Why do they say inverting op amp suffers from low input resistance

Discussion in 'Homework Help' started by nyasha, Jan 29, 2011.

  1. nyasha

    Thread Starter Active Member

    Mar 23, 2009
    For voltage amplifiers we need high input resistance, and for the inverting op amp it can be shown that the input resistance is equal to R1. What l don't understand if why does the textbook say the inverting op amp suffers from a low input resistance ? Please help[​IMG]
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    It depends on the value of R1. The inverting terminal runs at virtual ground, so R1 is all the signal sees.

    There are a lot of parameters not given in your example. If the voltage source is of small capacity, then an input resistance in the 10's of Gigaohms might be necessary to preserve signal fidelity. If the signal can source 10's of milliamps, then a couple of thousand ohms will do. Not all voltage amps work with similar voltage sources.

    Generally speaking, if you are dealing with a fragile signal, using an op amp as a buffer in the non-inverting mode followed by one in the inverting mode for gain is pretty common.
  3. Papabravo


    Feb 24, 2006
    Whatever the magnitude of R1 it is unlikely to be in the neighborhood of the of the actual input resistance of the amplifier which may be in the 100K to 10M Ohm range. So 1K to say 47K is much smaller than the input resistance of the amplifier itself. It is the actual connection of the terminals and the external components that produces this behavior.