The
following may seem Elementary and obvious, and a waste of the reader's
time.
You are
partly right, it is Elementary. And being elementary it is of the utmost
importance that you understand this on an intuitive
level: because everything that follows in engineering can be traced
back to this! well, almost
It's
True: If I'm Lying, I'm Dying!

Voltage Divider: 
If you tie two resistors of, say, the same value, end to endin
seriesacross a 10 volt power supply and measure the voltage at the junction
of the two resistors, you should measure 5 volts.
With these two resistors, you have created a Voltage Divider. The
two resistors do not have to be of equal value: their values are some Ratioone
to the otherhence a division of voltage.
Also, for sake of this discussion, let us say no matter what the
ratio of these resistor values, it is desirable that the total resistance
be the same. E.g., 1000 + 1000 = 2000; 1500 + 500 = 2000you get the idea! 

Potentiometer: 
Another form of Voltage Divider is the Potentiometer, or "Pot."
The pot is a variable resistor that allows the ratio of the two resistors
to change in a complementary and continuous fashion. That is, as one resistor
increases in value the other resistor is made to decrease in valuemaintaining
the total resistance the sameonly the Ratio Changes. Like the Fixed Voltage
Divider, the total resistance (LOAD) the pot places across the power supply
is constant. 

Rheostat: 
A rheostat is a variable resistor.
The rheostat and the Potentiometer are often confused. The difference
is, that a rheostatbeing a single variable resistor will cause current
to vary with its variation in resistance.
The Potentiometer "picks off" a particular potential (voltage) along
the Length of a non changing resistance, whose total "load" presented to
the power supply is constant regardless of its setting. 


Analog of Transistor Action




The Rheostat as Transistor
The transistor can be thought of as a device that is like a rheostat.
Imagine a rheostat tied to a fixed resistor as a transistor amplifier:
The rheostat is working against that fixed resistor just as a transistor
works against its collector load resistor.
As the rheostat's resistance is decreased, more current is made to flow
through the fixed Load Resistor, which causes a corresponding increase
in the voltage dropped across that Load Resistor. 

As the rheostat's resistance is increased, less
current is made to flow through the fixed Load Resistor, which causes a
corresponding decrease in the voltage dropped across that Load Resistor.
This may be "all well and good," but,
how do I amplify an Alternating CurrentA.C.with this Damn thing?
In the above, if we think of the extremes of the currentsmin/maxas
being the equivalent of the most positive and the most negative alternations
of a sine wave, for example, then we need only remove the Direct Current
(D.C.)a capacitor or transformer couplingto do the job! 

To add one more ingredient: in the simple example
above: we covered the two extremes of minimum and maximum
current, there is a third, or median value of current. This
can be thought of as the "resting" current. In the transistor world this
is referred to as the Bias Point.


It follows that any
symmetrical A.C. signal that progressively increases in amplitude will
swing about this bias point such that when one extreme of current is reached
the other extreme will also be reachedresulting in "symmetrical clipping." 
There's an Echo in Here
There's an Echo in Here
There's an Echo in Here
There's an Echo in Here
A NPN transistor connected as a common emitter amplifier:
The base needs current to do its thing. The collector cannot output
voltage, it can only cause more or less current to be drawn through its
load resistor.
If a voltage is applied to the base resistor a current
now flows into the base (base emitter junction). If a resistor is connected
between the collector and a positive supply voltage: the collector current
flowing through the collector load resistor causes a voltage
to be dropped across said load resistor. 
