I can not get the correct anser

[pretty]

Hi, Please help me.  As the following sch I can not get the correct sqrt value.  if changing resistor of R1 I always get 12v direct flow output.

[kittmaster]

Your knowledge of op amp theory leaves a lot to be desired.  First, its acting like a comparator because you  have no negative feedback (open loop mode), you are using a multiplier in the feedback path, what did you expect it to do?  Since 5V is greater than 0 and 5 times X Gain, it will clamp to the upper Vcc rail in this case, 12V.   You also did not ground the scope, where is your point of reference if it is not grounded?  What exactly are you trying to do here???????????? 

Message Edited by kittmaster on 04-07-2010 08:02 PM

[pretty]

Thanks for your answers.  I want the sch to do the operation of square root.

 1. in my sch the inverting input is DC -5v, I want to get the square root of 5?

 2. In my feedback path I use a multiplier, I Originally thought it is a negative feedback?

 3. it Already have ground in the sch, I do not understand that you say "did not ground the scope"?

Thanks again, Could you give me the further pointing. if possible Could you give me a correct sch.

(in fact, I get the sch from a rough sch in a textbook(the rough sch not for multisim). I just give the values for the all components)

[kittmaster]

1. Exactly, 0 is greater than -5, in open loop mode, the output will drive to the + Rail based on input X Gain.  I only quickly looked at the datasheet of that device in the feedback path and it looks like some type of amp.  Since the gain of open loop is >100, any mV offset will want the output to be mV X open loop gain value.  Unless I'm missing something.   2.  THe negative terminal of the scope is FLOATING, it needs to be referenced to ground. 3. Take a look here to see an example of a square root op amp circuit   http://www.edn.com/archives/1997/112097/24di_07.htm

[pretty]

Thanks!  Ok, the only muddle is "THe negative terminal of the scope is FLOATING".   I do not understand what is the negative terminal you pointed. Is that VEE -5? and how to do?  ( I have looked the website you give me. I just want to solve the problem.)

  I am a chinese man.  Maybe we have Differences in expression. could you give me help Patiently!!

[kittmaster]

the two terminals on the OSCOPE + and -   >  The - terminal should be tied to your GROUND.  The + terminal to where you want to measure.  I'm off to bed, I'll check back tomorrow.