Question

In a common emitter transistor amplifier circuit, the input resistance is 1.8 k$\Omega$ and output is obtained across a load resistance of 9 k$\Omega$. The alternating current gain is 70. Corresponding to an a.c. input voltage of 6 mV, the output voltage will be ______.

• A. 0.7 V
• B. 1.4 V
• C. 2.1 V
• D. 4.2 V

Hint:

Power Gain is the big brother here! $A_P = \beta \times A_v = \beta^2 \times (R_{\text{out}}/R_{\text{in}})$. Remember that Voltage Gain directly depends on both the current gain AND the ratio of the resistors!

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The voltage gain ($A_v$) of an amplifier is the ratio of output voltage to input voltage, also calculated as current gain ($\beta$) times the resistance gain.

Step 2: Formula Application:
$A_v = \beta \times \frac{R_L}{R_i}$.
$V_{out} = A_v \times V_{in}$.

Step 3: Explanation:
$A_v = 70 \times \frac{9000}{1800} = 70 \times 5 = 350$. $V_{out} = 350 \times 6$ mV $= 2100$ mV. $2100$ mV $= 2.1$ V.

Step 4: Final Answer:
The output voltage will be 2.1 V.