How does the FET feedback works

Discussion in 'Homework Help' started by vustudent, Mar 12, 2009.

  1. vustudent

    Thread Starter Active Member

    Mar 11, 2009
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    Here is a four resistor biasing circuit, I know that the RS resistor would provide negative feedback. However, I have problem in explaining how the whole feedback process works, please help.
    [​IMG]
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You should have ≈3V on the source, not 0V.
    If Ids tries to increase, Vgs will decrease (do you understand why?), which will tend to reduce Ids. Voila! Negative feedback.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your schematic looks awful.
    It was saved as a fuzzy JPG file type instead of as a very clear GIF or PNG file type.
    It is covered with Measles dots.

    Why does Multisim DO DAT?
     
  4. vustudent

    Thread Starter Active Member

    Mar 11, 2009
    38
    0
    I understand that
    Vgs= Vth-Rs(Id)

    when Vth increases Vgs will increase, which Id will increase and resulting a decrease in Vgs. However, how do you explain the correlation between such a negative feedback towards having smaller output gain at Vout?
     
  5. vustudent

    Thread Starter Active Member

    Mar 11, 2009
    38
    0

    Sorry my computer knowledge is awful, I just built my circuit in pspice and bracket, copy to clipboard and paste somewhere as a pic.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Your schematic looks fine here. Of course, I always use ICAP4, so I'm used to measle dots. :)

    Yes, this is classic CURRENT feedback, (acutally very commonly used in old vacuum tube class A power amplifiers). As current through the source increases, the voltage at the top end of the source resistor increases. This puts it at a higher voltage relative to the gate...in effect this LOWERS the gate voltage. This, in turn, tends to decrease the source current.
    The RC network from the drain to ground makes the overall gain frequency sensitive. This rolls off the gain of the device at higher frequencies, as it forms a voltage divider, lowering the source voltage progressively as the frequency is increased. This is quite independent of the feedback loop.

    Hope this helps.


    Eric
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    By the way...you must be a radio amateur, since "VU" is the prefix for India. :)

    eric
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    It looks perfectly fine on my system. Maybe you need an older computer.
     
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