Input Impedance of SIMPLE Circuit

I don't respond to problem definitions posted on sites requiring registration. Sorry.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
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|  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.

Impedance is impedance. It defines the vector relationship between voltage and current at the port. Doesn't matter whether it's "input" or "output" or whether you've "turned off" sources. Turning on the sources won't affect the impedance, but it may require you to use different measurement techniques. mike

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I second that. Jim's language is more diplomatic than what I had in mind, so I'll let it stand for me, too.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Post the gif/jpg on the newsgroup alt.binaries.schematics.electronic

[snip]

WOW! It's been a long time since someone referred to me as "diplomatic" ;-)

...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.

here:

Correction: Let me draw you a text version of the circuit:

100 ohms o---------/\/\/\----| | | ----- | + | 20Vdc | - | Zin -> ----- | | | o-------------------|

That's a voltage source in the circuit. Of course this could be a more complex circuit with more voltage sources in different places, but this two element circuit serves to explain my confusion. I don't know what the input impedance is for circuits like this (for output impedance of complex circuits with sources I can simply short voltage sources or open current sources before calculating, but what would I do for input impedance?)

we

if

Impedance is generally defined incrementally, so a 1V CHANGE at the input will produce a 1/20 Amp change in current... thus the input impedance is 20 ohms, irrespective of the voltage source behind it ;-)

...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.

more

this

what

of

input

mean

what

;-)

Yes, ie 100 ohms, so the OP is clear.

NT

Might help if I could read ;-)

...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.

Oops, time for another trip to the drugstore for stronger reading glasses. Or perhaps a trip to the kitchen so the good wife can give them a good cleaning.

--
 Thanks,
    - Win

-- and this is why you short a voltage source and open a current source, on an input _or_ output. Change the voltage by 1V into a voltage source and the incremental current change is infinite. Change the voltage by

1V into a current source and the incremental current change is zero.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Yes, the general definition of impedance is Z(s)=V(s)/I(s). Where Z(s), V(s), and I(s) are the Laplace transform of voltage, current and Impedance, If you can calculate the voltage and current Laplace transforms, then you can find the impedance provided the circuit is linear, which it usually is. This will work for alternating or DC sources. Ratch

OK, let's do the problem. Assume a constant DC voltage is applied to the terminals. V(s)=V/s. Writing the loop equation we get

100*I(s)=V/s-20/s=(V-20)/s==>I(s)=(V-20)/100s . Z(s)=(V/s)/I(s)=100V/V-20 . Notice from the last equation, that if the 20 volt source is removed, the impedance value becomes 100 ohms resistive regardless of the input source voltage. Also from the last equation, it can be seen that if the input voltage is 20 volts, 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. Ratch

wrote

Where

and

Is

No not KISS. Didn't you read my OP? I wanted to go passed the simple cases that everyone talks about and delve deaper. I was taught the calculation method for input impedances and have used it before but then I started questioning how voltage and current sources would be dealt with. Then I realized that I could make circuits (like the simple one I posted) that would not have a contast Vin/Iin ratio. This is why I wanted a more STRICT definition of input impedance.

Although I'm a Computer Science student, I do understand Thevenin and Laplace from my EE course. Thevenin applies to output impedances, and the method has specific treatments for voltage/current sources when looking BACK into a circuit. But I always wondered the method required to treat voltage and current sources for looking INTO a circuit (INPUT IMPEDANCE). V/I for this circuit is not constant, but other posters have noted that in such a case we only worry about the incremental effect on I for each Volt (which is the same result you get if you short voltage sources and open current sources - much like when calculating Thevenin output impedances).

I read in sci.electronics.design that Ratch wrote (in ) about 'Input Impedance of SIMPLE Circuit', on Mon, 25 Apr 2005:

The OP doesn't understand Thévenin and you want to sell him Laplace? Is this just an ego trip or do you really not understand the KISS Principle?

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I have to buy a new pair... dropped them on a concrete floor, and have a scratch right across the line of sight :-(

...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 | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Did you read the OP's request? "Does anyone know a THOROUGH definition of input impedance? output impedance?" He did not ask for a KISS, and therefore incomplete definition. So I gave a complete, general, thorough, and correct definiton. In another post, I worked his example and explained the result to show him how it is done. Ratch

wrote

Where

and

Is

No not KISS. Didn't you read my OP? I wanted to go passed the simple cases that everyone talks about and delve deaper. I was taught the calculation method for input impedances and have used it before but then I started questioning how voltage and current sources would be dealt with. Then I realized that I could make circuits (like the simple one I posted) that would not have a contast Vin/Iin ratio. This is why I wanted a more STRICT definition of input impedance.

Although I'm a Computer Science student, I do understand Thevenin and Laplace from my EE course. Thevenin applies to output impedances, and the method has specific treatments for voltage/current sources when looking BACK into a circuit. But I always wondered the method required to treat voltage and current sources for looking INTO a circuit (INPUT IMPEDANCE). V/I for this circuit is not constant, but other posters have noted that in such a case we only worry about the incremental effect on I for each Volt (which is the same result you get if you short voltage sources and open current sources - much like when calculating Thevenin output impedances).

I hope the Laplace definition of impedance and previous example have helped you. It is correct for input impedances, output impedances, and anything in between. It also works for active voltage/current sources, as the example illustrates. Ratch

about

and

that

each

and

want

the

IMPEDANCE (the impedance related to the THEVENIN VOLTAGE SOURCE). Can you point me to a reference that related THEVENING IMPEDANCE to INPUT IMPEDANCE?