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| Biasing in Transistor |
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| For proper working of a transistor, emitter base junction should be forward biased and CB junction should be reverse biased. |
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| The VEE supply is used to forward bias emitter base junction. As a result significant current flows, once the potential barrier is exceeded. The majority charge carriers diffuse from emitter into base and this results in emitter current IE as indicated in the above diagram. Once these reach the base, very few electrons or hole undergo recombination process and rest diffuse through collector base junction due to potential on the collector side. In a PNP the diffused holes reach the collector and at the same time an electron from the emitter enters into the positive pole of VEE thereby creating a hole in the emitter. Thus the current in PNP is caused by holes and the current in external circuit by electrons. |
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| Due to very less recombination process, the IC - IE. Apply Kirchhoffs current law. |
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| IE = IB + IC |
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| The action of NPN is similar to that of PNP. |
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| The three types of circuit connections for operating a transistor: |
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| i) Common base (CB) configuration |
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| ii) Common emitter (CE) configuration |
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| iii) Common collector (CC) configuration |
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| The common electrode is generally grounded and is common to the input and output circuit. |
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To study the transistor characteristics a transistor is either operated in common emitter configuration or in common base configuration. Let us take an npn transistor operated on common emitter mode. As discussed earlier a transistor works only when the input side is forward biased and output side is reverse biased. Ammeter are connected in series with base and collector to measure base current and collector current respectively. Voltmeters are connected in parallel to measure the input voltage (VBE) and output voltage (VCE). To study the characteristics, the output parameter, namely VCEis made constant and the variation of input current with input voltage is measured and the same plotted in the graph. (VBE v/s IB). A family of curves may be drawn by varying VCE.
The graph resembles the characteristics of a forward biased P-N junction.
The slope of the graph at a given point gives the input resistance |
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| Now to study the output characteristics, input current is kept constant and the variation of out voltage and output current are measure and a graph is plotted (adjacent page). It resembles characteristics of a reverse biased P-N junction diode. |
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| The output impedance can be found from the graph. Output impedance is the ratio of output voltage to output current at a constant input current. That is |
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The output characteristics show that IC changes rapidly in the beginning but soon IC becomes independent of VCE,
looking like a saturated one. The ratio  is nearly constant and it
is called as current gain. |
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