# Opamp circuit analysis problem

Discussion in 'Homework Help' started by anhnha, May 14, 2012.

1. ### anhnha Thread Starter Active Member

Apr 19, 2012
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48
Hi everyone,
I am sorry for my title.I am mistaken and can not change it.
Below is is part of solution of my problem, but there are some things make me confuse.
The solution is:

Eo=Up= E2. R4/(R3+R4) + E3. R3/(R3+R4)
If U1< Eo => Un< Up => Ud= Up - Un > 0 => Ua> 0
then D1 is conducted and D2 is not conducted.
If U1> Eo => Un> Up => Ud= Up - Un < 0 => Ua< 0
then D1 is not conducted and D2 is conducted.

What make me confuse is that
I think in order to know a diode is conducted or not we have to know the bias of diode or the voltage is appy to diode, we have to know both the voltage sides of diode.
In the solution why we can conclude when U1< Eo then Un < Up (Why Un is not equal to Up).
Why we can conclude when Ua <0 then D1 is not conducted and D2 is conducted.

Thanks a lot.

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Last edited: May 14, 2012
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But don't see any diagram

3. ### anhnha Thread Starter Active Member

Apr 19, 2012
776
48
I am sorry, I have just repost it.
Thanks.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,116
Try using the superposition and assume at first that Vp is connect to gnd.
And then try guess which diode "ON".
But first analysis this pictures

As you can see increasing non-inverting creates only a DC-offset.

As for why Vp is not equal to Vn. I think that this help us clearly defined the polarity of the output voltage.

See this post

and this imagine

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anhnha likes this.
5. ### anhnha Thread Starter Active Member

Apr 19, 2012
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48
Thank you so much.
The explaination is very detail and useful.Very nice pictures.