Diode Circuits and Applications Questions and Answers

Answer: A

The ideal model treats the diode as a perfect switch: no drop when conducting and no current when blocking, which is enough for deciding whether a diode conducts in a given circuit. The constant drop is the next model up, the piecewise-linear one. A current-dependent resistance belongs to the exponential law, and temperature-dependent leakage is a real effect outside all three models. Choosing the simplest model that answers the question is the whole skill in diode analysis, and the ideal model answers more questions than its crudeness suggests.

Answer: B

It adds a constant forward drop, typically 0.7 V for silicon and 0.3 V for germanium, and often a small series resistance as well. The temperature coefficient, the exponential law and reverse breakdown are all refinements beyond it. The model is the workhorse of hand analysis because it turns a non-linear device into a battery and a switch, which any linear method can then handle.

Answer: C

The thermal voltage is about 26 millivolts at room temperature, and it sets the steepness of the exponential characteristic - sixty millivolts per decade of current follows directly from it. The forward drop is a much larger quantity, breakdown is a reverse property, and thermal resistance is a packaging specification. A small voltage that governs an exponential is worth knowing numerically, because every logarithmic estimate in this topic depends on it.

Answer: D

Because the saturation current rises steeply with temperature, less forward voltage is needed to carry the same current - about two millivolts less per degree. Bulk resistance, depletion width and mobility all change with temperature too, but the saturation current dominates by a wide margin. That coefficient is used deliberately in temperature sensors and has to be compensated in reference circuits, so its sign and size are both worth remembering.

Answer: A

It is the small leakage that flows under reverse bias, nearly independent of the reverse voltage but strongly dependent on temperature, roughly doubling every ten degrees. The other three describe a forward characteristic feature, a rating and a breakdown condition. Its independence of voltage and dependence on temperature is what makes it the term that grows in the diode equation and the reason leakage matters in hot equipment. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

At breakdown the reverse current rises very steeply, and unless it is limited the diode is destroyed by the power dissipated. It does not begin conducting forwards, and the forward drop and junction capacitance are unaffected. The same mechanism is used deliberately in a Zener diode, where the current is limited externally, which illustrates that a failure mode and a design feature can be the same physics viewed differently. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

Under reverse bias the depletion capacitance dominates, and it falls as the reverse voltage widens the depletion layer - which is exactly the property a varactor exploits. Diffusion capacitance belongs to forward bias, where stored minority charge dominates. Package capacitance is small and fixed, and gate capacitance belongs to field effect devices. Which capacitance dominates depends on the bias, and that dependence is itself the basis of a whole class of tuning circuits.

Answer: D

The minority carrier charge stored during conduction must be swept out before the diode can block, and that reverse recovery time sets the speed. Depletion formation is fast, the package's thermal constant is far slower but irrelevant to switching, and establishing the forward drop is quick. Stored charge is why Schottky diodes, which store almost none, switch so much faster, and it is the property to look for in any fast switching application. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: A

With no load current flowing on the reverse half cycle, the diode stands off the full input peak. Twice the peak is the figure for a centre-tapped full-wave rectifier, where the non-conducting diode sees both halves of the winding. Half the peak and the peak less a drop arise from neither topology. Peak inverse voltage is the rating that most often decides a diode choice, and it differs between topologies that look superficially similar. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

The idle diode has one half-winding's peak on its cathode and the other's negative peak on its anode, so it stands off twice the peak of half the secondary. That is the topology's chief disadvantage against a bridge, which needs only the single peak. The remaining options understate it. Working out what the non-conducting device sees, rather than what the conducting one carries, is the habit this rating rewards. Checking the units of each quantity catches a wrong formula before the arithmetic begins.