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I am learning about differential amplifiers I found the circuit for AC analysis in a book as shown:

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They transformed it to a transistor model circuit as shown:

enter image description here

Though I am not excellent at circuit analysis but I suspect as why the direction of current source in right transistor is in the direction from emitter to collector. It is a NPN transistor, so shouldn't the current go from collector to the emitter? Where am I wrong?

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  • \$\begingroup\$ one useful way to consider the signal polarities thru a differential pair, is that the left-side base (input) polarity will be copied to the right-side collector (output). Does this circuit, with the drawn arrows, implement that view of the diffpair? \$\endgroup\$
    – analogsystemsrf
    Commented Sep 15, 2019 at 9:17
  • \$\begingroup\$ The direction of the arrow in the current source says nothing about the actual direction of current flow (which makes particular sense because oven you will find the direction only after the analysis but you have to decide for one of both possibilities before the analysis). The direction of the arrow only indicates the particular choice of two possible sign conventions: positive/negative if going into the collector. The current is always positive if it is going in the direction of the arrow and always negative if it is going the opposite way. \$\endgroup\$
    – Curd
    Commented Sep 15, 2019 at 11:29

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In terms of the AC analysis, the current in one collector is the inverse of the other collector hence, it makes perfect sense to show them as they are. But, in the bigger picture, the current direction as indicated by the arrow, will turn out to be negative if you followed through with the algebra so, it doesn’t really matter at all.

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  • \$\begingroup\$ Thanks Andy aka. But we must be already knowing that this is an npn transistor so the direction would be from collector to emitter. There must be no need to reverse direction when we already know it? Or would AC analysis change change something? \$\endgroup\$
    – Bhuvnesh
    Commented Sep 16, 2019 at 3:52
  • \$\begingroup\$ Try reading my answer again. \$\endgroup\$
    – Andy aka
    Commented Sep 16, 2019 at 7:00

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