Cut off, Active and saturation regions of a transistor:
1.Transistor Biasing:- The application of suitable dc voltages across the transistor terminals is called biasing. Each junction of a transistor may be forward biased or reverse biased independently. These are following three different ways of biasing a transistor, which is also known as modes of transistor operation.
2.Forward active:- Emitter-Base junction is forward biased Collector- base junction is reverse biased.
3.Saturation Region:- Emitter-Base junction is forward biased Collector- base junction is forward biased In this mode transistor has a very large value of current. The transistor is operated in this mode, when it is used as a closed switch. Here, there is a large change in the collector current IC with a small change in VCE.
4.Cut- off Region:- Emitter-Base junction is reverse biased Collector- base junction is reverse biased In this region both the junctions are Reverse Biased. In this mode transistor has zero current. The transistor is operated in this mode, when it is used as an open switch. Since the collector base junction is reversed biased, the current due to majority carriers flows from collector to emitter which is represented by ICEO
Output characteristics of Common Emitter Transistor
Output resistance: ro=ΔVceΔIc
IB = constant
Current gain = βdc=IcIb
Modes of transistor action:
Input characteristics:- It is the graph of input current Ib v/s input voltage Vbe at a constant output voltage Vce. It resembles the characteristics of a forward-biased diode. The input current Ib increases as the input voltage Vbe increases for a fixed value of Vce. As the Reverse-Bias voltage Vce increases, Depletion region in the collector –base junction increases. Hence the width of the base available for conduction decreases. Hence, Ib decreases due to early effect and the graph shifts towards X-axis.
Dynamic input resistance =
where VCE = constant
The family of output characteristics curves of a bipolar transistor is given below. The curves show the relationship between the collector current (IC) and the collector-emitter voltage (VCE) with the varying of base current (IB). We know that the transistor is ‘ON’ only when at least a small amount of current and small amount of voltage is applied at its base terminal relative to emitter otherwise the transistor is in ‘OFF’ state.