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Difficult, understanding Input and output impedance

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robsims:
Hi,

It's a bit difficult for me to understand the concept of input and output impedance. Iknow input and output impedances are thevenin equivalents. I have a picture attached. Is the input impedance of the voltage divider seen from Vcc, R1+R2? And is the output impedance seen from the load, R1 in parallel wit R2 (R1//R2)?

Audioguru again:

--- Quote from: robsims on August 18, 2019, 01:12:24 am ---Hi,

It's a bit difficult for me to understand the concept of input and output impedance. Iknow input and output impedances are thevenin equivalents. I have a picture attached. Is the input impedance of the voltage divider seen from Vcc, R1+R2? And is the output impedance seen from the load, R1 in parallel wit R2 (R1//R2)?

--- End quote ---
No, the input impedance is not R1 + R2 because R2 has the load in parallel with it.
Yes, the output impedance is R1//R2 because the Vcc power supply usually has a very low impedance.

robsims:
Ok with the load, the input impedance is (Zload//R2) +R1. Is that correct?

robsims:
What you stated "Audioguru again" is not true. Just watched a video:



Input impedance is R1 + R2
Output impedance is R1//R2

I think that's it

vk6zgo:
Audioguru has included the load, as per your diagram--- the video talks about the input impedance (actually resistance in this case) without the load connected.

Try it experimentally---- build up a voltage divider as shown on the video .
Place your DMM across the input terminals.--- you will now see R1+R2 in series as your reading.
Now place another resistor (let's call it R3) in parallel with R2 (a "load").

If we haven't shifted in time & space to another dimension, your DMM will now read a value equal to R1+R2//R3.

Now move R3 so it is now in parallel with R1.
You will now read a resistance value equal to R1//R3+R2.

I might lie to you, Audioguru might lie to you, the Professor's video might lie to you, but a real physical circuit measured with a DMM Ohms range is very, very, unlikely to do so.



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