Electrical – Transistors basic question, Voltage is confusing me

transistors

you can't go sticking a voltage across the base-emitter terminals,
because an enormous current will flow if the base is more positive
than the emitter by more than about 0.6 to 0.8 volt.

Base being more positive, is it saying that base has more holes ? And what does it mean that base is 0.6 volt more positive ? It also says: Vb= Ve+Vbe, the concept of voltage is really bothering me here, is this equation saying the voltage needed across base for current to flow throught it is equal to voltage across emitter plus the voltage drop from base to emitter ? What does this even mean ? These are probably very dumb questions but I'm really frustrated because anything I have read for the past 10 hours won't explain it in a more basic way.

Best Answer

They're not dumb questions but the concept isn't too difficult to understand.

A few notes:

  • The base emitter junction appears as a diode.
  • In forward conduction at moderate currents about 0.7 V is dropped across the junction.
  • The base current rises exponentially with the base-emitter voltage so we need to limit this applied voltage.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Various NPN transistor arrangements.

In (a) there is no current limiter on the base. We can turn the transistor on by applying about half a volt or more to it but slight variations in voltage will cause large variations in the current through R1. This makes it very difficult to control. If the base voltage rises too much the transistor will be destroyed.

In (b) we have added a current limiting resistor to the base. Now we can apply, for example, 5 V to NODE1 and with a 1k resistor be sure that a maximum of almost 5 mA will flow into the base.

In (c) we can apply a voltage to NODE1 and current will flow into the base turning on the transistor. Current will flow through R3 causing a voltage drop across it and the emitter voltage will rise to about 0.7 V below that of NODE1.

(a) and (b) are common emitter mode as their emitters are connected to a fixed voltage. (c) is a common collector arrangement and is also called an "emitter follower" as the emitter voltage follows the base voltage (with the 0.7 V drop).

To answer your questions:

Base being more positive, is it saying that base has more holes?

No. And thinking of holes and electrons doesn't help much. Just think of voltages and currents.

And what does it mean that base is 0.6 volt more positive?

Explained above.

It also says: Vb = Ve+Vbe.

It's just saying that when you're calculating voltages at any point in the circuits above you can reckon that the base voltage will be 0.6 or 0.7 V above the emitter voltage (when current is flowing into the base).

enter image description here

Figure 2. Horowitz and Hill, The Art of Electronics. "Transistor Man" looks at the current at the base, and adjust the current at the collector so as to be a multiple of the base current.

The image of Figure 2 may help a little.