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Working of MESFETs

  1. Feb 9, 2010 #1
    friends, i have got stuck in MOSFETs which we did today in class.
    i could not follow how MESFETS work- all i remember and my notes say is that i use a tungsten metal instead of metal oxide as we use in MOSFETS. but i could not detergent the mechanism. can anyone please help me a bit.....

    also, can anyone tell me what is impedance matching? i dont know if its related to this topic,sorry. but i found it in my notes.
    thanks a lot for any help.:biggrin:
  2. jcsd
  3. Feb 9, 2010 #2
    In short, impedance matching maximizes power transfer between two devices. Take a voltage source with an equivalent internal resistance (Ri) in series, hook it up to a load resistor (Rl), and note how the power dissipated in the load changes with different Rl/Ri ratios (Hint: the power peaks at some point -- where?).

    For transmission lines, an impedance mismatch causes reflections on the line. I.e., some fraction of the power you intended to transfer is not transmitted.
    Last edited: Feb 9, 2010
  4. Feb 9, 2010 #3
    thank you,gnurf. well, i think when internal impedance is the complex conjugate of the external impedance, then power transfer is max.for simple circuits, i can write this as internal resistance=ext resistance. so that is impedance matching....
    but how is it applied in electronics/mosfets/mesfets?
    Last edited: Feb 9, 2010
  5. Feb 9, 2010 #4
    I'm not sure what you mean by 'how is it applied in electronics' (I thought I answered that), but an example would be finding the thevenin equivalent resistance of some device and matching your load to it. Another example would be matching your antenna to the characteristic impedance of your coax feed with a balun.

    As for FETs, I'm unfamiliar with their use in impedance matching, but operated in linear (ohmic) mode they do act as variable resistors, which perhaps in combination with some kind of feedback could be used to maintain maximum power transfer for a variable load, source or even both maybe.

    I think that's the best I can do. With some luck, someone might chime in and add something of more value. Good luck.
  6. Feb 19, 2010 #5
    MESFETs are a form of JFET - they are depletion FETs. The primary difference is the semiconductor (III-V rather than Si) and junction type (Schottky metal-semiconductor diode rather than PN junction).

    MESFETs are generally microwave amplification devices. At microwave frequencies, lumped equivalent model concepts of discrete components like resistors, capacitors and inductors start to completely fail and the distributed model of transmission lines and s-parameters is required. The latter require impedance matching, which is assuring that Thevenin impedances and load impedances are conjugate matched to the transmission lines and devices they are connected to. The mismatch of input or output impedance of a MESFET to earlier or later circuits affects it performance as an amplifier.
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