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DC & Op Amps

Discussion in 'Electronic Basics' started by zero, Mar 25, 2005.

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  1. zero

    zero Guest

    In PSPICE I created a circuit with a VDC = 6V feeding into a uA741 op-amp in
    a inverting amplifier configuration, with both the input and feedback
    resistors at 1K.

    I expected Vout = -6V but instead got Vout to be about 0V. I also expected
    the V- to be 0V because of virtual ground but it came out about 1/2 of Vin.

    I was wondering why the op-amp doesn't amplify at DC.
     
  2. Paul Wagner

    Paul Wagner Guest

    It does, but only in the given bounds. For the OPAmp to go to negative
    voltages, it has to be supplied with a negative voltage, which means that
    you have to use a symmetric power supply (two DC-sources in series, the
    connection point being ground).

    So use

    +6V
    |
    |
    -----
    ---
    |
    |
    |
    o--------+
    | |
    | GND
    -----
    ---
    |
    |
    |
    -6V

    instead of

    +6V
    |
    |
    -----
    ---
    |
    |
    |
    GND

    and it will work. By the way, the 741 does reach its supply limits only
    within a margin of about 2..3V, so if supplied with +/-6V, you can only
    expect +/-4V at the output.

    Paul
     
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