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Input Impedance of SIMPLE Circuit

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?

volt source

terminals

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.

Reply to
Dr. Polemic
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agree in some

the answer to

without the

volt source

input terminals

according to

balances the

making the

short and

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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I read in sci.electronics.design that Rich Grise wrote (in ) about 'Input Impedance of SIMPLE Circuit', on Thu, 28 Apr 2005:

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

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.

Reply to
Reg Edwards

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). 
--
  Keith
Reply to
keith

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