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Op amp circuit

Discussion in 'Electronic Basics' started by Jim Donaldson, Jan 5, 2004.

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  1. Hi All,

    I've just encountered an op-amp circuit that has confused me slightly.
    The input signal comes into the non-inverting input, and there is
    feedback via a potential divider into the inverting input (so far, so
    good - a standard non inverting amplifier.)

    However, there is in addition a DC voltage into the inverting input
    which is set by a two resistor potential divider between the power
    supply rails (0VDC and 5VDC.)

    I presume that this provides a DC bias for the circuit (is this
    correct?), however, I am a little confused as to how to analyse it. My
    problem being that if we use the standard ideal op amp analysis
    technique, then V- = V+ (the voltages on the op amp inputs become
    equal because of -ve feedbak.) However, we have this DC voltage input
    connected directly to V-, which means that V- doesn't equal V+.

    Thanks for all help in advance,

  2. You are correct in your understanding, so far. The only other
    equation that you need to finish is based on the assumption that the
    V- input has a very high impedance, the current into this pin can be
    considered to be approximately zero, so any current arriving at that
    node via one branch (the resistor to the pot, for example) must exit
    via some other branch (the feedback resistor, for example).
  3. Ian Bell

    Ian Bell Guest

    The DC bias to the non inverting input is a standard way of biasing an op
    amp for use in single supply situations. Normaly though, the resistor you
    have from the inverting input to ground would be decoupled by a capacitor.
    In the way the DC level at the output would be the same as the pot divider
    connected to the non inverting input. The pot divider creates a false
    ground for the op amp, usually at about half the supply volts. If the
    resistor fron the non inverting input really is connected directly to 0V
    then this effectively is a negative input voltage relative to the false
    ground. SO just subtract the false ground potential from all inputs and
    analyse as normal.

  4. I sometimes use an offset bias like this even when powered from dual
    supplies. Its fairly common in video amps and error amplifiers in
    feedback loops.
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