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GATE 2018 EE – Question 51

Analog and Digital Electronics · Amplifiers: biasing, equivalent circuit and frequency response · 2 marks · Numerical answer

In the circuit shown in the figure, the bipolar junction transistor (BJT) has a current gain $\beta=100$. The base-emitter voltage drop is a constant, $V_{BE}=0.7$ V. The value of the Thevenin equivalent resistance $R_{Th}$ (in $\Omega$) as shown in the figure is ________ (up to 2 decimal places).

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Correct answer: 89.65 to 90.55

Explanation

Looking into the emitter, the base resistance of $10\ \text{k}\Omega$ is reflected as $\frac{10\text{ k}\Omega}{\beta+1}=99.0\ \Omega$. This is in parallel with the $1\ \text{k}\Omega$ resistor, so $R_{Th}=\frac{1000\times99.0}{1099.0}=90.1\ \Omega$.