Thread Starter


Joined Nov 15, 2006
can anyone explain why do we call BJT as current controlled while actually they are voltage controlled...the base current is actually a function of base emitter voltage
then why not call it voltage controlled


Joined Jan 28, 2005
I believe the answer to your question resides in the fact that once the transistor's base emitter junction is fully forward biased in a common emitter configured BJT stage, the application of more voltage to the base results in an increase in the base current. The voltage at the base changes very little.

It is only by providing a base current limiting resistor between the base and the input voltage source that the applied voltage can be safely increased without damaging the transistor.



Joined Oct 14, 2005
It is actually viewed from the semiconductor angle. Inside, the current that flows through the base-emitter junction causes or controls much larger current that flows through the collector-emitter junction. So, although on the outside the base-emitter current is ultimately due to base-emitter voltage, the terminology refers to what's going on in the inside.

In voltage controlled devices, such as FETs, the gate-source voltage alters the channel width thus controlling the drain-source current.

Ron H

Joined Apr 14, 2005
This is a debate that shows up in forums frequently, and there is no definitive answer. It's a little like asking whether the current through a resistor is due to the voltage across it, or is the voltage across it due to the current through it. It all depends on how you are driving it.
It is true that the emitter current is defined as a function of the base-emitter voltage (the diode equation, I=Is*e^(qV/kT)-1. This is very valuable if you are making log amps, or analog multipliers (or other Gilbert cells), but if you are making a switch or an inverter or any other saturating BJT circuit, the diode equation is pretty much irrelevant, other than the fact that it will predict that that Vbe(sat)~0.7V. Beta becomes much more important.
If you want to waste your time predicting the behavior of a saturating switch with the diode equation, go for it.:D