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In this problem, it's asking us to draw the small signal equivalent of an amplifier but I think the answer they gave is wrong. In the second picture (b) I don't think the emitter of Q2 should be grounded because then the current source from Q1 would be between two nodes that are grounded which is impossible unless the current is 0.

The circuit and the equivalent small signal model that the textbook gives.

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    \$\begingroup\$ Yes, it is a typo. Remove this GND and you will be fine. \$\endgroup\$
    – G36
    Sep 3, 2022 at 17:05
  • \$\begingroup\$ Thanks, I kept on feeling like I was missing something and it was bothering me a lot. \$\endgroup\$
    – snowball
    Sep 3, 2022 at 17:08
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    \$\begingroup\$ Look here seas.ucla.edu/brweb/teaching/… \$\endgroup\$
    – G36
    Sep 3, 2022 at 18:12

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Yes, that's a typo, but you don't have to go that far in your reasoning. "because then the current source from Q1 would be between two nodes that are grounded" -- maybe that's true, maybe that's not, and you're depending on the circuit being sensible.

What is true, always, is that if you look at picture (a), the emitter of Q2 is not grounded, but when you look at picture (b) it is. There is an exact 1:1 correspondence* between the nodes in a hybrid pi model of a transistor and the actual bits of metal sticking out of the case -- and those two pictures simply don't match.

So -- there's definitely a typo. Now you can question which circuit is wrong, and here you can apply the "sensibility" test -- if you ground the emitter of Q2 then you're also grounding the collector of Q1, and basically shorting out the signal path of the circuit. So, that's not sensible, and someone tossed an extra ground into the side showing the hybrid pi models.

* Up to modeling errors -- that's not the issue here.

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