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Explain the input and output characteristics curves for a common emitter transistor. |
Answer» Solution :COMMON emitter characteistics. The circuit diagram for common emitter p-n-p transistor is shown in fig.(a). The input is applied ACROSS the emitter and base and output is TAKEN across collector and emitter. Here emitter is common to both input and output circuits. The most important characteristics of common base connections are input characteristic and output characteristics.  `(i)` Input Characteristics A graph showing the relationship between base-emitter voltage and base-current and constant collector-emitter is called input characteristics of the transistor. To obtain input characteristic , the emitter is forward biased by base emitter voltage `V_(be)` and the collector is reverse biased by collector emitter voltage `V_(ce)`. The collector voltage `V_(ec)` is KEPT constant at a suitable value. The base voltage `V_(be)` is varied in small steps and the base current `I_(b)` is noted corresponding to each value of the base-emitter voltage. A curve is then plotted between `V_(bc)` and `I_(b)` for a given value of `V_(ce)`. This is one characteristic. Similar characteristic curves can be drawn for different fixed VALUES of `V_(ce)("say"-2V,-4V"etc)` as shown in fig. (b) .  `(ii)` Output characteristics . A graph showing the relationship between the collector base voltage and collector current at constant output characteristic of the transistor. To obtain output characterisitics the base current is kept constant at a suitable value the collector emitter voltage `V_(ce)` is varied in small steps and the collector current `I_(c)` is noted corresponding to each value of the base current `I_(b)` . A curve is then plotted between `V_(ce)` and `I_(c)` for a given value `I_(b)`. This is one characteristic. Similar characteristic curves can be drawn for different fixed  values of `I_(b)` (say `10muA`, `15muA`, `20muA` etc. as shown in fig.(c) |
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