# about Rail-to-Rail Opamp

Discussion in 'Homework Help' started by firstman, Apr 5, 2010.

1. ### firstman Thread Starter New Member

Mar 19, 2010
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We know that output cascade of Rail-to-Rail opamp use common emitter.Common emitter has very high output resistance because two collectors connect in output. But when I measured output resistance of Rail-to-Rail opamp in micro-cap 9,I get low out put resistance . I want to know , why I get this result? Please explain me.The measurement result of output resistance of Rail-to-Rail ( AD8031 ) is shown in below.

AD8031

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I think Rout is low thanks to voltage negative feedback.
Negative feedback decreases the output resistance by a factor ( 1 + β A_ol)
A_ol is a open loop gain
β - feedback factor ( feedback gain )
So
Rout = Rout_open_loop/(1+A_ol*β)

Last edited: Apr 5, 2010
3. ### firstman Thread Starter New Member

Mar 19, 2010
6
0
thanks your answer. But it is without negative feedback. It is open loop output resistance measurement of rail-to-rail(AD8031).

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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Post the schematic of your measurement technique.

5. ### firstman Thread Starter New Member

Mar 19, 2010
6
0
Output measurement of rail-to-rail (AD8031) in micro-cap 9 is shown in below.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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How did you come up with 531μV for the input offset voltage?

7. ### firstman Thread Starter New Member

Mar 19, 2010
6
0
Input offset voltage effect the output resistance and other parameters.In this case, the offset voltage is so small and can neglect it. Output resistance measurement of rail-to-rail (AD8031) is shown in below, as we assume input offset voltage is zero.Clearly, I get same value of open loop output resistance.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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674
I think your op amp is saturated at one of the rails, so you are seeing the resistance of a saturated transistor.
For the simulation, try running the op amp with unity DC feedback, but open loop for AC. You can do this by connecting a 10k resistor from output to -in, and a 1000 Farad cap from -in to GND. Connect +in to GND, and use the power supplies you show in your schematic.
I have done this with many op amps. LTspice does not converge for this op amp. Perhaps your simulator will.

9. ### firstman Thread Starter New Member

Mar 19, 2010
6
0
I did simulation as you said , but result is same.If my simulation is wrong,please reply me.Thanks for your help.

10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
674
Your sim circuit looks OK. How do you measure Rout? Is there an equation you use?