Question

In a common emitter (CE) amplifier having a voltage gain G, the transistor used has transconductance 0.03 mho and current gain 25. If the above transistor is replaced with another one with transconductance 0.02 mho and current gain 20, the voltage gain will be

• A. 2/3 G-
• B. 1.5 G
• C. 1/3 G
• D. 5/4 G

Answer 1

The Correct Option is A

To determine the new voltage gain when the transistor in a common emitter (CE) amplifier is replaced, we'll analyze the relationship between voltage gain, transconductance, and current gain.

The voltage gain G of a CE amplifier can be expressed as:

G = \beta \cdot g_m \cdot R_C

where:

• \beta is the current gain.
• g_m is the transconductance.
• R_C is the collector resistance.

Initially, the transistor has:

• Transconductance, g_m = 0.03 \text{ mho}
• Current gain, \beta = 25

Therefore, the initial voltage gain G is:

G = 25 \cdot 0.03 \cdot R_C = 0.75 \cdot R_C

Now, the transistor is replaced with another having:

• Transconductance, g_m = 0.02 \text{ mho}
• Current gain, \beta = 20

The new voltage gain G_{new} is:

G_{new} = 20 \cdot 0.02 \cdot R_C = 0.4 \cdot R_C

The ratio of the new voltage gain to the original voltage gain is:

\frac{G_{new}}{G} = \frac{0.4 \cdot R_C}{0.75 \cdot R_C} = \frac{0.4}{0.75} = \frac{2}{3}

Therefore, the new voltage gain is:

G_{new} = \frac{2}{3} G

Thus, the correct option is \frac{2}{3} G, which corresponds to the option 2/3 G-.

As a tip, remember the relationship between amplifier parameters, since changes in transconductance and current gain directly affect the voltage gain in amplifiers.