Which of the following equations correctly defines the input resistance of the common emitter junction transistor?
Correct Answer:
C.
Explanation:
The input resistance of a common emitter junction transistor is defined by the ratio of the change in base-emitter voltage to the corresponding change in base current, while the collector-emitter voltage remains constant. Mathematically, this is expressed as Rin = ?VBE / ?IB. This dynamic resistance, often denoted as rbe or hie in small-signal models, determines how much current the amplifier draws from the signal source. For a basic configuration, it can be approximated as ß times the internal emitter resistance (re), where re is roughly 25mV divided by the emitter current. Including an external emitter resistor (Re) significantly increases this value through the resistance-reflection rule, making the total input resistance approximately (ß + 1)(re + Re).
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