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I have been solving BJT circuits on my textbook and stumbled upon a PNP BJT small signal model question. It asks to build the small signal model of a PNP transistor circuit. When I checked the proposed solution, I was a bit confused. Following are the original circuit and the proposed small-signal model.

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My confusion is that, this is a PNP transistor, so the current flows from the emitter to the base and to the collector, unlike an NPN transistor. But in the solution it looks like the current source is directed from the collector to the emitter, which is contradicting.

What am I missing here?

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1 Answer 1

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Instead of writing -gmV, they just reversed the direction of the current source in the circuit. Look at the pi models of BJTs here. Notice the current source direction is from emitter to collector and the value is gmVeb and not gmVbe.

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  • \$\begingroup\$ IOW, reverse the reference '+' and '-' on \$r_\pi\$, and reverse the direction of the dependent source. If you want to follow current American standards (and I think English-language standards), make the dependent source diamond-shaped while you're at it. \$\endgroup\$
    – TimWescott
    Nov 22, 2018 at 18:32

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