The following java simulate a "Common Emitter Transistor Amplifier".
The Gain is mainly control by RE1 and Rc, G=RC/RE1
(1st order approximation: Assume RL>>Rc, and R1,R2 >> RE1,Rs<
R1 and R2 is used to set up the bias voltage. VB=Vcc*R2/(R1+R2);
RE1+RE2 in series to make the circuit more stable (provide feedback).
RE2 in parallel with a CE to short circuit the AC signal, so the gain is not dependent on RE2.
You can adjust RE1 and RC to control the GAIN of the amplifier, however if the Gain is too large (or biased voltage is not set up property), some voltage will be cutoff or make the transistor saturate (it will burn out in real life, the color will turn RED in the simulation).
(Because VC output can not be larger than Vcc (or IC=0 which is cutoff region),
and VC can not be smaller than VE+0.2V which will run into saturation and the transistor will burn out.
So the proper adjustment with R1,R2 will increase the working region of the amplifier.
Have fun with the following circuit!
You are welcomed to check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=2116.msg7909#msg7909]The Common Emitter Amplifier Circuit[/url] to learn how to set up the DC biased circuit setup.