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Here's my problem. enter image description here

i've been trying to draw the small signal equivalent circuit. And this is what i have

enter image description here but i feel something's wrong because when i calculate voltage gain i dont have to use gm of the BJT. can you guys show me where did i do wrong?

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  • \$\begingroup\$ The gain is 2 by simple inspection and I never used the model or gm. I can't tell from your scrawlings what your conclusion was. \$\endgroup\$ – Andy aka Jun 4 '16 at 9:40
  • \$\begingroup\$ sorry for scrawling. i have edit the image. i want to ask if my small signal equivalent circuit is drawn right? \$\endgroup\$ – Nguyễn Tiến Thành Jun 4 '16 at 15:02
  • \$\begingroup\$ By inspection - M2 is a source follower (unity gain), without C2 whatever voltage drop there is across the emitter resistor ( 18k) you'll get twice that voltage across the collector resistor (2x18k). (Hint Ic is almost equal to Ie) \$\endgroup\$ – JIm Dearden Jun 4 '16 at 19:40
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Your small-signal equivalent circuit is wrong. In your sketched circuit you connected the mosfet's drain to the bjt's collector and the mosfet's gate to the bjt's emitter.

To draw a small-signal equivalent circuit you should substitute each device with its small-signal equivalent, respecting the position of the terminals.

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