Electronic – Why is a BJT considered “current-controlled”


With BJTs, we can control base current using Vin (from diagram). Why do textbooks state that BJTs are current controlled when it's obvious that changing the voltage controls the current through the collector? enter image description here

Best Answer

In the above circuit Vin is controlling the current going to the base, not the voltage drop across the base and emitter of the transistor itself.

The voltage drop across Vbe will always be around 0.7V for Vin > 0.7; the excess voltage will be dropped across the R1.

By changing Vin, you are actually controlling the current going to the base based on the equation:

$$I_B = (Vin-0.7V)/R1$$