Why is the ac output voltage negative for a common emitter Bjt network?

[Evilinside]

I get that collector current is fixed so as collector current goes up, the voltage drop across the collector resistor goes up and the collector-emitter voltage goes down. Which is why it is basically zero at saturation. But for ac it is not zero but negative when input is positive. why is this so?

Also why is ac output current chosen to come up, from ground, through the collector resistor for the common-emitter configuration and comes up from ground through the emitter resistor emitter-follower configuration. what actually determines the output current direction when modeling the configuration for ac analysis?

 

[vk6kro]

The output of an common emitter amplifier is negative of there is a capacitor in series with the output. This is because a series capacitor removes the DC component that existed on the collector and the voltage then alternates above and below zero after the capacitor.


The convention when doing AC analysis is to take a reference direction ( in this case the input signal) and assign a polarity to it (usually upwards or positive)

The other polarities then follow from this.

Usually it is oposite to what you describe, though. A more positive input produces an increase in collector current which goes from the positive to the negative supply.
An increase in collector current produces a decrease in output voltage, so this would be shown as a downward arrow as well.

Maybe you mean something else?