Question

Parameters of the transistor shown in the circuit below are $\beta=100$, $I_{Cq} = 1$ mA. 
Input resistance $R_i$ of the circuit is:

• A. 5
• B. 2.6
• C. 400
• D. 3

Hint:

The input resistance in a BJT amplifier depends upon the base current \(I_B\), remember the formula \( R_i = \frac{V_T}{I_B} \) and use that to solve problems like these.

Answer 1

The Correct Option is B

Step 1: Given, \( \beta = 100 \) and \( I_{Cq} = 1 mA \). We need to find input resistance \( R_i \). The input resistance \( R_i \) is given by \[ R_i = \frac{V_T}{I_B} \] where \( V_T \) is thermal voltage = 26mV. 
Step 2: First we need to find \(I_B\), which is given by \( I_C = \beta \times I_B\) \[ I_B = \frac{I_C}{\beta} = \frac{1mA}{100} = 0.01 mA \] 
Step 3: Calculate the input resistance: \[ R_i = \frac{V_T}{I_B} = \frac{26 \times 10^{-3}}{0.01 \times 10^{-3}} = \frac{26}{0.01} = 2600 \Omega = 2.6 k\Omega \] Thus the input resistance is 2.6 k$\Omega$.