D-MOSFET input behavior vs. frequency: How does the input impedance of a depletion-type MOSFET change as signal frequency varies, considering the gate's capacitive effects?

Introduction / Context:Although MOSFET gates are insulated (extremely high DC resistance), the input is not purely resistive. Intrinsic gate–source and gate–drain capacitances make the input impedance frequency-dependent in AC applications.

Given Data / Assumptions:

• Depletion-type MOSFET (D-MOSFET) with intact gate oxide.
• Gate leakage is negligible; capacitances dominate AC input impedance.
• Small-signal conditions.

Concept / Approach:Capacitive reactance is Xc = 1 / (2 * π * f * C). As frequency f increases, Xc falls, lowering input impedance; as frequency decreases, Xc rises, increasing input impedance. Thus, input impedance is higher at lower frequencies and lower at higher frequencies.

Step-by-Step Solution:Model gate path as capacitors to source/drain.Compute reactance trend: increase f → decrease Xc → lower impedance.Conclude: decreasing frequency increases input impedance.

Verification / Alternative check:Network analyzer or impedance meter measurements show input magnitude falling with frequency due to capacitive paths, matching Xc behavior.

Why Other Options Are Wrong:Statements claiming constancy or increase with frequency contradict Xc behavior.

“Independent of frequency” ignores the well-documented Cgs and Cgd effects.

Common Pitfalls:Assuming near-infinite impedance at RF; at high f the capacitive path can significantly load preceding stages, especially with Miller multiplication in voltage-gain stages.

Final Answer:As frequency decreases, input impedance increases.