Diode Circuits and Applications Questions and Answers

Answer: C

Below roughly five volts the Zener effect, a direct tunnelling process, dominates and has a negative temperature coefficient; above it avalanche multiplication takes over with a positive one. Thermal runaway is a failure rather than a mechanism, and punch-through belongs to transistors. The crossover is why diodes near 5.6 V have almost zero temperature coefficient, which is exactly why that value is chosen for references. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: D

At about that voltage the negative coefficient of the Zener mechanism and the positive one of avalanche cancel, giving a reference that barely drifts with temperature. Dynamic resistance is lowest around six or seven volts, power depends on the current chosen, and the series resistor follows from the input voltage. A parameter that passes through zero where two mechanisms cross over is worth exploiting, and reference diodes are the clearest example. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Dynamic resistance is the slope of the breakdown characteristic, and it decides how much the output moves when the load changes - a few ohms in a good device. The series resistor is external, the sub-breakdown resistance is enormous and irrelevant, and the power to current ratio is just the voltage. Regulation quality in a shunt regulator is set by this slope and the series resistor together, which is why both appear in the line and load regulation figures.

Answer: B

A Schottky's metal-semiconductor junction stores almost no minority charge, so it offers a forward drop around 0.3 V and negligible recovery time, which is why switching supplies use them. High reverse ratings are not their strength; they are usually modest. Voltage-dependent capacitance belongs to the varactor and negative resistance to the tunnel diode. Each special diode exists for one property, and naming that property is the whole of choosing between them.

Answer: C

A varactor is used as a voltage-controlled capacitor, because the depletion layer widens with reverse bias and the capacitance falls. That is what allows electronic tuning and the voltage-controlled oscillator at the heart of a phase-locked loop. The low drop belongs to a Schottky, light emission to a light emitting diode, and negative resistance to a tunnel diode. Every junction has this capacitance; the varactor is simply built to make it large and predictable.

Answer: D

The forward characteristic is so steep that a small voltage change produces a large current change, so the current must be set externally rather than by the applied voltage. The low reverse rating matters for protection, the temperature dependence affects brightness, and the colour-dependent drop matters when choosing the resistor's value - but none of them is why the resistor is needed. Any device with a steep characteristic must have its operating point set by something in series.

Answer: A

A photodiode is usually reverse biased, and the reverse current then rises in proportion to the incident light, which gives a linear and fast response. It can also be used unbiased in photovoltaic mode, which is what a solar cell does, but that is slower and less linear. Forward bias and breakdown are not used. The same device operated in two different modes trades speed and linearity against the need for a supply. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: B

A tunnel diode has a region where current falls as voltage rises, and that negative resistance can sustain oscillation or provide gain. Its heavy doping does give it unusual behaviour near zero bias, but the negative resistance is the defining feature. Low forward drop belongs to the Schottky and bias-dependent capacitance to the varactor. A device whose slope is negative over part of its range can do things no passive component can, which is why it is worth the awkwardness of using it.

Answer: C

A Schottky is the choice, because its fast recovery and low drop keep switching and conduction losses down at the tens or hundreds of kilohertz such supplies run at. A Zener regulates rather than rectifies, a varactor tunes, and a tunnel diode oscillates. Matching the device to the duty means asking what the circuit demands most - here speed and efficiency together - and then finding the diode built for exactly that. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: D

A varactor gives a capacitance controlled by a voltage, which is precisely what electronic tuning needs and is how every phase-locked loop's oscillator is steered. A Schottky rectifies quickly, a light emitting diode indicates, and a Zener regulates. The fact that the same physical effect - a bias-dependent depletion width - is a nuisance in a rectifier and the whole point of a varactor is a good illustration of how devices are specialised. Framing the question as which quantity the change acts upon makes the answer follow without calculation.