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Homework Help: Pspice opamp DC open loop gain

  1. Dec 31, 2011 #1
    1. The problem statement, all variables and given/known data
    Hi, im trying to use pspice student to work out a ideal opamp. This is what i did:
    qsp3bk.png

    2. Relevant equations
    I have find out the gain for my opamp using simulation. But the problem is how can i find out the DC open loop gain using this software? i have google it and it just show me the way of "spice" to find out the open loop gain. In my courework, i need to use the simulation.

    Thanks for the help
     
  2. jcsd
  3. Dec 31, 2011 #2
    There are a lot of ways.

    The easy way is to open the loop then manually adjust the input voltage until you get a finite output voltage that's not at the rails. Then take the ratio of output to input.

    This is a "C" answer. There are better ways; see if you can figure one out.
     
  4. Jan 1, 2012 #3
    Thank you for your time
    I am not quite understand what you mean by "that's not at the rails"?
     
  5. Jan 1, 2012 #4
    The amplifier in your diagram is "closed loop" because you have a feedback resistor of 100k from the output ( terminal 6) to the - input ( terminal 2)
    Open loop means removing this feedback resistor ( the open loop gain will be very, very high !!!!!!!)
    "not at the rails" means the output voltage is less than the power supply voltages +/- 10V ( power supply connections are sometimes called " the rails")
     
  6. Jan 1, 2012 #5
    Thanks for the info. I have simulate the open loop gain out but it seems like wrong.
    waveform: http://www.sendspace.com/file/2mrn69
    Do you see what is the problem with that?

    I have connected a cap paralle with R2 to decrease the noise. And how can i find out the offset voltage for my opamp? It said i have to connect the terminal 1 and 5 to zero to get the offset voltage. But i am still confusing how can i do that?
     
  7. Jan 1, 2012 #6
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