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23rd June 2004, 15:09 #1
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what is "negative impedance"?
Can somebody tell me what the "negative impedance" is ? and what is its usage?

23rd June 2004, 15:24 #2
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Re: what is "negative impedance"?
Ok let me describe shortly.
the negative can be only the diferential impedance.
diferential impedance definition:
Suppose you have the voltamper diagram .
The derivative of this curve at given point is the differenetial impedance.
You can aproximate the derivative as division of the small changes of the voltage to the corespondent small changes of the current in certain point
Rdiff=dU/dI
Now if you have increasing voltamper curve the Rdiff is positive but if you have decreasing curve
Rdiff will be negative.
The Tunnel Diodes has reagion on their voltamper function with negative slope so they hav negative Rdiff at that region. Tunnel Diodes are used for implementation of the oscilators.
dora

23rd June 2004, 18:46 #3
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Re: what is "negative impedance"?
HI fatwolfoning :I think that as Dora explained the negative impedace is a differential ./ a rate of change and this can be negative or positive !
Could it be that what you are interested is in NEGATIVE RESISTANCE .. ??
SO useful in Physics and in theory of Oscillators
There is active research in NEGATIVE RESISTORS .. a DEVICE that absorbs heat when current flows through .. Is a device that delivers energy .. by stealing it from AMBIENT HEAT !
A dream ENGINE with 100% EFFICIENCY
take a look at :
http://jlnlabs.online.fr/cnr/index.htm
ps if this is NOT what you wanted to know .. well .. IS OK !

23rd June 2004, 18:46

23rd June 2004, 19:39 #4
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Re: what is "negative impedance"?
I remember once in Analog CMOS IC design course, there are some certain CMOS applifier which can work as negative resistences under some specificial conditions, that can aslo be used to make an osilator, or something. but I forgot the details about it.
Maybe Razavi's Analog book has some related examples.

24th June 2004, 03:48 #5
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what is "negative impedance"?
I know from book that the op amps can compose of a negative impedance,but the book doesn't tell me its usage.
With your help, I know the defination and the answer to my doubt.
To eltonjohn:I am learning analog design,so I am not clear of the ENGINE with 100% EFFICIENCY.Any way, thank you very much!
thanks evryone who gave me help!

28th June 2004, 07:17 #6
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Re: what is "negative impedance"?
A negative resistance is one which does not obey Ohm's Law.
An inverse relation exists between current and voltage rather than
direct relation (as in Ohm's Law)
In other words it is a property of certain devices whereby a portion of
the currentvoltage characteristic has a negative slope, i.e.
the current decreasing with the increasing applied voltage.
Devices that exhibit this property include thyristor,tunnel diode
and magnetron.

1st July 2004, 23:29 #7
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Re: what is "negative impedance"?
Hello....
At the moment which you matematically have a negative impedance, you are looking a signal source, being this purely resistive. But, if it's an imaginary one, we have a capacitor at frequency domain. Also a negative impedance have two applications:
*As a controlled source (no linear device).
*As a compensator of an oscillator circuit.

1st July 2004, 23:29

2nd July 2004, 08:57 #8
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Re: what is "negative impedance"?
visit the following link to get the proper answer online 24 hrs a day...
http://<b><a href="http://www.techno...lt.asp</a></b> 8O

2nd July 2004, 09:00 #9
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Re: what is "negative impedance"?
An example of negative resistance device is tunnel diode. It has a certain region on VI static characteristic where increasing voltage result in decreasing current. This is what was said negative (dynamic) resistance.
Tunnel diode can be used as generator or amplifier

2nd July 2004, 09:00

2nd July 2004, 09:01 #10
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Re: what is "negative impedance"?
An example of negative resistance device is tunnel diode. It has a certain region on VI static characteristic where increasing voltage result in decreasing current. This is what was said negative (dynamic) resistance.
Tunnel diode can be used as generator or amplifier

2nd July 2004, 09:25 #11
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Re: what is "negative impedance"?
When you connenct two DCDC converters (inputoutput in cascade) in your system matrix you will get negative elements in it (like impedance) and it means that you will have instability in your system.

