Input/Output Impedance

Discussion in 'General Electronics Chat' started by joshgreaves332, Jul 1, 2014.

  1. joshgreaves332

    Thread Starter New Member

    Jun 28, 2014
    Could somebody please do me the favor of explaining the REASON that input impedance of say an amplifier would be high? What function does this serve?

    Also, why would an amps output impedance preferably be low? Would this ever be reversed? What function would that serve?

    I understand an ideal opamp has infinite input Z and 0 output Z. What exactly does this mean for surround circuit elements?
  2. antonv


    Nov 27, 2012
    Sometimes you want in- and output impedance to be equal, if you are interested in transferring maximum power.

    When this is not important then you want input impedance to be high so that when you attach the circuit to a source of some kind then it will not draw enough current from the source to change the source's characteristics.

    Likewise, you want an output impedance to be low so that anything that draws current when connected to the output will not change the output voltage due to an IR drop across the output impedance.

    In both cases you are minimizing sources and loads effects on each other.
  3. joshgreaves332

    Thread Starter New Member

    Jun 28, 2014
    Thanks for the reply: so drawing current from the source and hence changing its characteristics, as you said, is that what is known as the 'loading' of that source?
  4. crutschow


    Mar 14, 2008
    That is correct.

    It means that the input impedance won't affect the signal coming from the input and feedback elements and the output voltage won't be affected by any load.

    In general it means that the opamp operates ideally and the output voltage will be as calculated from the values of the input and feedback impedances. You don't have to put in any fudge-factors to account for non-ideal opamp characteristics.
  5. ericgibbs

    Senior Member

    Jan 29, 2010
    hi 332,
    When designing and building 'filter networks', the output impedance of the element driving the the filter and the load impedance imposed by the receiving element will/can alter the filters characteristics.

    So ideally for a filter network, the driving source will have a very low output impedance and the filter output will be driving a very high input impedance.

  6. anhnha

    Active Member

    Apr 19, 2012