Opamp circuit analysis problem

Thread Starter

anhnha

Joined Apr 19, 2012
905
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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Jony130

Joined Feb 17, 2009
5,487
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
http://forum.allaboutcircuits.com/showpost.php?p=444315&postcount=9

and this imagine
 

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