The BJT has a large input resistance, known as rΠ, with magnitude of order of kΩ.
Moreover, it also has a large output resistance, known as ro, which is usually even larger than rΠ.
Since both of its Rin and Rout are large, then when a BJT is used as an amplifier, for example a Common Emitter amplifier, the amplifier is considered to be a transconductance amplifier, meaning the input signal is voltage (vBE).
So why is it said (at least in the Semiconductor's Basics course) that the BJT amplifies current, if its input signal is voltage?
Moreover, in what I managed to read so far about BJT amplifiers, they always calculated the collector current using VBE, not IB.
Hi there,
The transistor is said to be 'current controlled' because that is the
main way the transistor operates when looked at from a certain point
of view where you can see the base current vary widely when the
base emitter voltage only varies slightly. It's almost like saying that
an LED is current controlled, because the main feature to look at
in most cases is the current, not the voltage.
This doesnt mean that voltage is totally out of the picture, because
of course there has to be enough voltage to start with, but if you
picture the current source on the input as theoretical then you dont
really think as much about the input voltage because it goes to
whatever it needs too. In fact, in many designs you would regard
that small voltage change as a disturbance rather than something
you design for.
There are views that look at the voltage vs collector current as you
already noticed. That's an attempt to make some sense out of
how the transistor operates with respect to its input voltage rather
than the current, but for many devices like this they are often
characterized by their dominate features rather than the smaller
features that change only a little. This also aids in the design
sometimes where you can ignore certain features and concentrate
more on others.
Part of engineering is noting both the small and the large, but knowing
when the small can be mostly ignored. The nature of the application at
hand is the most important dictator of how you should look at a circuit
element...sometimes the voltage is very important (log converter)
and sometimes it is not important at all (AC amplifier).