Op-Amp Resistor Consideration

Discussion in 'Homework Help' started by pdunc, May 4, 2010.

  1. pdunc

    Thread Starter New Member

    Feb 26, 2010
    Good afternoon!

    I have built an active filter for part of a project. I know that you cannot use resistors of above ~1 MΩ or op-amp behaviour strays from the ideal conditions.

    But why is this? Does it reduce current to unacceptable levels?

    This might be simple but I am struggling to work it out myself.


  2. mik3

    Senior Member

    Feb 4, 2008
    A real op amp draws current through its inputs which has to be over a minimum limit for the op amp to work properly. If the external resistors are very high then the voltages internally will not be able to adjusted in such a way that the input current is what required.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    While input current in some op amps can be a problem if large resistor values are used, CMOS op amps draw so little input current that 1MΩ will not be a problem. However, parasitic capacitance combined with high resistance values can result in nonideal behavior.
    Also, resistors generate voltage noise proportional to the square root of the resistance, all other factors being equal. In high gain, wide bandwidth circuits, high resistor values can result in excessive noise.