# Working with Op Amp mV

Discussion in 'Homework Help' started by Bob O, Feb 16, 2008.

1. ### Bob O Thread Starter New Member

Jul 3, 2007
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I am studying Op Amps and need to determine the fine characteristics of a 741 using the range of +/- 1mV.
I have 1.5 V source on a radio shack setup and I was planning on building a voltage divider to drop the 1.5 V to 1 mV.
My calculation show two resistors R1 = 6.7 ohm and R2 = 10kOhm
Vout = Vs*R2/(R1+R2)

Does this seem practical/correct or am I missing something?

Thanks for the help and or advice.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The input to the 741 has only about 200K resistance, which could throw your calculations off. Rather than try to make a chain of resistors in parallel and series to produce the exact voltage drop. use a fixed resistor and a trim pot - adjust the wiper to the exact level you need.

It's still sloppy and source voltage dependent, but it works.

I guess they must have been purchased by schools in the zillions so they are readily available, but using 741's to demo op amps is like teaching about cars using a 1972 Chevy Nova.

3. ### Audioguru New Member

Dec 20, 2007
9,411
896
The input resistance of an old 741 opamp is so much higher than the voltage divider's 6.7 ohm resistor that it won't make any difference in the measurement.
The input offset voltage of the old opamp which is a max of 6mV at room temperature will make a huge difference.

4. ### Bob O Thread Starter New Member

Jul 3, 2007
9
0
Thank you for the replies.
Sorry it took me so long to get back, watching the race yesterday and the now the big race today.

Another question, I created a bipolar power supply by following the directions but used two different voltage sources and then I tried with just one source and inverted the signals but it didn't work like I had hoped.
Did I wire it to the bread board incorrectly or can't I use the same source the way I had hoped. Just trying to remove or cut down on all these different voltage source for some of these experiments.

Thanks for your help and time.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
What do you mean by “fine characteristics”? Can you post a schematic of what you are trying to do, with some explanations?

6. ### Bob O Thread Starter New Member

Jul 3, 2007
9
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fine characteristics is stated in the lab manual. They want me to vary the voltage from -2 to 2 volts at the Vin and measure the Vout and watch the change and the drop the Vin to around +/-10 mV but try and get close to 1 mV and then vary that and watch Vout.
I was able to build a bipolar power supply -9 to 9 and watch the Vout change but now I am trying reduce the number of batteries I have plugged.

Hope this make sense, it is for class.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
If you understand feedback, and have a multimeter, there might be a better way to do this. Or are you stuck with a scripted procedure?

8. ### Bob O Thread Starter New Member

Jul 3, 2007
9
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With "scripted procedure", I assume you mean understanding and setting up the basics if so, I don't mind jumping through the hoops, It forces me to think about things I have just been able to get by with in the past, not electronics related but electrical. Hope that made sense.

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I assume you are trying to look at the open-loop Vout vs Vin characteristics of the op amp. It might be more easily done when the op amp is in a closed loop (feedback) configuration. However, if you don't yet understand feedback, it probably wouldn't make any sense to you.

10. ### Bob O Thread Starter New Member

Jul 3, 2007
9
0
Yes, I took readings of Vin and Vout and plotted the results. From these results I 'm suppose to calculate the slope of Vout where it crosses zero and determine the Gain.
I have my plot and have calculated the slope but need to check it again and try to make sense of the equations.