Introduction / Context:
Designers often estimate FET small-signal parameters by order of magnitude. Typical room-temperature values help with quick checks before detailed datasheet review or SPICE extraction. The original options mixed units ambiguously (e.g., 'mW' for a conductance). Here, units are corrected for clarity.
Given Data / Assumptions:
- gm (transconductance) is measured in siemens (S).
- rd (drain resistance) is measured in ohms (Ω).
- gg (gate conductance) is extremely small (near leakage), in siemens (S).
- Typical magnitudes: gm for JFET in the mS to 10 mS range; rd for JFET on the order of tens of kΩ; rd for MOSFET (small-signal, triode-region excluded) can be hundreds of kΩ to mega-ohms; gg for JFET around 10^-8 S (or lower).
Concept / Approach:
Map each parameter to a plausible magnitude used in hand calculations and conceptual exam problems.
Step-by-Step Solution:
A (gm for JFET) → choose 0.01 S (10 mS) as a representative exam value → 2.B (rd for JFET) → tens of kΩ typical → 25 kΩ → 1.C (rd for MOSFET) → often higher than JFET → pick 0.5 MΩ → 3.D (gg for JFET) → tiny leakage conductance → ~10^-8 S → 4.
Verification / Alternative check:
Sanity check: gm * rd for JFET gives intrinsic gain order gmrd ≈ (10 mS)(25 kΩ) ≈ 250, a reasonable small-signal estimate. MOSFET's higher rd aligns with channel-length modulation effects.
Why Other Options Are Wrong:
- A-1, B-2, C-3, D-4: Assigns JFET gm to 25 kΩ (wrong unit and concept).
- A-4, B-2, C-3, D-1: Gives gm an unphysically tiny conductance and swaps resistances.
- A-2, B-3, C-1, D-4 (distractor): Misplaces which device typically has larger rd.
Common Pitfalls:
- Confusing resistance (Ω) with conductance (S); they are reciprocals.
- Using power units (W) by mistake for small-signal parameters.
Final Answer:
A-2, B-1, C-3, D-4
Discussion & Comments