Question

For a common emitter transistor amplifier, the voltage across the collector resistance is 2.8 V. If the current amplification factor of the transistor is 70 and the collector resistance is 2 k$\Omega$, then the base current is

• A. 20 $\mu$A
• B. 30 $\mu$A
• C. 10 $\mu$A
• D. 40 $\mu$A

Hint:

Transistor formulas: $I_E = I_B + I_C$ and $I_C = \beta I_B$.

Answer 1

The Correct Option is A

Step 1: Calculate Collector Current ($I_C$):
Voltage across collector resistance $V_{RC} = 2.8$ V. Collector resistance $R_C = 2 \, k\Omega = 2000 \, \Omega$. \[ I_C = \frac{V_{RC}}{R_C} = \frac{2.8}{2000} \, \text{A} \] \[ I_C = 1.4 \times 10^{-3} \, \text{A} = 1.4 \, \text{mA} \]
Step 2: Calculate Base Current ($I_B$):
Current amplification factor (Current Gain) $\beta = 70$. Relation: $\beta = \frac{I_C}{I_B}$. \[ I_B = \frac{I_C}{\beta} \] \[ I_B = \frac{1.4 \times 10^{-3}}{70} \] \[ I_B = \frac{1.4}{70} \text{ mA} = 0.02 \, \text{mA} \] Convert to micro-amperes ($\mu$A): \[ I_B = 0.02 \times 1000 \, \mu\text{A} = 20 \, \mu\text{A} \]
Step 3: Final Answer:
The base current is 20 $\mu$A.