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Opamp question

Discussion in 'Electronic Basics' started by Anders Nesheim Vinje, May 10, 2005.

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  1. I was wondering why in an inverting opamp configuration, sometimes there is
    resistor from the + input to ground.
    The resistor often matches the resistor going into the - input in where the
    signal goes trough. Does this improve the overal perfomance of the opamp?
    Has this something to do about the matching input impedance and therefore
    amplify both the postive and negative part of the singnal equalley and
    theirby rejecting more noise??

    Anders N. Vinje
     
  2. tlbs

    tlbs Guest

    The resistor from the + input to ground will act to equalize the errors
    due to bias current (Ib). Bias current flows through both the feedback
    resistor and the resistor from the - input to ground (through the
    impedance of the input source), and generates a small voltage. If
    there is no resistor present at the + input, that voltage due to the
    bias current produces an error at the output.

    By placing a resistor from the + input to ground, the bias current
    flowing from the + input generates a voltage across the resistor equal
    to that in the negative side and the error voltage cancels out at the
    output.

    The ideal value for the resistor is: the parallel combination of the
    feedback resistor, and the sum of the input resistor with the source
    input impedance. Any value other than zero-ohms will reduce the bias
    current error, though (as is the case in your example).
     
  3. The resistor from + input to ground is used only when the opamp bias
    current causes an objectionable offset at the opamp inputs when it
    passes through the input resistances. If the total parallel
    resistance of the network on the - input is matched by a resistor on
    the + input, the approximately equal input bias currents will produce
    similar voltage drops and there will be a cancellation. The resistor
    on the + input actually increases the noise a bit, because the bias
    current has some noise in it, and the resistor will drop a variable
    voltage as this noisy current pases through it. The resistor also
    contributes a bit of its own noise to the input.
     
  4. Ban

    Ban Guest

    Anders,
    in older opamps you will find that the bias current is much bigger than the
    offset current. Then this resistor might be of some impact in high ohmic
    circuits.
    Lets look at the venerable LM324: Ibias is typically 45nA, whereas Ioffset
    is only 5nA. With a 100k source resistance the resistor will reduce the bias
    induced offset error from 4.5mV to 0.5mV. Additionally you have to add the
    2mV offset voltage. This is input related, a gain in the circuit also
    amplifies these offsets.
    But many modern precision opamps have some cancellation circuit built in, so
    the bias current is no more larger than the offset current. In this case
    there will be no improvement.
    The OPA177 has 0.5nA bias and 0.3nA offset current. This will cause 50uV
    offset in the 100k resistor which could be reduced only to 30uV with the
    resistor. This opamp has only 20uV additional offset voltage in the cheap
    version.
    Of course inserting the resistor has also disadvantages: the source resistor
    noise will go up 3dB unless you bypass it with a cap, and also the input is
    more prone to induction, because its impedance is higher now.
     
  5. Guest

    Question , John,
    Would a capacitor from + input to ground reduce the bias and resistor
    noise?
    Thanks
    Glenn Gundlach
     
  6. For frequencies higher than 2*pi*R*C it would for the resistor and
    current noise. It doesn't help the input voltage noise, since that is
    effectively a source in series with the capacitor.
     
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