TRANSISTOR

A Transistor is a Current In/Current Out Device

A Transistor can be thought of as a device that is active in only One Direction: It can draw more or less current through its load resistor (sometimes referred to as a pull-up resistor).
It can either Source Current or it can Sink Current
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Since the Transistor is a Current device, any signal Voltage must first be
Converted to a Current

Voltage to Current Convertor
First, you must convert the input voltage to a current by
using a Voltage to Current Convertor--a resistor

Since the Transistor is a Current in/Current out device, any Current Output is
Converted to a Voltage Drop by the Current flowing thru a Load Resistor
Current to Voltage Convertor
Next, you convert the output current into a voltage by
using a Current to Voltage Convertor in the collector circuit--you guessed it--a resistor.

Note voltage to current convertor in the base circuit. A.K.A., current limiting resistor

The Results when driving the transistor's base directly with no voltage to current convertor

A N I M A T I O N S
If a picture is worth 1024 words, how many words if it moves?
Spring as Load Analogy

Common Emitter

Common Base

Differential Amplifier

Differential input

Common Mode input

The Rheostat as a Transistor
The transistor can be thought of as a device that is like a rheostat (potentiometer). If you think of a pot tied to a fixed resistor as a transistor amplifier: the pot is working against the fixed resistor--the collector load resistor. This means the transistor cannot generate a positive and a negative signal, it can only draw more or less current, e.g., the pot decreases its resistance, causing more current through the "load" resistor, thus causing the voltage dropped across that resistor to increase; the pot increases its resistance, causing less current through the load resistor, and this causes less voltage to be dropped across the load resistor. If we think of the extremes of current as being the equivalent of the positive and negative alternations of a sine wave, then it follows that the equivalent of zero is some current equidistant between the two

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 or load resistor causes a voltage to be dropped across said load resistor.
Diodes as Transistor
We can simulate a NPN transistor using two diodes and connecting both anodes together. One cathode is tied to common (the emitter); the other cathode (the collector) goes to a load resistor tied to the positive supply. Now connect a 1k resistor to the junction of the two anodes (the base), and using a signal generator, apply a 0 to 2 volt P-P sine wave to the other end. Using a dual beam oscilloscope, observe the signal at both ends of the resistor, i.e., the generator and the "base."

The results should resemble the figure: the diode signal starts up unimpeded until it reaches ~ 0. 6 volts peak (1.2 volts P - P), at which point the voltage at the "base" appears to stop increasing, even though the signal generator is still increasing in amplitude. No matter how much the voltage applied from the generator increases (within reason), the "base" voltage appears to not increase. However, the current into that junction (two anodes) increases linearly: I = [E - 0.6]/R.
Now at this point, the analogy falls apart: these two diodes have no gain, as the transistor we are trying to simulate would have. However, let us pretend that it does: the "collector" is a high impedance current source and if a resistor (the load resistor) is connected between the "collector" and the positive supply, a voltage is seen at the collector. This changing voltage drop across the resistor--caused by the changing collector current--will change correspondingly to the "base" current.

Now follow me, just a few more words, and You've got it! As the voltage at the generator goes more positive; the base current increases; the collector current increases; the voltage drop across the collector resistor increases; and the voltage at the collector goes less positive or lower.
Hang on! Stay with me!
Conversely, when the voltage at the generator goes less positive; the base current decreases; the collector current decreases; the voltage drop across the collector resistor decreases; and the voltage at the collector goes more positive or higher. Feel better now OK, So I Lied: There is just a little more to the story. Remember when the base reached ~0.6 volts? well the collector output is only that part of the signal that caused the base to conduct current. In other words: until the base rises to ~ 0.6 volts and there is base current, there is no change at the collector--no collector output.
Make a List
The following list of attributes may, at first glance, seem confusing and contradictory, however they are all true and are offered as clues to the puzzle of: 'how does a transistor really work?'
Abstractly, here are some Characteristics:
1. An equivalent circuit of a NPN transistor is two diodes tied anode to anode; one cathode being the emitter, the other the collector, and the junction of the anodes is the base.

2. When a NPN transistor is doing-its-thing, there is always a constant 0.6 volt drop between the base and emitter, i.e., the base is always ~ 0.6 volts more positive than the emitter--always!

3. There is no output at the collector, until the base has reached ~ 0.6 volts and the base is drawing current, i.e., any signal that appears at the base that is not up to ~ 0.6 volts (and not drawing base current), is never seen at the collector.

4. The base requires a current, not a voltage to control the collector current.

5. The collector is a current source: it does not source a voltage.

6. The collector appears to output a voltage when a resistor is connected between it and power.

7. The collector is a high impedance when compared to the emitter.

8. The transistor can output an amplified signal either from the collector or the emitter (or both).

9. When operating with a collector resistor (RL): the output voltage from the collector is an amplified voltage.

10. When operating with only an emitter resistor (Re): the output voltage from the emitter is not an amplified voltage, because it is always ~ 0.6 volts, below the input (base) voltage--hence the name voltage follower. But because the emitter can source large amounts of current to the "LOAD," it can be said, there was CURRENT amplification.

11. The collector--being high impedance--cannot drive a low impedance load.

12. The emitter--being a low impedance--can drive a low impedance load.

13. The voltage gain from the collector is greater than one (Gv > 1).

14. The voltage gain from the emitter is less than one (Gv < i =" P." href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6WKjjR_jH1uhrJufiMcHcZCMbd2Zpn-q6jY54XptI4BmElSe_UIk3fC4GyJ5qtZL4y4t2yS3oOCJpucaCUXwdQPOFc0hvKxZEL0HBHZxQn7MMrN713K9_hj_7OB-nO_9VlL6r7qmX7mu9/s1600-h/1.gif">

Because a transistor is a current device: if you cause some current to flow in the base, a larger amount of current is caused to flow in the collector. There's that pesky echo again.
Looking at the common emitter circuit in the figure: while measuring the voltage and the current, one starts to apply a voltage to the base of the transistor through the base resistor.

As the voltage increases from, zero there is no current flowing. At 0.1 volt, no current; 0.2 volt, no current; 0.5 volt, still no current; as the voltage at the base approaches 0.6 volts--where there was no current--all of a sudden a small current starts to be drawn by the base, and the voltage at the base slows its rate of increase--and remains at ~ 0.6 volts. As the voltage from the source increases, the voltage at the base remains ~ 0.6 volts, and the current increases--as well as the corresponding collector current.

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