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Negative resistance is use to generate power.
If you see the graph(I-V characteristics odf device) of tunnel diode you will see the negative resistence, see the slope of the graph.
This devices are mostly used in oscillator that means,
You can also generate -ve resistace by positive feedback.
Certain semiconductor devices, under certain biasing condition (or in a certain mode), show the behavior that INCREASING the "signal" voltage across them causes REDUCTION in the current through them...Clearly, such a behavior (if it is linear enough) can be modelled by a negative resistance (current increases when voltage decreases).
Such devices are called negative resistance devices, and are very commonly used in oscillators.
However, I believe that the NEGATIVE RESISTANCE characteristic of the device itself has nothing to do with POSITIVE FEEDBACK, as some ppl replied (correct me if I'm wrong! - they are probably confused b/c +ve f/b is present in oscillators, however it has nothing to do with -ve resistance)....and such devices can operate in their "negative resistance mode" under proper biasing conditions, whether or not feedback is applied.
In oscillator you usually make I/P port of transistor(S11) >>1, which means Gamma>>1.
So if you see relation between Zin & Gamma, I suppose You will get -ve resistance at the Input port of transistor.
You generate negative resistance by feed back at the port of the transistor, so this does not mean you make transistor a negative resistance device.
I never said that you can MAKE transistor (BJT or MOS) a negative resistance device....and I know that it is possible to construct a positive f/b circuit USING a transistor which shows negative resistance properties.....
However, the question asked is related to negative resistance DEVICES, not circuits whose input resistance appears to be negative.
...and my point is that, such DEVICES do exist which act as show -ve resistance under certain bias conditions.[/quote]
Meanwhile, if i remember correctly my Oscillators' course.....a device called UJT (Uni-Junction Transistor) used to have this type of characteristic.
Yes. this is absolutely correct.
The details about this devices can be found in Watson's book or Streetman's books.
It explains in good details about negative resistance devices such as tunnel diode or gunn,Esaki diodes etc.