Op amp with fet in his feed back

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Mar 23, 2012.

  1. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
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    Please can some one help me why we use the fet with op amp in the feedback instead of using resistors
     
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  2. #12

    Expert

    Nov 30, 2010
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    The fet is pretending to be a variable resistor. R61 is pouring 8 microamps into the non-inverting input from a 5 volt source and the fet is keeping that input at zero volts so it matches the voltage on the inverting input...which is R96 to ground.

    As the input signal changes, the fet dumps more or less current in order to keep the non-inverting input at zero volts.
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The source of the MOSFET is a current source. As #12 said, the +input to the op amp is a virtual ground, so any current from R61 and R101 will flow through the D-S of the MOSFET . The source current will be independent of the value of R90, unless the current tries to force the source voltage to go above zero volts.
     
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  4. #12

    Expert

    Nov 30, 2010
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    As Ron said, R90 is almost insignificant. 8 microamps can only create 1.6 millivolts across R90 and the op-amp will simply increase the gate voltage on the fet (by 1.6 millivolts) to compensate for that tiny voltage.

    The parts of the circuit that we can't see might have a lot more effect than the parts we CAN see, but the question is answered: The fet is acting as a variable resistor.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    We know what the FET is doing but it's not clear why it's there. :confused: As #12 said the parts of the circuit we don't see may have a significant effect on the circuit operation and could help explain what the FET's purpose is.
     
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