- Bipolar Transistors are NOT current amplifiers. Most books get this wrong. In fact they're voltage-controlled insulators.

- Overall, bipolar transistors act like a thin layer of insulator which has a variable thickness. The thickness of the insulating layer can be electrically altered in order to control electric current. It's like closing a switch: make the insulator thin enough, and the transistor turns "on." Make it slightly thicker, and the transistor turns partially on.

- A bipolar transistor is like three hunks of silicon in a row. Usually it's NPN; two hunks of n-type silicon separated by a hunk of p-type. Doped silicon (n-type and p-type silicon) is a conductor. At the places where the n-type silicon is touching the p-type, a very thin layer of insulator spontaneously appears. Because the three hunks of silicon are normally separated by these insulating layers, a transistor is normally turned off.

- The center region is called the "Base." One of the end regions is called the "Emitter." By applying a small voltage between Base and Emitter, we can make the thin layer of insulator become even thinner. If it's thin enough it stops insulating and charges flow across it. (Imagine bringing two wires closer and closer until the electrons start jumping across the microscopic gap.)

- The transistor's Base/Emitter voltage controls the insulator, which then controls the whole transistor. A large voltage will shrink the insulating region and turn the transistor on. A smaller Base/Emitter voltage can turn the transistor half-way on, and this gives us linear amplifying action rather than just on-off switching.

- But a transistor is not a diode! Exactly. There's a second insulating region between the Base region and the third hunk of silicon (called the "Collector.") This insulator acts very weird, since it doesn't block the charges coming from the Base. It's not even an insulator? However, it does block the effects of any voltage placed between Collector and Base. Whether the Collector voltage is low or high, we get the same amperage through that layer. Therefore the Collector acts as if it is insulated from the Base. Yet charges flow right through the insulating region, from Emitter through Base and into Collector. Therefore the Collector acts as if it's NOT insulated from the Base. Which is it? Both!

- Why do transistors act as amplifiers? That's simple to explain: all valves are amplifiers. Water valves, air valves, and vacuum tubes are amplifiers. Carbon microphones are amplifiers (paired with loudspeakers, they were used as the first audio amps, placed in long distance phone lines. Thomas Edison made big bucks!) It takes very little energy to open or close a valve, yet this controls huge pressures and huge flows. A transistor is like a valve, a valve where a tiny change in voltage can open or close the valve by varying amounts. A small amount of "signal" energy causes the transistor to control the large current which is pumped by a battery. The shape of the output waveform is the same as the shape of the input waveform, so we say that the signal has been "amplified," when really it has been created from the battery's energy. And if battery voltage stays the same, then any change in charge flow is a change in energy flow. Wattage. Small watts in, big watts out. That's what amplifiers are all about. But any valve can do this.

- Most transistors are Silicon, and Silicon junctions "turn on" at around 0.4V to 0.8V. Place half a volt on the Base of a transistor and the transistor barely starts turning on.

- But why do so many books say "Transistors Amplify Current?" It's because the authors are confused by the leakage current coming out of the Base wire. Yes, the Base/Emitter voltage controls the thickness of the insulating region and thus controls the main transistor current. But also a tiny bit of charge leaks out through the Base connection. It SEEMS like this leakage affects the collector current.

- It just so happens that the tiny leakage current in the Base connection is proportional to the transistor's main Emitter/Collector current. (This makes perfect sense, since both the Base leakage current and the main Emitter/Collector current are controlled by the insulator thickness, which is set by the Base/Emitter voltage.) ...so it SEEMS as though the Collector current is controlled by the Base current. You can even simplify things by pretending that this is true. But in reality, it's the Base/Emitter voltage which runs things. You'll never understand the simple physics if you think that Base current can control Collector current. It can't.

- A bipolar transistor has a voltage-controlled input, while it's output is a variable controller which creates a constant current. It's vaguely like a resistor, but where the voltage in one place creates a current in a second place ...yet the voltage in that second place does not affect the current. It's not like a transformer where volts and current are swapped, but wattage stays the same. The effects of voltage are TRANSFERRED to another spot in the circuit. "Transfer Resistor." Transistor.