# what is "negative impedance"?

1. ## what is "negative impedance"?

Can somebody tell me what the "negative impedance" is ? and what is its usage?

2. ## 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 volt-amper 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 volt-amper curve the Rdiff is positive but if you have decreasing curve
Rdiff will be negative.

The Tunnel Diodes has reagion on their volt-amper function with negative slope so they hav negative Rdiff at that region. Tunnel Diodes are used for implementation of the oscilators.

dora

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3. ## 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 !

4. ## 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.

5. ## 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!

6. ## 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 current-voltage 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.

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7. ## 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.

8. ## Re: what is "negative impedance"?

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9. ## Re: what is "negative impedance"?

An example of negative resistance device is tunnel diode. It has a certain region on V-I 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

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10. ## Re: what is "negative impedance"?

An example of negative resistance device is tunnel diode. It has a certain region on V-I 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

11. ## Re: what is "negative impedance"?

When you connenct two DCDC converters (input-output 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.

12. ## Re: what is "negative impedance"?

Originally Posted by fatwolfning
Can somebody tell me what the "negative impedance" is ? and what is its usage?
I assume you are familiar with some of concepts I will discuss. Just follow through to clarify this wonder concept of negative resistance.

negative resistance is better suited we the discussion is about with solid-state devices but not always to them. Remember even the simplest op-amp has at least 10 solid-state 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 vice-verca. 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! :)

13. ## 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.

14. ## Re: what is "negative impedance"?

to build oscillator one of the condition is :

Zin+Zout=0 so Zin=-Zout

15. ## 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.
ALL electricity obeys 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.
Op-amps do not compose of a negative resistance! They compose only of transistors, diodes, semiconductor resistors and semiconductor capacitors.

An active device like an op-amp can behave like a negative resistance by using "positive feedback". An op-amp 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 op-amp output to the "+" terminal through a zero phase-shift (resistive) feedback network, you will have a two-terminal device across the "+" and "-" inputs which looks like a negative resistance!

This is how an op-amp can be used as an oscillator! All oscillators work using the "negative resistance" principle. If you use a -tuned- feedback network which is a band-pass filter, then you will only get oscillation at the centre frequency of the filter where there is zero phase-shift!

You will also get oscillation if you connect the output of an op-amp to the "-" terminal through a feedback network which provides 180 degrees phase shift at a single frequency (ie. the op-amp provides the other 180 degrees to make 360 degrees which is the same as zero phase-shift!). 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 op-amp, and with oscillators in general.

16. ## Re: what is "negative impedance"?

Originally Posted by smyback
A -purely- negative impedance IS a negative resisistance!

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!).
This perfect smyback! All that you need to talk for negative resistance & oscillation.

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???

17. ## what is "negative impedance"?

Oh you also need also bias circuitry to drive any kind of active device.

18. ## what is "negative impedance"?

Negative impedance:
An impedance that displays the same behavior as that of NEGATIVE RESISTANCE.

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19. ## Re: what is "negative impedance"?

From the energy stand point of view, does the negotive impedance absorb energy?

20. ## Re: what is "negative impedance"?

this site might be useful

h**p://whatis.techtarget.com/definition/0,,sid9_gci212333,00.html

taring

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