BJT Differential Pair

Thread Starter


Joined Mar 8, 2005
Hi all,
In the beginning of chapter6 'Differential and multistage Amplifier', it explains about differential signals. The part I dont understand is that
why if VB2=0 and VB1=+1, Q2 is off and Q1 is on? and why if Vb1=-1 and VB2=0, Q2 is on and Q1 is off?
(this is on page 488 and 489)
Could anyone help me explain this?
tingyu :rolleyes:


Joined Jan 28, 2005
Anytime the voltage difference between the two bases in an NPN transistor differential pair is greater than 0.7V (the forward voltage of a base-emitter junction) then the base with the higher voltage will conduct all of the available current leaving none for the remaining transistor. Thus the remaining transistor is in the off state since its base-emitter junction is reverse biased.