direct coupled amplifier

Thread Starter

yifei87

Joined Sep 7, 2008
16
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
 

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studiot

Joined Nov 9, 2007
4,998
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.
 

Thread Starter

yifei87

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

mik3

Joined Feb 4, 2008
4,843
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.
 

hgmjr

Joined Jan 28, 2005
9,027
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.
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
 

mik3

Joined Feb 4, 2008
4,843
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.
 
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