For a varactor (varicap) diode used as a voltage-controlled capacitor, how should the applied reverse bias relate to the device’s avalanche breakdown voltage during normal operation?
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AIt must remain less than the avalanche breakdown voltage
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BIt should exceed the avalanche breakdown voltage for maximum capacitance swing
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CIt may be above or below breakdown without affecting reliability
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DIt must be much greater than the breakdown voltage to minimize loss
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EIt should alternate between forward conduction and breakdown each cycle
Answer
Correct Answer: It must remain less than the avalanche breakdown voltage
Explanation
Introduction / Context:Varactor diodes provide a voltage-dependent junction capacitance used in tunable filters, VCOs, and phase shifters. Safe biasing is essential to achieve wide tuning ranges without damaging the device or injecting excessive noise.
Given Data / Assumptions:
- Operation in reverse bias to exploit depletion-region capacitance.
- Breakdown voltage V_BR is specified by the manufacturer.
- Goal: maintain reliable, low-noise capacitance control.
Concept / Approach:
The depletion width increases with reverse bias, reducing junction capacitance approximately as C_j ∝ (V_R + V_bi)^−n (with n depending on doping profile). However, exceeding V_BR causes avalanche breakdown with large currents, potential thermal damage, and severe noise, rendering the varactor unsuitable for linear tuning.
Step-by-Step Solution:
1) Bias the diode in reverse (no forward conduction) to access the variable C_j(V_R).2) Choose a tuning range with V_R below V_BR, including margin for ripple and RF swing.3) Add series resistance or limiters if necessary to protect against transients.4) Verify linearity and Q across the intended bias range.Verification / Alternative check:
Datasheets show recommended reverse-bias ranges safely below V_BR, with capacitance–voltage curves specified only in that region. Circuits violating V_BR suffer breakdown and device degradation.
Why Other Options Are Wrong:
- Exceed breakdown: Destroys the device and increases noise.
- Above/below without effect: Reliability is strongly affected by breakdown.
- Much greater than breakdown: Unsafe and unnecessary.
- Alternating with forward conduction: Not how varactors are used; they are not switch-mode capacitors.
Common Pitfalls:
Ignoring RF swing on the junction, which adds to DC bias; failing to account for temperature variation of V_BR; omitting ESD protection.
Final Answer:
It must remain less than the avalanche breakdown voltage