TRANSISTORS

COMMON-EMITTER (Input and Output signal is common to the Emitter)

Current through the transistor is controlled by the base-to-emitter bias. If both the base and emitter become more positive by the same amount at the same time (signal through emitter and base), current will not increase. It is the difference between the base and emitter voltages that controls the current flow through the transistor.

To eliminate this negative feedback caused by the emitter resistor, some way must be found to
remove the signal from the emitter. If the signal could be coupled to ground (decoupled) the emitter of the transistor would be unaffected. That is exactly what is done. A DECOUPLING CAPACITOR (C3 in view B) is placed between the emitter of Q1 and ground (across the emitter resistor). This capacitor should have a high capacitance so that it will pass the signal to ground easily. The decoupling capacitor (C3) should have the same qualities as the coupling capacitors (C1 and C2) of the circuit. Decoupling capacitors are also called bypass capacitors.

COMMON EMITTER AMPLIFIER

In a Common Emitter Amplifier, the output signal is 180Degrees out of phase to the input signal - why ?
As the Base Emitter of the signal goes positive, the forward bias current increases which causes an amplified signal to appear at the collector.  Now the Load which is based on the collector (e.g a resistor) - its voltage increases, causing a DECREASE at the collector point - this is the negative alteration of the signal at the collector point, since the voltage drop across the Load has increased.

When the Base Emitter signal goes into the negative (assuming an AC sinusoidal signal), the forward bias is reduced causing a reduction in the current across the collector, which in turn is felt across the Load resistor, which causes a reduction in voltage across the Load, which in turn causes an INCREASE in voltage across point C (collector) of the Transistor.

Power (WATTS)

The Power output of a transistor is calculated from the Source Power (Vcc) over the Load of the transistor, (RL) usually in the Collector.  The law is

P = V * I

Which means, the greater the Source power, then the greater the power output.
The power dissipated in the load comes from the powersupply (Vcc)

Resistors and Capacitors in Parallel on BASE of Transistor

Adding a capacitor in parallel with the base resistor will permit transistorQ2 to switch at a higher speed. The capacitor passes a current spike that clears the current carriers out of the junction area and permits the transistor to cut off quickly.When the transistor is switched on via a base voltage or signal, the capacitor passes the needed voltage directly to the base and drives the transistor quickly into saturation.

SATURATION

When a transistor is in the SATURATED state, the COMMON-EMITTER Junction is acting like a short (or a piece of wire), therefore, the VOLTAGE DROP is very very small at the COLLECTOR, and hence the voltage drop across the transistor is close to ZERO.