NPN Transistor With Three LED's as a Switch

 Three modes of Operating Region of Transistors are:

1. Active Region
2. Saturation Region
3. Cut-off Region

1- Active Region Characteristics:

• In 'Active Region' , the transistor operate as an 'Amplifier'
• The Junction Base-Emitter is forward Biased.
• The Junction Collector-Base is Reverse Biased.

2- Saturation Region Characteristics:

• ​​In 'Saturation' region, the transistor operates as an 'Closed Switch'.
• The 'Base' Electrode is Connected to Supply Voltage [ Vcc ]
• The Junction Base-Emitter Voltage is greater than 0.6V i.e V_BE > 0.6V.
• The Junction Base-Emitter is Forward Biased.
• The Junction Collector-Base is Forward Biased.
• Maximum Saturation Collector Current [ Ic ] flows, i.e [ Ic=Vcc/R_L ]
• This condition of transistor is act as a 'Fully-ON' and this is 'Saturation Region'.
• Collector to Emitter Voltage [ V_CE ] is approximately equal to Zero ideally. i.e [ V_CE = 0V ]

​3- Cut-Off Region Characteristics:

• In 'Cut-off' region, the transistor operate as an 'Open Switch'.​
• The 'Base' Electrode is Grounded to zero Voltage [ 0V ].
• The Junction Base-Emitter Voltage is less than 0.6V i.e V_BE < 0.6V.
• The Junction Base-Emitter is Reverse Biased.
• The Junction Collector-Base is Reverse Biased.
• No Collector Current [ Ic ] flows, i.e [ Ic=0 ].
• This condition of transistor is act as a 'Fully-OFF ' and this is 'Cut-Off Region'.
• Collector to Emitter Voltage [ V_CE ] is approximately equal to Supply Voltage [ V_CC ] [ V_CE = V_CC ]

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