Negativ differentiel modstand
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En komponent eller et kredsløbs dynamiske modstand eller differentielle modstand er defineret ved: R(I)=dU/dI eller R(U)=dU/dI, hvor R er den dynamiske modstand ved hhv. strømmen I eller spændingen U, U er spændingen over komponenten og I er strømmen gennem komponenten.
Der findes simple komponenter, som har negativ differentiel modstand (NDR), på visse dele af deres modstandskarakteristik. Nogle af disse er tunneldiode, Gunn diode, IMPATT-Diode, mikrobølge avalanchediode, resonant tunneling diode, unijunction transistor (ujt), resonant tunneling transistor, (Usikkert: plasma (f.eks. lysstofrør) - plasmas modstand falder med stigende strøm - men om det skyldes en evt. stigende temperatur vides ikke).
Selvom en komponent er ulinear og har negativ differentiel modstand, er den som regel i mindre strøm- og spændings-intervaller linear, hvilket betyder at den differentielle modstand i intervallet er konstant.
I netop et nogenlunde lineart interval med negativ differentiel modstand, kan komponenten forstærke signaler nogenlunde lineart. En af de simpleste eksempler er at sætte komponenten i serie med en svingningskreds.
Afhængig af om svingningskredsens samlede tab (incl. evt. ekstern belastningsmodstand) delvis ophæves, netop ophæves eller mere end ophæves, så vil kredsløbet hhv. fungere som en afstemt forstærker, bibeholde en igangsat svingning eller fungerer som en oscillator.
Der findes også kredsløb, som udviser negativ differentiel modstand, på visse dele af deres modstandskarateristik. Nogle af disse er: Lambda diode og vistnok gyrator, Negativ impedans konverter (eng. Negative Impedance Converter) (NIC), Genereliseret impedans konverter (eng. Generalized Impedance Converter) (GIC), Frekvens afhængig negativ modstand (eng. frequency-dependent negative resistor) (FDNR).
[redigér] Anvendelse
Af anvendelser af negativ differentiel modstand er der f.eks. superregenerativ modtager og NDR oscillator.
[redigér] Kilder/referencer
- Matematisk orienteret: Jim Lesurf: Negative Resistance Oscillators
- God: Negative Resistance Revived. by Lloyd Butler VK5BR. (Condensed version of article originally published in Amateur Radio, November 1995)
- The Negistor & Negative Resistance - 04/07/00 courtesy Marcelo Puhl
[redigér] Eksterne henvisninger
- God information om anvendelse af dimser med negativ modstandskarakteristikker, før de blev opdaget i tunneldioder i 1957: Cristadyne: Semiconductor archaeology or tribute to unknown precursors Citat: "...In 1923, Oleg Losev [O. V. Lossev, Lossew] (1903-1942) ( See link below ) managed to make a high frequency generator using such a detector. But it was polarized. This indicates that this diode had a characteristic curve in which a negative slope was present. And this makes one think of the tunnel effect diode invented a half a century later...These layouts where part of what one called CRYSTADYNE [eller Cristadyne, Crystodyne ] systems. But in those days, the technical performance and industrial ease of the new increasing valve technology made these layouts to be ignored, and then forgotten..."
- By Nyle Steiner K7NS 2001. Hjemmelavede dimser med negativ modstandskarakteristikker:
- The Wireless World and Radio Review. October 1, 1924 and October 8, 1924: "The Crystal As A Generator And Amplifier" by Victor Gabel.
- Radio News, September, 1924, pages 294-295, 431: The Crystodyne Principle
- Radio News, September, 1924, page 291: A Sensational Radio Invention By HUGO GERNSBACK
- US PATENT SUBCLASS 331 / 132: Negative resistance or negative transconductance oscillator
- "A compilation of negative resistance circuits generated by two novel algorithms", Active and Passive Electronic Components, 1 January 2002, vol. 25, no. 3, pp. 211-214(4), Kumar U. Citat: "...Abstract: There are two algorithms to generate a negative-resistance device which exhibits either a type-N shaped V-1 characteristic similar to a tunnel diode, or a type-S shaped V-1 characteristic similar to a four layered pnpn diode. We present here a selection of these circuits using bipolar, JFET or MOSFET or their combinations..."
- 'Energy-sucking' Radio Antennas, N. Tesla's Power Receiver Citat: "...The truth is quite strange...By manipulating the EM fields, we can force an electrically-small receiving antenna to behave as if it was very, VERY large...In theory a tiny loop antenna can work as well as a longwire 1/2-wave antenna which is 10KM long...The energy doesn't vanish, instead it ends up INSIDE the atom. By resonantly creating an "anti-wave", the tiny atom has "sucked energy" out of the enormously long light waves as they go by...When all is said and done, our oscillating coil has absorbed half of the incoming EM energy and re-emitted (or "scattered") the rest..."
- C. F. Bohren, "How can a particle absorb more than the light incident on it?", Am J Phys, 51 #4, pp323 Apr 1983 Citat: "...A particle can indeed absorb more than the light incident on it...In the former, strong absorption is associated with excitation of surfaces plasmons; in the latter it is associated with excitation of surface phonons. In both instances, the target area a particle presents to incident light can be much greater than its geometrical cross-sectional area...."
- Regeneration revisited. The Tesla Connection by Gary L. Peterson Citat: "...So it may be said that Tesla anticipated the technique of regenerative feedback to increase detector sensitivity...A detailed description of how the negative resistance, negative inductance circuit works, including a differential form of the active antenna circuit and other pertinent information, can be found in U.S. Patent No. 5,296,866, Mar. 22, 199..."
