diiferential amp(p spice)

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Mar 5, 2014.

  1. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    I am trying to do simulation for differential amplifier by using P Spice ,but the output is not right ,can please someone check what is the problem
    the output should be 10 Vdc
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    What output are you getting? The differential gain of the amp is 100k/3010k = 0.0332 and thus the output for the 600V at the input should be 19.93V, not 10V.
     
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  3. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    thanks ,I got 14.4 Vdc,i attach the p spice
     
    Last edited: Mar 5, 2014
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Of course. Your amp is saturated. It's trying to get to 20V but can only get to 14.4V with a 15V supply.
     
  5. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Takymounir, your circuit cannot work as a diff. amplifier because the input signals have no ground reference.
     
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  6. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    the gain is 100k/6010 =.0166 so vo= 600 * .0166=9.983 is this right or no
    I get 14.4 not 9.98
     
  7. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Kilo-Volts, Milli-Volts, Volts or Amperes or something else?

    Question: Did you correct the circuit (input ground reference)?
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,991
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    The input doesn't need a separate ground reference for this circuit. That's provided through R5, R6, and R7.

    TAKYMOUNIR, the differential gain is 100k/3010K (look here under Differential Amplifier). Don't know how you arrived at your value.
     
  9. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    The resistor values in the second circuit are different than the first circuit. 6 Meg vs 3 Meg for R3 & 6.
     
  10. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Crutschow, thank you. You are right. There was something like an obstacle in my head. Perhaps, because the circuit looks rather uncommon?
     
  11. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    OK, I missed the resistor values being changed in the second circuit. For that the calculated output should be 9.98V, and that's what my simulation also showed, so don't know why your simulation is different. What op amp is that?
     
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  12. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    sorry I change it it is 6 mega instead of 3 mega
     
  13. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    I use LF411,I WAS USING LETTER M AS MEGA AND THAT IS WHY MY SIMULATION WAS WRONG,SO SORRY FOR THIS
     
    Last edited: Mar 6, 2014
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