negative feedback and impedance

Discussion in 'General Electronics Chat' started by veenife, May 28, 2014.

  1. veenife

    Thread Starter Member

    Jan 16, 2014
    42
    0
    hey,

    how does exactly negative feedback changes impedance???

    i understand that the ouput signal is reversed in phase and fed back into an earlier stage of the amp ... in the case of a transistor is that the emitter???....

    but that then causes a drop in voltage??? ... or current???

    then after that how exactly this will influence the input impedance???
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    It depends upon the nature of the feedback circuit as to whether it affects the input impedance. Post a schematic.
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Look at example amplifier without the feedback

    [​IMG]

    As you can see Zin = Vin/Iin = Rin because all Vin voltage is apply directory across Rin. But if we apply Voltage series feedback to our amplifier. We see that now the bottom end of a Rin resistor is connected Va node.
    And voltage at this node is small portion of the output voltage.
    And this means that the voltage across Rin is now longer equal to Vin but Vin - Va. So if voltage across resistance is smaller current also must be smaller. And this means that Rin is larger with the feedback.

    But for the voltage shunt feedback we have different situation.

    [​IMG]

    Zin drops, because now signal source must provide current not only for the Rin but also for feedback resistor.

    http://forum.allaboutcircuits.com/attachment.php?attachmentid=38845&d=1326553766
     
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  4. veenife

    Thread Starter Member

    Jan 16, 2014
    42
    0
    ... okay.. but it must be a standard way that it works...

    i just want to understand...

    lets say.,... how does it affect the input impedance of a transistorized amp (guitar amp)...
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    Again, we need the circuit.
     
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