Question

Consider a BJT transistor acting as a switch. How to determine the design resistances if we want to transition between cut off and saturation region to active region?

Answer 1

Transistor is nothing but a variable resistor. The resistance value of Transistor is changed by the Base current (BJT) or Gate voltage (FET). The value of resistance varies between Zero and Infinity. When the resistance is Zero the transistor operates in Saturation mode.For specific Base current, if the resistance reached Zero after that increasing the Base current will not have any effect on the resistance. The same way when the Base current is Zero the Transistor resistance is Infinity.

    Now we change the variable resistor block as Transistor.

              This Transistor acts as a Regulator (Variable Resistor) or a switch(ON/OFF).

              The Transistor has 3 operating modes.

               1. Cut-off (Switch - OFF)
               2- Saturation (Switch - ON)
               3- Active (Regulator).

        1. Cut-off Mode

        Vb < Vbe (Generally 0.7V)
         So Ib = 0A
              Ic = 0A
              Vc = Ic x Rc = 0V
              Vo = Vcc - Vc = Vcc

      2.  Saturation Mode

        Ic > Ic.max
        Ic.max = Vcc/Rc
        Ic = β Ib
        Ic = Ic.max
        Vc = Ic x Rc = Vcc
        Vo = Vcc - Vc = 0V

       3. Active Mode

        0 < Ic < Ic.max

        Ib = (Vb - Vbe)/Rb
        Ic =  β x Ib
        Vc = Ic x Rc
        Vo = Vcc - Vc
   
         0V < Vo < Vcc

      When a Transistor acts as a Regulator, it is called an Amplifier.
      When a Transistor acts as a Switch, it is called a Gate.

       Transistor in Active Mode - Analog Electronics
       Transistor in Cutoff/Saturation Mode - Digital Electronics