The voltage gain of a D-MOSFET amplifier is dependent on

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all of the above

Explanation: The voltage gain of a D-MOSFET amplifier is determined by the transconductance ($g_m$) of the device and the effective load resistance at the drain terminal. Since the total load is the parallel combination of the drain resistor ($R_D$) and the external load resistance ($R_L$), the voltage gain is a function of all three parameters.

Working Principle: In a common-source D-MOSFET amplifier, the output voltage is generated by the product of the drain current variation and the effective load resistance. The transconductance $g_m$ represents the sensitivity of the output current to the input gate-source voltage. Thus, the voltage gain $A_v$ is defined by the product of $g_m$ and the effective parallel resistance $R_D || R_L$.

Key Formulas: $A_v = -g_m (R_D \parallel R_L)$ — Voltage gain of common-source D-MOSFET; $g_m = g_{m0} \left( 1 - \frac{V_{GS}}{V_P} \right)$ — Transconductance dependence on gate bias

Key Points: The gain is directly proportional to $g_m$, which varies with the quiescent bias point.; $R_D$ acts as the internal drain load, while $R_L$ is the external load.; Parallel combination of $R_D$ and $R_L$ reduces the total effective load, thereby limiting gain.; A negative sign indicates a 180-degree phase shift between input and output.

Advantages: High input impedance; Linearity in transconductance

Disadvantages: Gain is sensitive to bias point stability; Gain is affected by load variations

Applications: RF Amplifiers; Voltage-controlled resistors

Additional Info: The transconductance $g_m$ is defined as the change in drain current for a change in gate-source voltage at a constant drain-source voltage.. If $R_L$ is absent, the gain is simply $-g_m R_D$.

Answer: D is correct — The voltage gain is defined by $A_v = -g_m(R_D || R_L)$, making it explicitly dependent on $g_m$, $R_D$, and $R_L$.

Exam Tip: In any FET amplifier, remember that the total output impedance is always the parallel combination of all resistive elements connected to the drain terminal.

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