Question

In a common emitter amplifier, which one of the following statement is TRUE?

• A. In a mid-band region, the coupling capacitors and junction capacitances are short circuited.
• B. In a mid-band region, the coupling capacitors are open circuited and junction capacitances are short circuited.
• C. In a mid-band region, the coupling capacitors are short circuited and junction capacitances are open circuited.
• D. In a mid-band region, the coupling capacitors and junction capacitances are open circuited.

Hint:

Mid-band means the "sweet spot" of the amplifier:
Large external capacitors have already become short circuits.
Small parasitic internal capacitors have not yet started acting as shorts (they remain open circuits).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This is a qualitative question about the behavior of capacitors in a BJT common emitter amplifier within the mid-band frequency range.

Step 2: Key Formula or Approach:
The impedance of a capacitor is inversely proportional to frequency:
\[ X_C = \frac{1}{2\pi f C} \]
We analyze how different types of capacitors (large external coupling/bypass vs. small internal junction capacitances) behave at mid-band frequencies.

Step 3: Detailed Explanation:

• Coupling and Bypass Capacitors ($C_C$, $C_E$): These are physically large capacitors, typically in the microfarad ($\mu\text{F}$) range. At mid-band frequencies, their reactance $X_C = \frac{1}{\omega C}$ is extremely small (approaching zero). Thus, they are treated as short circuits.

• Junction Capacitances ($C_{be}$, $C_{bc}$): These are internal parasitic capacitances of the transistor, typically in the picofrad ($\text{pF}$) range. At mid-band frequencies, their reactance $X_C$ is extremely large (approaching infinity). Thus, they are treated as open circuits and only become relevant at high frequencies.

Step 4: Final Answer:
In the mid-band region, the coupling capacitors are short-circuited and junction capacitances are open-circuited, which corresponds to Option (C).