Silly transistor question, Voltage to the base

[nuttaphong191]

I have this transistor 3904. My question is: what is the max voltage I can apply to the base? Is EB voltage it? Why is CB voltage is so high? And why is there any voltage there at all? CE voltage makes sense but CB voltage?

 

[audioguru2]

The base-emitter junction of a transistor is a diode. It limits the voltage to about 0.7V when it is forward-biased and conducts  like a zener diode when its reverse voltage exceeds about 6V.

You do not apply a voltage to the base of a transistor. You apply a forward current.
An emitter-follower can have a voltage input because the emitter is the variable output.

Your circuit must limit the current to the base or the transistor will be destroyed.
If you have +10,000V then a 1M resistor in series to the base will limit the current to only 10mA which is fine.

The max CB voltage rating is higher than the max CE voltage rating because leakage current turns on the transistor at high CE voltages. Then the transistor would short the supply and melt. The leakage current is low enough so that CB voltage can be higher.

 

[AN920]

The max CB voltage rating is higher than the max CE voltage rating because leakage current turns on the transistor at high CE voltages. Then the transistor would short the supply and melt. The leakage current is low enough so that CB voltage can be higher.

The collector Vceo implies the breakdown c-e rating with the base connection open. Transistors are hardly ever used like this, as normal operation always requires the b-c junction to be reverse biased. With reverse bias the c-e breakdown rating can be as high as 3 x Vceo. Designers like to use the Vceo rating because it has a automatic safety factor added.