Dear all,
can anybody tell me selection of opamps resistors..??
on wot basis the resistors are been decided from ohms to megaohms..
i am looking for the answers as i coudnt get the idea to select a proper resistors & capacitors...
plz plz plz
There are also inputs' bias currents and offset voltages to worry about, with many op amps (Check for differences between op amps with FET inputs and BJT inputs). And the resistor values you use can determine how those might affect your circuit's behaviors. In some types of circuits, it can be important. For example, maybe sometimes you would want the impedances seen by the two inputs to be matched, to minimize the effects (at the output) of the input bias currents. Those are sometimes reasons, for example, to use a resistor in the feedback loop of a unity-gain op amp buffer, or in series with an input that might usually be tied directly to ground.
This CLASSIC application note will explain a lot about how the resistor values are chosen:
https://www.ti.com/lit/an/snoa621c/snoa621c.pdf
And this one is an essential old classic, too:
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Here is one of Walt Jung's (Analog Devices) books, on line for free ("Op Amp Applications Handbook"):
https://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
Here is another great free book ("Op Amps for Everyone"):
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Those books can answer all of the questions you have asked, here, and far more.
Cheers,
Tom
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Besides the resistor values, one extremely important thing that you will need to do, to get your op amp circuits to work correctly (and sometimes just to get them to work at all), is to ALWAYS use bypass and decoupling capacitors, at the power supply pins.
You should always connect a physically-small capacitor very close to each power pin, usually one from each power pin to ground. These are bypass caps and are needed for stability, i.e. so the circuit won't oscillate or "ring" (oscillation that dies out). The bypass capacitors need to connect VERY close to the power pins; within 2 mm or less is good. Their connections and lead lengths should be as short as possible, to minimize stray inductance, so they will be able to be effective for high frequencies. If in doubt, just use an X7R ceramic cap of 0.1 uF or 0.01 uF. (The reason these bypass caps are needed is that there is usually a hidden positive feedback loop through the power rail, and the cap will short the HF to ground so the circuit won't oscillate at high frequency.)
You should also always connect a larger cap from each power supply pin to ground. This is for decoupling. It acts as a small, local "point-of-load" current supply, for fast transient currents that are needed by the power pin. If these transient currents had to try to come through the power rail conductor, all the way from the main power supply, they would induce voltage spikes across the parasitic self-inductance of the conductor, possibly polluting everything else connected to the power supply, and might also not be delivered to the device's power pin with accurate amplitude or timing. if you're just building an op amp circuit with no really-demanding requirements, you can usually just use a 10 uF or larger electrolytic, for the decoupling cap for each power pin. It doesn't need to be as close to the power pin as the bypass cap. But it should be as close as possible.
Later, you can learn to easily calculate the required values of the decoupling capacitors, and the maximum lengths that their power pin connections should be allowed to be.
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