Electronics > Beginners
Difficult, understanding Input and output impedance
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robsims:
Of course the output impedance has nothing to do with input impedance, but that's not my point now. My point is do you include the impedance of the load when calculating the output impedance of a circuit? I want an answer based on theory. Refer to a vid or book i can read?
StillTrying:

--- Quote from: robsims on August 18, 2019, 01:15:08 pm ---My point is do you include the impedance of the load when calculating the output impedance of a circuit? I want an answer based on theory. Refer to a vid or book i can read?
--- End quote ---

No you assume the load is not there at all.

1st few minutes of this vid looked OK.

rstofer:
You don't usually include the load (which is often application specific) in the output impedance calculation.  The circuit would be considered a 3 terminal 'black box' (see bottom link).

It is important to know that maximum power transfer occurs when the output impedance and the load impedance are equal.  For the idea to make any sense (and it's an important concept), the output impedance must not include the load impedance.

I wouldn't include the load in the input impedance calculation either simply because  the load impedance isn't always known or is likely to vary with application.

https://www.electronics-tutorials.ws/dccircuits/dcp_9.html

Problem 1 here:

http://pleclair.ua.edu/ph102/Homework/Sum09/HW5_21Jul09/HW5_magn_21Jul09_SOLN.pdf
robsims:
Very nice rstofer. That's what i thought. The professor is right too. You gave an answer based on theory and logic. "OUTPUT IMPEDANCE MUST NOT INCLUDE THE LOAD IMPEDANCE". See attached picture. This is from the book: Grob's Basic Electronics, Mitchel E. Schultz, 11th Edition, page 920.  From a math point of view it's impossible to include he load impedance when matching output and load impedances. I think we all learned again. Many thanks
magic:
Output impedance includes impedance of the source.*
Input impedance includes impedance of the load.
Not the other way around.

Simple :D

*Yes, that Vcc thing you have drawn will have some output resistance in practice, which adds to R1.
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