A 741 can be okay for certain applications as long as you (to paraphrase Dirty Harry) "know its limitations".

 

@#12
- No.
- No, I don't.
- Unfortunately no. I always wanted to have a nice oscilloscope but I couldn't purchase one.
- Well I would like to see how this Op-Amp behaves with the gain of around 400 and I knew that it would has problem in the output with this gain. then this would be a good chance to me to get familiar with the problems of Op-Amps and its solution.
- For supply I use a ATX PSU(Power supply unit) like this:

And for posetive input(for Op-Amp), I use two simple potentiometer. one 100kohm and one 500ohm. I give the +12 output of PSU to 100kohm and then from 100kohm to 500ohm and finally from 500ohm to posetive input of the Op-Amp. in this way I can make voltage even less than 1mv. look at the circuit:

(please ignore the red part. that's irrelevant.)

- I'm not sure.

@djsfantasi
Yes. What does mean series1 and 2?(in other word, What are those two curve?) you meant measured curve and calculated curve?

@bertus
Thanks but I'm not going to use OP484 or something else. Ironically I think LM324 is better. because I want to learn how to reduce the error!

@Alec_t Thanks dude. yes, this Op-Amp would be a good choice for learning.

Okay guys, now let's discuss about the solutions. the our problems are the Input Offset Voltage and Current. how to figure it out? can you explain it to me via calculation?

Thanks in advance

 

Yes, series1 is the measured values and series 2 are the calculated values. My last graph is labelled more clearly and is a better representation of your situation. The latter graph ignores the strange value and shows your measured value is a linear function. It also shows the percent difference between the measured and calculated values, which is approximately 0.8%

 

You have found that there is a small (within tolerance) positive offset voltage. In other words the opamp behaves as though its non-inverting input is slightly above its inverting input, even when it really isn't. So how do you think you can adjust an input to compensate that?

 

You have found that there is a small (within tolerance) positive offset voltage. In other words the opamp behaves as though its non-inverting input is slightly above its inverting input, even when it really isn't. So how do you think you can adjust an input to compensate that?

I have to add a voltage between 3 to 7mv to the inverting input. am I right? What is the simplest way to do it? is there any way to add a circuit or part to do it automatically? like an auto-zero-derift Op-Amp.

 

Another question,
Can supply voltage influence on Gain? if so, how?

 

am I right? What is the simplest way to do it? is there any way to add a circuit or part to do it automatically?

1) You're right.
2) In a similar way to how you obtained your 1mV input (i.e using a potential divider). Apply the compensation via a high value resistor.
3) It could be done automatically, but not simply .
4) Ideally supply voltage does not affect gain. Realistically, you can only drive the output to within ~1.5V of the supply voltage, so for a given input signal that will limit the gain that can be used without clipping the output signal.