BJT Differential Amplifier DC Analysis

Discussion in 'Homework Help' started by TheRedDevil18, Feb 27, 2016.

  1. TheRedDevil18

    Thread Starter New Member

    May 23, 2015
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    diff amp.PNG
    Vi1 = Vi2 = 3V dc
    It = 2mA

    For the diff amp above I have to perform a DC analysis and check if Q3 and Q4 are in active mode, so I have two equations

    KVL at input
    3 = Ib*R3 + Vbe + Ve
    Ve = 3 - Ib*R3 - Vbe

    KVL at supply
    12 = Ic*R1 + Vce + Ve

    Ie = It/2 = 2/2 = 1mA

    If Ic approx = Ie then Ic = 1mA

    The Beta value wasn't given so how do I find Ib ?
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
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    The tail is a 2ma ccs. So each leg gets 1ma. Base is at 3v, emitters at 3 minus 0.7v, and collector at 12v minus 4.7k x 1ma.

    Pretty clear and shut.

    As to beta, you typically assume that it is sufficiently large here.
     
  3. TheRedDevil18

    Thread Starter New Member

    May 23, 2015
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    Yes thanks, that's what I thought initially. The resistance R6 (200 ohm), is that the emitter resistor for both the transistors ?
     
  4. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490
    Hi,

    Just a quick note...

    R6 is a pot and with a completely symmetrical circuit that would be adjusted exactly half way, which puts 100 ohms in series with each emitter, but they would be considered separate resistors too.

    Find the Beta from the data sheet for whatever transistors you are using.

    Watch out that the collector voltage calculation is reasonable.
     
    Last edited: Feb 27, 2016
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    R6 allows you to counter the imbalance between the two legs, and also to. Provide some degenerative power (DC feedback).
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    R6 also provides AC negative feedback.
     
  7. TheRedDevil18

    Thread Starter New Member

    May 23, 2015
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    Ok, for the second part I have to determine the differential and common mode voltage gains. So is that R6 resistor just a separate 100 ohm resistor for each emitter of the bjt's ?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    That's the equivalent circuit, yes.
     
  9. TheRedDevil18

    Thread Starter New Member

    May 23, 2015
    14
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    Will Rid (input differential resistance) be

    Rid = Rib(input resistance from base) + R3 ?

    Where Rib = ie(re + R6/2)/ib
     
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