Consider this circuit.
Transistor is 2N2222A. Ic is chosen to be 10 mA. Hfe is 225 for that current. Resistors are calculated to have Vbe = 0.7V and Vc = 4.5V.
I am trying to understand what happens when I inject an AC signal at the base.
Suppose an AC signal that is 1Hz, 0.02V of peak voltage (varies between +0.02V and -0.02V). The capacitor is huge (100F), just to be transparent to 1Hz.
What I think happens is this: as the input increases from 0 to 0.02V, base increases from 0.7 to 0.72V. So the base resistor is now connected between Vcc (9V) and 0.72V. Base current is now less than before, because the difference in potential across base resistor is less than before.
If base current is less, collector current is less, right? If collector current is less, the difference in potential across the collector resistor is less. If so, Vc increases, right?
My simulator says Vc (collector voltage) decreases...
What is wrong with my analysis?
YES, I know this transistor configuration for amplifiers is bad. I am just trying to see what happens to this circuit.