FET Small-Signal Amplification: Based on the V–I characteristics of a Field Effect Transistor (FET), identify the region in which the device should be biased for small-signal amplification.
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AAB
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BBC
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CCD
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DBD
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EAD
Answer
Correct Answer: CD
Explanation
Introduction / Context:For FET amplifiers, proper biasing ensures linear operation with good gain. The device characteristics (drain current vs. drain–source voltage) show several distinct regions: cutoff, ohmic (linear), saturation (active), and breakdown. Amplification is only feasible in one of them.
Given Data / Assumptions:
- Typical n-channel JFET or MOSFET characteristic curve is implied.
- Regions: AB (cutoff), BC (ohmic), CD (saturation/active), BD (includes breakdown).
Concept / Approach:
The small-signal amplification region corresponds to the saturation (also called active) region, where drain current is controlled primarily by gate–source voltage and remains nearly constant over a range of drain–source voltages.
Step-by-Step Solution:
Cutoff region (AB): No conduction; cannot amplify.Ohmic region (BC): Acts as a variable resistor, not suitable for linear voltage amplification.Saturation region (CD): ID depends on VGS; current is nearly constant w.r.t. VDS, allowing linear amplification.Breakdown (BD): Device fails; not a usable region.Verification / Alternative check:
Practical FET amplifier bias points are always chosen in the saturation region, validating CD as the correct choice.
Why Other Options Are Wrong:
- AB: Cutoff, no current.
- BC: Resistive behavior, not linear gain.
- BD: Beyond safe operation.
Common Pitfalls:
- Confusing MOSFET digital switching (cutoff & saturation in digital sense) with analog small-signal operation (saturation = active region).
Final Answer:
CD