A DC power supply is provided for the operation of a transistor. This DC power supply is provided to the two P-N junctions of a transistor which affects the actions of the majority carriers in these emitter and collector junctions.
The junction is forward-biased and reverse-biased depending on our requirements. When in the forward biased state, a positive voltage is applied to the P-type semiconductor material and a negative voltage is applied to the N-type semiconductor material. When in the reverse biased state, a positive voltage is applied to the N-type semiconductor material and a negative voltage is applied to the P-type semiconductor material.
Transistor biasing
The power supply of the appropriate external DC voltage is called biasing. Either forward or reverse biasing is done at the emitter and collector junctions of the transistor. These biasing methods render transistor circuits rarely used in four types of regions: active region, saturation region, cutoff region, and the inverse active region.
In these regions, the inverse active region, which is the inverse of the active region, is not suitable for any application and is therefore not used.
Related Tutorial: Transistor Biasing and Operations.
Active region
This is the region in which transistors have many applications. It is also called a linear region. A transistor, while in this region, functions better as an amplifier.
The region between cut-off and saturation is known as the active region. In the active region, the collector-base junction remains in reverse-biased while the base-emitter junction remains forward-biased. As a result, the transistor will function normally in this region.
In this region operation, the current flowing from the collector to the emitter is proportional to the current flowing in the base.
This region lies between saturation and cutoff. The transistor operates in the active region when the emitter junction is forward-biased and the collector junction is reverse-biased. In the active state, the collector current is β times the base current,
Where,
- IC = Collector current.
- β = Current amplification factor.
- IB = Base current.
Saturation Region
This is the region in which the transistor behaves as a closed switch. The transistor has the effect of shorting its collector and emitter. Collector and emitter currents are maximum in this mode of operation.
The transistor acts like a short circuit. Current flows freely from collector to emitter.
The transistor operates in a saturation region when both the emitter and collector junctions are forward-biased. In the saturation region, the transistor behaves as a closed switch, we can say that,
Where,
- IC = Collector current.
- IE = Emitter current.
Cutoff Region
This is the region in which the transistor behaves as an open switch. The transistor is affected by the opening of its collector and base. In this mode of operation the collector, emitter, and base currents are all zero.
The transistor acts like an open circuit. No current flows from collector to emitter.
The transistor operates in the cutoff region when both the emitter and collector junctions are reverse-biased. Like the cutoff region, collector current, emitter current, and base current are zero, write them as
Where,
- IC = Collector current.
- IE = Emitter current.
- IB = Base Current.
Transistor Load Line Analysis
The transistor conducts well in the active region and hence is also called a linear region. The outputs of the transistor are collector current and collector voltage.
The load line analysis of a transistor means that for a given value of collector-emitter voltage we find the value of collector current. This can be done by plotting the output characteristic and then determining the collector current IC with respect to the collector-emitter voltage VCE. Load line analysis can be easily achieved by determining the output characteristics of load line analysis methods.
Transistor Output Characteristics
When considering the output characteristics of a transistor, the curve for different input values can be represented by a suitable figure.
By the above figure, the output characteristics between collector current IC and collector voltage VCE can be drawn for different values of base current IB in the transistor. These are considered here for different input values to obtain different output curves.
Operating point
When considering a value for the maximum possible collector current, that point will be present on the Y-axis, which will be nothing but the saturation point. Also, when a value is considered for the maximum possible collector-emitter voltage, that point will be present on the x-axis, which will remain the cutoff point.
When a line is drawn joining two points, such a line can be called a load line. It is also called so because it denotes the output on load. This line, when drawn on the output characteristic curve, makes contact at a point called the operating point.
This operating point is also called a quiescent point or simply Q-point. There can be many such intersection points, but the Q-point is selected in such a way that the transistor remains in the active region, regardless of the AC signal swing.
A load line has to be drawn to get the Q-point. A transistor will act as a good amplifier when it is in the active region and when it is made to operate at the Q-point, faithful amplification is achieved.
The process of receiving the entire portion of the input signal by increasing the signal strength is called Faithful Amplification. This is done when an AC signal is applied to its input.
Transistor DC Load line
When the transistor is biased and no signal is applied to its input, the load line drawn in such a condition can be understood as a DC condition. Here there is no amplification because the signal is absent. The circuit can be better understood by the above figure,
The value of the collector-emitter voltage at any given time will be
Since VCC and RC are fixed values, the above is a first-degree equation and will therefore be a straight line at the output characteristics. This line is called the DC load line. The DC load line is shown in the above figure.
To obtain the load line, the two end points of the straight line have to be determined. Let those two points be A and B.
To obtain A
When the collector-emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. it is shown as,
This collector gives point A (OA = VCC/RC) on the current axis.
To obtain B
When the collector current is IC = 0, the collector-emitter voltage is maximum and will be equal to VCC. It gives the maximum value of IC. it is shown as,
This gives point B, which means (OB = VCC) on the collector-emitter voltage axis.