INDEX

TRANSISTOR
When we talk of transistor in robotics, we talk about the cut off and saturation region only, while in your course you study transistor in active region. So here I am talking about transistor as a switch. When we say transistor as a switch, we talk of cut off or not because the typical cut off voltage is around .5V and the saturation voltage(vbe) is around .8V. There is regions between them. Let's start with transistor to glow an LED.

Connect this ckt and see. Connect multimeter at the base of the transistor and see the voltage. In this circuit we can see that Ve=Vbe. For the transistor to be switched ON Ve=.5V. Vary the potentiometer to make Vbe=.5V, you can see that LED starts glowing(but it is less brightness). Vary the potentiometer to make Vbe to around .8V , you can see that the LED brightness increases. This is because when Vbe=.5V it starts with cut off and when Vbe=.7V in active and Vbe=.8V it become saturation region. Transistor is a current controlled device. In active region Ic=hfe Ib and in saturation region Ic>hfeIb. That is why the brightness of the LED changes.