negative feedback and impedance

Thread Starter

veenife

Joined Jan 16, 2014
42
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???
 

Jony130

Joined Feb 17, 2009
5,022
Look at example amplifier without the feedback



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.



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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Thread Starter

veenife

Joined Jan 16, 2014
42
... 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)...
 
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