GATE EE 2018 Set 1 — Question 51
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Analog & Digital Electronics → Amplifiers & Feedback → Small-Signal Equivalent Circuits
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Question
In the circuit shown in the figure, the bipolar junction transistor (BJT) has a current gain . The base-emitter voltage drop is a constant, . The value of the Thevenin equivalent resistance (in ) as shown in the figure is ______ (up to 2 decimal places).
Correct answer
89 to 91.5
Solution
To find the Thevenin equivalent resistance at terminals a-b, we deactivate all independent sources (the 15 V and 10.7 V DC sources are shorted to ground).The resistance looking into the emitter of the BJT is given by:
where is the resistance in the base path.From the DC bias conditions:
.
.
.
.Calculating :
.The Thevenin resistance is the parallel combination of the emitter resistor and the resistance looking into the emitter :
.Using the approximation :
.
This value falls within the official range of 89.0 to 91.5.
where is the resistance in the base path.From the DC bias conditions:
.
.
.
.Calculating :
.The Thevenin resistance is the parallel combination of the emitter resistor and the resistance looking into the emitter :
.Using the approximation :
.
This value falls within the official range of 89.0 to 91.5.
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