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?