direct coupled amplifier

Discussion in 'General Electronics Chat' started by yifei87, Nov 1, 2008.

  1. yifei87

    Thread Starter Member

    Sep 7, 2008
    16
    0
    hey, guy..
    here my question. what Q2 function in this circuit? why it is not neccessary to be drawn or calculated in ac analysis to get gain from source to output voltage? thanks a lot
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Check your drawing carefully.

    It looks as though you have drawn a standard long tail pair but shorted the collectors together.

    This also shorts out collector load Rc1.
     
  3. yifei87

    Thread Starter Member

    Sep 7, 2008
    16
    0
    o, yaa...some mistake of the circuit i draw. the node at collector of Q2 does not connected to Q1 collector or Q3 base.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Q1 and Q2 form a differential amplifier and the one input of the differential amplifier is just connected to ground. You dont include the effect of Q2 in the calculations because you assume the transistors are identical and when you apply a differential voltage Vs on the inputs, each transistor will have the half of Vs applied on its base and with opposite polarity. Also, the node above R3 will have zero voltage with respect to ground and thus you can connect the emitter of each transistor individually to ground and thus you have the output affected only by Q1 in the calculations.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Mik3,

    You say "that the node at the top of R3 is at zero volts with respect to ground."

    Aren't the emitters of both of the transistors in the differential pair actually one Vbe below ground?

    hgmjr
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Actually, if we ground the input of Q1 too, the voltage at the node above R3 will be -0.7V and for small signal operation it will swing around -0.7V.
     
  7. yifei87

    Thread Starter Member

    Sep 7, 2008
    16
    0
    Thanks Mik3
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Yep...good catch. :)
     
Loading...