Draw the transfer characteristics of a common emitter n-p-n transistor. Point out the region in which the transistor operates as an amplifier. |
Define the following terms used in transistor amplifiers. |
(i) Input resistance |
(ii) Output resistance |
(iii) Current amplification factor |
Answer:
The transfer characteristic curve of common emitter n-p-n transistor is drawn between \[{{I}_{C}}\] and \[{{I}_{B}},\] when \[{{V}_{CE}}\] is kept constant. Transfer characteristic of CE \[n-p-n\]transistor A transistor works as amplifier when it is operated in an active region. In this region, emitter-base junction is forward biased, while collector-base is reverse biased. (i) Input resistance: It is defined as the ratio of change in base-emitter voltage \[(\Delta \,{{V}_{BE}})\] to the resulting change in the base current \[(\Delta \,{{I}_{B}})\] at constant collector-emitter voltage \[({{V}_{CE}}).\] It is reciprocal of slope of \[{{I}_{B}}-{{V}_{BE}}\] curve. Input resistance, \[{{R}_{i}}={{\left( \frac{\Delta \,{{V}_{BE}}}{\Delta \,{{I}_{B}}} \right)}_{{{V}_{CE}}=\,\,\text{constant}}}\] (ii) Output resistance: From the output characteristics, we define output resistance of transistor as the ratio of change in collector-emitter voltage to the resulting change in collector current at constant base current. Thus, Output resistance, \[{{R}_{0}}={{\left( \frac{\Delta \,{{V}_{CE}}}{\Delta \,{{I}_{C}}} \right)}_{{{I}_{B}}\,\,=\,\,\text{constant}}}\] = Reciprocal of slope of \[{{I}_{C}}-{{V}_{CE}}\]curve. (iii) Current amplification: The current amplification factor \[(\beta )\] of a transistor in CE configuration is defined as the ratio of change in collector current to the change in base current at a constant collector-emitter voltage when the transistor is in active state. + \[\therefore \] \[{{\beta }_{AC}}={{\left( \frac{\Delta \,{{I}_{C}}}{\Delta \,{{I}_{B}}} \right)}_{{{V}_{CE}}\,\,=\,\,\text{constant}}}\] Its value is very large \[\left( {{\beta }_{AC}}>>1 \right).\]
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