Small signal model analysis - Enquiry

crutschow

Joined Mar 14, 2008
23,314
Do you mean "why is the output voltage inverted with respect to the input voltage"?
That's because an increase in input voltage causes an increase in the output current.
This then causes the output voltage at the resistor collector junction to decrease.
 

Jony130

Joined Feb 17, 2009
4,969
Vout is negative because gm*vgs current source "sink" the current from RD resistor. Note the gm*vgs direction and this mean that RD current will flow from reference point (GND) up through RD resistor (from bottom to top ) into gm*vgs current source down through RS to GND.
 

HunterDX77M

Joined Sep 28, 2011
104
Basically your current (gm * V1) is flowing in the opposite direction from what you are taking your voltage reading across: resistor RD.
 
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