# real op-amp with a given open-loop gain

Discussion in 'General Electronics Chat' started by ronybhai, Sep 18, 2012.

1. ### ronybhai Thread Starter Member

Sep 18, 2012
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How to Draw the input-output voltage transfer characteristics of a real op-amp with a given open-loop gain.

2. ### crutschow Expert

Mar 14, 2008
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Look on page 7 of this for an example.

3. ### ronybhai Thread Starter Member

Sep 18, 2012
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there are 4 pictures on that page. Is it the 1st one Input Voltage Range?

4. ### Papabravo Expert

Feb 24, 2006
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No! It is the 4th one that shows gain in dB as a function of supply voltage and output load. So tell me -- what does 120 dB of voltage gain in round numbers work out to be?

If you want to draw the voltage transfer characteristic it is a line through the origin with a nearly vertical slope up to the positive saturation voltage and dwon to the negative saturation voltage. In both the negative and positive directions the slope is pretty close to zero after reaching saturation.

Last edited: Sep 18, 2012
5. ### ronybhai Thread Starter Member

Sep 18, 2012
97
0
according to the picture, about 15 supply voltage?

6. ### ronybhai Thread Starter Member

Sep 18, 2012
97
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is the image looks like this

7. ### Papabravo Expert

Feb 24, 2006
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Yes. That is the voltage transfer picture for the open loop configuration.
How much voltage gain is 120 dB in round numbers?

8. ### ronybhai Thread Starter Member

Sep 18, 2012
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is it 1000000?10^6

9. ### Audioguru Expert

Dec 20, 2007
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Then at DC or a very low frequency, only 10uV input gives 10V output.
Where will you ever find a signal that is only 10uV?
Opamps are NEVER used open-loop in a linear circuit.

10. ### Papabravo Expert

Feb 24, 2006
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Yes -- 120 dB is a voltage gain of 1 million
Now do you understand why they are never used in the open loop configuration?
If you look at them funny the output WILL go to the rails, and it might even oscillate like a scalded duck.

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11. ### ronybhai Thread Starter Member

Sep 18, 2012
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isnt the input-output voltage transfer characteristics of an ideal op-amp same as a real op-amp?

12. ### Papabravo Expert

Feb 24, 2006
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The deviation from ideality is very slight. The gain of an ideal op-amp is infinite!

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13. ### ronybhai Thread Starter Member

Sep 18, 2012
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but image is the same...it luks the same...whn u draw the characteristics?

Last edited: Sep 18, 2012
14. ### Papabravo Expert

Feb 24, 2006
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Except the slope of the transition between the negative rail and the positive rail is absolutely vertical. That is, the slope is ∞.

15. ### ronybhai Thread Starter Member

Sep 18, 2012
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thts for ideal op-amp?

16. ### Ron H AAC Fanatic!

Apr 14, 2005
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See the attachments.

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• ###### op amp infinite gain.png
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17. ### ronybhai Thread Starter Member

Sep 18, 2012
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thx man..thts what I was luking at.
1st one is Ideal and 2nd one is real.
my h.w. was due today..I kind of got the 2nd one wrong...but now I go it.