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Draw a circuit diagram to obtain the characteristics of a n-p-n transistor in common base configuration . Give shape of input and output characteristic curves. |
Answer» Solution :`(i)` Common base characteristics. The CIRCUIT diagram for common base n-p-n transistor is shown in fig. (a). The INPUT is applied across the emitter and base and OUTPUT is taken across collector and base. Here base is common to both the input and output circuits. The most important characteristics of common base connections are input characteristic and output characteristic.  Input characteristics (or emitter characteristics) A graph showing the relationship between the emitter base VOLTAGE `(V_(eb))` and the emitter current `(I_(e))` at constant collector base voltage is called input characteristics of emitter characteristics of the transistor. To obtain input characteristics, the emitter base circuit is forward baised by emitter to base voltage `V_(eb)` and collector base circuit is reverse biased by collector to base voltage `V_(eb)`. The collector voltage `V_(cb)` is kept constant at a suitable value. The emitter voltage `V_(eb)` is VARIED in small steps and the emitter current `I_(e)` is noted corresponding to each value of the emitter voltage. A curve is then plotted between `V_(eb)` and `I_(e)` for a given value of `V`. This is one characterisitic. Similar characteristic curves can be drawn for different fixed values of `V_(cb)` as shown in fig. (b).  `(ii)` Output characteristic (or collector characteristics). A graph showing the relationship between the collector base voltage `(V_(cb)` and collector current at constant emitter current `(I_(e))` is called the collector characteristic or the output characteristic of the transistor. To obtain ouput characteristics the emitter current `I_(e)` is kept constant at a suitable value. The collector voltage `V_(cb)` is varied in small steps and the collector current `I_(c)` is noted corresponding to each value of the emitter current. A curve is then plotted between `V_(cb)` and `I_(c)` for a given value `I_(e)`. This is one characteristic. Similar characteristic curves can be drawn for different fixed values of `I_(c)` as shown in fig (c) . 
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