Summary about the lecture on 19-08-2014
Common Base Input Characteristics
- It is essentially the characteristics of a FB PN junction
- The input characteristics shifts towards the ordinate when the collector-base reverse bias voltage is increased, because as the |VCB| increases the depletion region in the JC deeply penetrates into the base region and reduces the effective base width causing an increase in the emitter current IE. Hence for a given value of |VEB| the input current increases with increase in |VCB|.
Common
Base Output Characteristics
- IC is almost equal to IE.
- IC appears to be almost constant when VCB is increased. This is due to the fact that with IE held constant, the increase in IC due to the reduction in the effective base width on account of increase in VCB is so small and noticeable only at large values of VCB. Hence the output curves appears to be almost horizontal.
- It is noteworthy from the output characteristics, that even when the potential VCB is zero, the collector current IC still flows. This is due to the fact that there exists an inherent barrier voltage across the JC, even when the external reverse bias voltage VCB is zero. This inherent barrier voltage aids the flow of IC, because the electric field established in the JC acts in such a direction to pull the minority charge carriers in the base region into the collector region constituting the current IC. Hence in order to reduce the collector current IC to zero it is essential to apply a forward biasing voltage to the junction JC.
- In the cut-off region, the magnitude of the current ICB0 is so small such that the curve, corresponding to IE=0, seems virtually lies on the horizontal axis. Remember that the current ICB0 is sensitive to temperature.
References:
David A. Bell, Electronic Devices and Circuits - 5Ed, Oxford University Press, New Delhi.
Robert L. Boylestad and Louis Nashelsky, Electronic Devices and Circuit Theory - 10Ed, Pearson, New Delhi.
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