Question

In a CE transistor, a change of $8.0 \text{ mA}$ in the emitter current produces a change of $7.8 \text{ mA}$ in the collector current. What change in the base current is necessary to produce the same change in the collector current?

• A. $200 \; \mu\text{A}$
• B. $50 \; \mu\text{A}$
• C. $100 \; \mu\text{A}$
• D. $150 \; \mu\text{A}$

Hint:

Always keep this rule in mind: The base current is a tiny bottleneck gate current, representing the difference between the large outer currents ($I_B = I_E - I_C$). Simply subtracting the coefficients ($8.0 - 7.8 = 0.2$) instantly gives you the fraction needed to match the options.

Answer 1

The Correct Option is A

Step 1: Read the data.
In a common-emitter transistor, a change in emitter current $\Delta I_E = 8.0\ \text{mA}$ produces a change in collector current $\Delta I_C = 7.8\ \text{mA}$. We need the matching change in base current $\Delta I_B$.
Step 2: Fundamental current relation.
For any transistor the three currents satisfy $I_E = I_B + I_C$.
Step 3: Apply it to the changes.
Since the relation holds at every instant, it also holds for the changes: $\Delta I_E = \Delta I_B + \Delta I_C$.
Step 4: Rearrange for base current.
$\Delta I_B = \Delta I_E - \Delta I_C$.
Step 5: Plug in the numbers.
$\Delta I_B = 8.0 - 7.8 = 0.2\ \text{mA}$.
Step 6: Convert to microamperes.
Since $1\ \text{mA} = 1000\ \mu\text{A}$, $\Delta I_B = 0.2\times 1000 = 200\ \mu\text{A}$.
\[ \boxed{\Delta I_B = 200\ \mu\text{A (option 1)}} \]