# Double OP AMP linearity

Discussion in 'Electronics Homework Help' started by Integrator741, Jun 16, 2013.

1. ### Integrator741

125
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Jun 16, 2013
Good day everybody!

My teacher gave me double op amp circuit. The first problem was that output was too small, basically i had to describe how to increase voltage gain and I did it.

The second question says "analyze its effects on the linearity over an input voltage range" and voltages are "10mV pk, 100mV pk, 500mV pk and 1Vpk"

I will level with you and tell you that I have no clue what linearity over an input voltage means. I tried to google it, i couldn't find anything.

Please, can somebody help me to understand this question?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
OK, if the amplifier has a nominal gain of 10, for these inputs, the output will be 100mV 1V, 5V, and 10V.

Those outputs would make the output linear -- the same gain is applied to each.

Does that rule apply to your amplifier?

3. ### Integrator741

125
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Jun 16, 2013
Yes, that is exactly how it works with the voltage gain of 10.

Bu then I increased my voltage gain to 20. And weirdest thing happed. Basically when I put my input of 10 mV peak to peak i get 200 mV peak to peak. Then again I put 100 mV peak to peak and i get 2 V peak to peak. I put 500 mV peak to peak i get 10V peak to peak.

And then i came to 1 V peak to peak. I put in 1 V peak to peak and what i get is about 14.5 v peak to peak and my output wave looks like trimmed off.

P.S. Picture is from http://jeelabs.org/2012/05/20/a-better-sine-wave/
But my output wave looks just like it.

So my question would be, why is that? Shouldn't my output of 1V peak to peak be 20V peak to peak??

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Indeed!

The basic op-amp equations will suggest that for an input of 1V with a gain of 100 should result in 100V output. Would you expect that?

You need to consider a couple of things, but the most important (and the only one I'll draw your attention to) is the power supply voltage.

What relationship do you imagine the output has to the power supply voltage(s)?

5. ### Integrator741

125
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Jun 16, 2013
Last edited: Jun 18, 2013

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Sep 5, 2009
exactly

Dave

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
And in many cases you can't even reach the rail. It's not uncommon for the output swing to be only to within 2V of the supply rails.

In addition, it's not always symmetrical (especially for comparators).

...And it can be related to load.

You might want to investigate the difference between a bipolar vs mosfet output stage.

125
4
Jun 16, 2013