4th July 2004, 03:42 #12
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Re: what is "negative impedance"?
Originally Posted by fatwolfning
negative resistance is better suited we the discussion is about with solidstate devices but not always to them. Remember even the simplest opamp has at least 10 solidstate devices (transistors etc) in the chip.
definition: Negative resistance of a device(a simple transistor for example) in which the voltage and current are 180 degrees out of phase with respect to each other.
Now think what would come out of this definition?
I can guess that an increase in voltage across the device will produce a decrease in current and viceverca. Right? This leads by multiplication of current and voltage into being negative.
If I am right on this assumption then I did say this is the concept of power being generated by the device. Negative resistance is used in so many places. What I said is a vidid start to work on designing an oscillator.
have fun! :)

4th July 2004, 05:12 #13
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what is "negative impedance"?
The negative impedance region is the unstable region. So the device will try to pass to the next stable region. We use this behavior to design the latching characteristic of the triac and to design the astable oscillator operation.

4th July 2004, 10:24 #14
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Re: what is "negative impedance"?
to build oscillator one of the condition is :
Zin+Zout=0 so Zin=Zout

7th July 2004, 01:51 #15
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Re: what is "negative impedance"?
A purely negative impedance IS a negative resisistance!
Hamidzia wrote:
A negative resistance is one which does not obey Ohm's Law.
Z = V/I, if Z is purely negative you have negative resistance!
If a voltage source is applied to a negative resistance load, then current will flow from the load to the source, instead of the other way!
A positive resistance dissipates energy, a negative resistance sources energy! Therefore it is impossible to realise a negative resistance using passive components, it must have it's own power source (ie. it needs to be an active device!).
Fatwolfing wrote:
I know from book that the op amps can compose of a negative impedance,but the book doesn't tell me its usage.
An active device like an opamp can behave like a negative resistance by using "positive feedback". An opamp has two input terminals "+" and "".
* An increasing voltage applied to the "+" terminal will increase the output voltage.
* An increasing voltage applied to the "" terminal will decrease the output voltage.
It stands to reason, that if you connect the opamp output to the "+" terminal through a zero phaseshift (resistive) feedback network, you will have a twoterminal device across the "+" and "" inputs which looks like a negative resistance!
This is how an opamp can be used as an oscillator! All oscillators work using the "negative resistance" principle. If you use a tuned feedback network which is a bandpass filter, then you will only get oscillation at the centre frequency of the filter where there is zero phaseshift!
You will also get oscillation if you connect the output of an opamp to the "" terminal through a feedback network which provides 180 degrees phase shift at a single frequency (ie. the opamp provides the other 180 degrees to make 360 degrees which is the same as zero phaseshift!). This is the technique used by most discrete transistor oscillators!!!
I hope this clarifies the meaning of negative resistance and it's use with an opamp, and with oscillators in general.

7th July 2004, 16:20 #16
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Re: what is "negative impedance"?
Originally Posted by smyback
Once again I must repeat myself this occurs when in device voltage and current are 180 degrees out of phase.
I must add a crarification smyback: active devices also do not have their own power sources. Remember BIAS circuitry and DC supply???

7th July 2004, 16:21 #17
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what is "negative impedance"?
Oh you also need also bias circuitry to drive any kind of active device.

14th July 2004, 02:34 #18
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what is "negative impedance"?
Negative impedance:
An impedance that displays the same behavior as that of NEGATIVE RESISTANCE.

15th July 2004, 03:59 #19
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Re: what is "negative impedance"?
From the energy stand point of view, does the negotive impedance absorb energy?

15th July 2004, 04:42 #20
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Re: what is "negative impedance"?
this site might be useful
h**p://whatis.techtarget.com/definition/0,,sid9_gci212333,00.html
taring

15th July 2004, 04:42
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