BJT Biasing and Stability Questions and Answers

Answer: C

Both are true and the second is the reason: with the load impedance no longer dominant, the gain rises to the device's intrinsic limit of transconductance times output resistance, a few thousand rather than a few hundred. Discrete designs cannot easily afford the extra devices or the matching. What is cheap in one technology and expensive in another is what makes the two design styles diverge as much as they do. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: D

All three are correct, and together they are the complete account of one component: one benefit and two costs, one of which a capacitor can remove and one of which nothing can. That the swing penalty is the irreducible cost is what makes a negative supply attractive when headroom is tight. A component with three distinct effects has to be judged on all of them rather than on the one it was added for. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

The pairing is A-P, B-Q, C-R. Each measure attacks a different weakness, which is why a well-designed stage uses more than one of them: the emitter resistor for thermal feedback, the divider stiffness for beta immunity, and the negative supply to escape the headroom cost of the first. Matching a remedy to the specific weakness it addresses is what stops a design accumulating components that all do the same job. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: B

The first three are correctly paired and the fourth is not, so three match. The bypass capacitor costs nothing in thermal stability, since it is an open circuit at direct current; its price is a low-frequency limit and the physical size of a large capacitor. Being able to name the cost of each choice as readily as its benefit is what turns a list of techniques into a design method.

Answer: C

Over temperature the operating point's drift dominates, so Design B's stability factor of 1.4 against 6 is decisive, and the 1.2 volts of swing given up is a fair price. Design A would be the better choice where the temperature is controlled and the output swing is the binding constraint. A comparison becomes decidable only once the application names which parameter is limiting, and this table is readable either way without that. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: D

What is recovered is headroom: the emitter can sit near earth while the resistor still drops several volts, leaving the whole positive supply for the collector to swing into. Gain is restored by the bypass capacitor instead, and bandwidth and input impedance are affected by other things entirely. Naming precisely which resource a technique recovers is what prevents it being applied to the wrong problem. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

A data sheet quoting one hundred to three hundred means any circuit whose operating point follows beta will differ threefold across a production batch, so both extremes must be checked. The measurement current is stated and can be allowed for, beta does not degrade in normal service, and the figure is quoted at a stated temperature rather than the maximum. Designing against a range rather than a value is what makes a circuit manufacturable in quantity rather than merely functional once.

Answer: B

Beta independence means the expression for the operating point contains only voltages and resistances, so any device in the family gives the same result. Measuring and allowing for beta, or selecting devices, are both ways of coping with dependence rather than removing it, and a low stability factor is a related consequence. A design property that survives changing the device is worth far more than one that has been tuned around a particular sample.

Answer: C

Even a perfectly matched device changes its beta as it warms and as its operating current moves, so selection fixes only the initial spread and not the drift. The cost and effort of selection are real objections but secondary. Frequency response is unaffected and selection is easy enough to do. A remedy that addresses only one of several causes leaves the others in place, which is why the circuit rather than the component is the right place to solve this.

Answer: D

The usual criterion is a tenth of the reflected impedance:

\[ \begin{aligned} R_{th} &\le \frac{150 \ \mathrm{k}}{10} \\ &= 15 \ \mathrm{k\Omega} \end{aligned} \]

giving about 15 kilohms. Matching the two would leave the base voltage moving noticeably with beta, and a larger value would be worse still. A factor of ten is the conventional margin for treating one impedance as negligible beside another, and it recurs throughout circuit design.