One way of understanding the basic operation of a bipolar transistor is by looking at its Vcb-Ic curves, i.e., the plots of its collector current Ic versus the voltage Vcb across its collector and base for different values of emitter current Ie.
These curves are derived from a transistor in common-base configuration, and basically describe the transistor's common-base output characteristics.
A transistor circuit is said to be in common-base configuration if the base is the terminal common to both the input and the output.
In the analysis of transistor circuits, the input current and output voltage are usually considered the independent variables.
Thus, for the common-base configuration, the independent variables are the input current Ie and the output voltage Vcb, while the dependent variables are the input voltage Veb and the output current Ic.
The family of input characteristic curves may therefore be described by the function f1 wherein Veb = f1(Vcb, Ie), while the family of output curves may be described by the function f2, wherein Ic = f2(Vcb, Ie).
The output curves corresponding to f2 are drawn with the collector-to-base voltage Vcb as the abscissa, and the collector current Ic as the ordinate. Different output curves are generated for different values of emitter current Ie, which are all drawn on the same plot.
These common-base Vcb-Ic curves are useful in distinguishing between the different regions of operation of a bipolar transistor, namely, the active, saturation, and cut-off regions.
The active or linear region, wherein the collector junction is reverse-biased while the emitter junction is forward-biased, is characterized by the flat or horizontal portions of the curves.
The saturation region, in which both the collector and emitter junctions are forward-biased, lies in the portions of the curves wherein Vcb is just very slightly above 0 V and Ie > 0.
Lastly, the cut-off region, wherein both the collector and emitter junctions are reverse-biased, is where the curves are below Ie =0.
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One way of understanding the basic operation of a bipolar transistor is by looking at its Vcb-Ic curves, i.e., the plots of its collector current Ic versus the voltage Vcb across its collector and base for different values of emitter current Ie.
These curves are derived from a transistor in common-base configuration, and basically describe the transistor's common-base output characteristics.
A transistor circuit is said to be in common-base configuration if the base is the terminal common to both the input and the output.
In the analysis of transistor circuits, the input current and output voltage are usually considered the independent variables.
Thus, for the common-base configuration, the independent variables are the input current Ie and the output voltage Vcb, while the dependent variables are the input voltage Veb and the output current Ic.
The family of input characteristic curves may therefore be described by the function f1 wherein Veb = f1(Vcb, Ie), while the family of output curves may be described by the function f2, wherein Ic = f2(Vcb, Ie).
The output curves corresponding to f2 are drawn with the collector-to-base voltage Vcb as the abscissa, and the collector current Ic as the ordinate. Different output curves are generated for different values of emitter current Ie, which are all drawn on the same plot.
These common-base Vcb-Ic curves are useful in distinguishing between the different regions of operation of a bipolar transistor, namely, the active, saturation, and cut-off regions.
The active or linear region, wherein the collector junction is reverse-biased while the emitter junction is forward-biased, is characterized by the flat or horizontal portions of the curves.
The saturation region, in which both the collector and emitter junctions are forward-biased, lies in the portions of the curves wherein Vcb is just very slightly above 0 V and Ie > 0.
Lastly, the cut-off region, wherein both the collector and emitter junctions are reverse-biased, is where the curves are below Ie =0.
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