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 Fun with Resistors

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 end--in series--across 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 Ratio--one to the other--hence 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 = 2000--you get the idea! 

Voltage Divider
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 value--maintaining the total resistance the same--only the Ratio Changes. Like the Fixed Voltage Divider, the total resistance (LOAD) the pot places across the power supply is constant.  Potentiometer
A rheostat is a variable resistor. 

The rheostat and the Potentiometer are often confused. The difference is, that a rheostat--being 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
Rheostat as Transistor

Transistor max current

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. 

Transistor min current 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 Current--A.C.--with this Damn thing?

In the above, if we think of the extremes of the currents--min/max--as 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 coupling--to do the job!

Transistor nom/bias current 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. 

Transistor dynamic currents 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 reached--resulting 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. 

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