BJT(Bipolar Junction Transistor )

BJT has three modes or regions of operation that is
1. Active
2. Cut off
3. Saturation

2.1.1. Active Mode
If the base-emitter junction is forward biased and the base-collector junction is reverse biased, the
transistor is active in the "linear region." There is controllable current flow through the transistor, with the collector and emitter currents being controlled by the base current.


Equations for Active Mode-
In active mode, we have:
(1) VBE ≈ 0.7 V
(2) IC = β IB; where β is a parameter of the particular BJT and is called the common-emitter current gain.

Two types of active mood, that’s are :-
2.1.1.1. Forward Active

In forward active mode, that is the base-emitter junction diode is forward biased (Vbe=0.7V), and the base-collector diode is in cut-off mode (Vbc<0.6V). The current through the base terminal is directly proportional to the collector current;
Ic=*Ib, (- common-emitter current gain.).
The emitter current is the sum of the two other terminal currents; Ie=Ib+Ic.
Slightly modifying the circuit voltage and/or resistor characteristics can drive the BJT into saturation.[7]

2.1.1.2. Reverse Active
In reverse active mode that is the base-collector junction diode is forward biased (Vbc=0.7V), and the base-
emitter diode is in cut-off mode (Vbe<0.6V). The current through the base terminal is directly proportional to the emitter current;
Ie=R*Ib.
The collector current is the sum of the two other terminal currents; Ic=Ib+Ie.
2.1.2. Cut-Off
BJT is in cut-off mode, meaning there is an insignificant amount of current flowing through its three terminals. In order for the transistor to begin operating in forward active (linear) mode, the base-emitter voltage must be at least 0.6V (approx.), also known as the edge of conduction point.
When the base emitter voltage is below the edge of conduction point, the BJT could operate in reverse active mode, if the base collector voltage is larger than cut off voltage (i.e. VDD<-0.6V) [7].
2.1.2.1 Edge of Conduction
The BJT is at the edge of conduction, that is a slight rise in the input voltage will drive the BJT in to forward active (linear) mode.
The current at the terminals at this point is still insignificant, in the microamperes range (and lower)
2.1.3. Saturation.
When both PN junctions are forward biased, the transistor appears as a short circuit. The collector current is maximum and is controlled by external components, not the base current. VCE, the voltage between the collector and emitter will be about 0.1V (near zero).[4]
The BJT is operating in saturation mode, meaning both base-collector and base-emitter p-n junctions are forward biased resulting in
Vbe=0.8V and Vce=0.1V (approx.).
The collector current cannot keep up with the base current, thus the linear base-collector relationship is lost, and
Ic < *Ib.
The emitter current is still equal to the sum of the base and collector currents. Ic is at a maximum for the specific circuit resistors and voltage sources, and is constant when the BJT is saturated.
Slightly modifying the circuit voltage and/or resistor characteristics can drive the BJT into forward active mode [6].

(frm my thesis 1-2)