Electronics > Beginners
Difficult, understanding Input and output impedance
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fourfathom:

--- Quote from: rstofer on August 18, 2019, 01:53:43 pm ---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.
--- End quote ---

The OP's circuit is a passive attenuator.  In a passive circuit you absolutely do include the load impedance when calculating the input impedance.  If you don't know the load impedance then neither the input impedance nor the attenuation can be determined.

With an amplifier in the circuit then things may be different.  In some RF designs (for example) the load impedance is reflected in the input impedance, even with an amplifier between input and output.  In these cases you do need to include the load when calculating input impedance.  In other cases the amplifier provides essentially perfect isolation so the load has no effect in input impedance
robsims:
Magic, of course Vcc has output resistance. That's the internal resistance of Vcc which is very low for a good Voltage source.
rstofer:
As long as you state whether the impedances calculated include a load or some source impedance, neither being a part of the 'black box', you're still good.  In a practical world, of course the load is important.  In a classroom, maybe not.  It depends on what is being taught.

If you have the 3 terminal characteristics, it is easy enough to add in source impedance and load impedance.  The important thing in a classroom is to get the answer the instructor wants you to get.  Their game, their rules.
chrisl:
The input impedance is what the source sees (in this case the Vcc) and includes everything after the source. 
The output impedance is what the load sees and it includes everything right before the load.
GerryR:
I think if the OP would have drawn the circuit as a "Two-Terminal-Pair Network" it would have been a little clearer.  See the attachment from R. E. Scott's. Linear Circuits.
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