • \$\begingroup\$ The equation might exist, but the givens - I mean the actual voltages? They come from your teacher, no? They aren't on the web, are they? If so, provide a URL please. \$\endgroup\$
    – mike65535
    May 4, 2018 at 19:25

1 Answer 1

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1) For a "normal" op amp, start with R1 and R2 at 10k. This will produce currents of about 1 mA for 10 volt differences, and that's a perfectly normal current. Unless you also want super-low amplifier current, as you might well do if you're using batteries. And 9 volts is a standard battery voltage.

2) Use the input voltages for your two stated conditions. Find the output difference and divide by the input difference, and that's your gain.

3) Now you have to produce your input offset. Given your input conditions, this should be easy.

4) Finally, a caution. You should note that, for V+ equals 9 volts, and V- equals zero, most real op amps will not produce an output swing from zero to 9 volts. Those that do are called rail-to-rail output amps, and even those don't do well at each extreme if called on to produce much current.

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  • \$\begingroup\$ Would i use the same resistors values for a current of 100mA ? also i will be using a DC power supply. Could you please clarify a bit more step 3) ? \$\endgroup\$
    – Corn
    May 4, 2018 at 16:54
  • \$\begingroup\$ If you had 100 mA, what would be the voltage drop across the resistors? And no, I'm not going to do your work for you. If you cannot figure it out, start another question and ask that. \$\endgroup\$
    – WhatRoughBeast
    May 4, 2018 at 20:23

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