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Input Impedance of SIMPLE Circuit

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Rich Grise

Jan 1, 1970
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I read in sci.electronics.design that Rich Grise <[email protected]>


You don't need to be too modest. It's a 'tech' question, which is why I
moaned about the introduction of Laplace and other exotica. The thread
was lengthened by my assumption that 'Ratch' was a perverse tech rather
than the (newly enlightened) student he now appears to be.

One of my Dad's favorite Ziggy cartoons (which he had pinned to his cork
board) had Ziggy looking into a mirror, asking himself rhetorically, "I
wonder if I'd be modest, if I had a choice?" ;-P

Cheers!
Rich
 
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Dr. Polemic

Jan 1, 1970
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Well, what have you and others decided about this question? I have seen you agree in some
measure to explanations given by John Woodgate and Jim Thompson, but what is the answer to
the question in the last line of the paragraph just above? Is there a theory without the
inconstencies found in your line of reasoning?

Not if you try to measure the impedance by applying 20 volts externally according to
Ratch. Pasting in his earlier explanation, we see how the reasoning would go:

"...the impedance is infinite. This is because the 20 volts of input voltage balances the
20 volt source voltage, so that no current exists in the circuit, thereby making the
impedance infinite."

So it would seem that a voltage source can have infinite impedance under certain
circumstances, even though the texts tell us to replace voltage sources with a short and
current sources with an open.
 
J

Jim Thompson

Jan 1, 1970
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Well, what have you and others decided about this question? I have seen you agree in some
measure to explanations given by John Woodgate and Jim Thompson, but what is the answer to
the question in the last line of the paragraph just above? Is there a theory without the
inconstencies found in your line of reasoning?


Not if you try to measure the impedance by applying 20 volts externally according to
Ratch. Pasting in his earlier explanation, we see how the reasoning would go:

"...the impedance is infinite. This is because the 20 volts of input voltage balances the
20 volt source voltage, so that no current exists in the circuit, thereby making the
impedance infinite."

So it would seem that a voltage source can have infinite impedance under certain
circumstances, even though the texts tell us to replace voltage sources with a short and
current sources with an open.

I think the "infinite" crowd have themselves thoroughly confused by
the differences in meaning between _impedance_ and
_Thevenin_equivalence_.

Heaven help us when they discover Norton ;-)

...Jim Thompson
 
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Reg Edwards

Jan 1, 1970
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Reggie's Theorem -

To calculate the input impedance of any network simply replace
internal current and voltage sources with their own internal
impedances and carry on as normal.

Can't imagine what all the reams of fuss from the old wives is all
about.
 
K

keith

Jan 1, 1970
0
I guess my prof's mere Vin/Iin isn't a broad enough definition of input
impedance (as I suspected). Thank you for clearing things up (I'll
ignore the insults in your post).

It's not Vin/Iin (I suspect you've misunderstood your prof). It's (delta
Vin)/(delta Iin). Put a 1Mohm load on the input node and measure the
*change* in voltage divided by the *change* in current. Now make that
resistor infinite (limit).
 
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