BJT Biasing and Stability Questions and Answers

Answer: D

Fifty-five degrees is between five and six doublings:

\[ 1 \times 2^{5.5} \approx 45 \ \mathrm{\mu A} \]

so about 50 microamps. Amplified by beta in a common-emitter stage that becomes milliamps, which is why leakage matters so much more in hot power stages than on a cool bench. Exponential dependences must be evaluated rather than estimated linearly.

Answer: A

Multiplying the leakage change by the factor:

\[ \Delta I_{C} = 10 \times 2 = 20 \ \mathrm{\mu A} \]

so 20 microamps. Fixed bias with a beta of 150 would give a factor of 151 and a shift of 300 microamps from the same cause. A sensitivity figure exists precisely so that this multiplication can be done, and comparing arrangements means comparing those factors.

Answer: B

No drop across the collector resistor means no collector current, so the device is cut off - an open base circuit, a missing divider resistor, or no bias at all. Saturation would put the collector near the emitter, and mid-line would put it near half supply. Reading the collector voltage alone distinguishes all three states, which makes it the first measurement to take on any suspect stage.

Answer: C

The device is saturated: too much base current for the load to support, typically from a divider resistor that has changed value or a leaky coupling capacitor injecting bias. Cut-off would show the collector at the supply. An open load resistor would also read low but with no current, which a measurement across the resistor settles. Two readings rather than one distinguish every case here.

Answer: D

Drift with temperature and only with temperature points at a bias arrangement that does not oppose the three thermal effects, so the current climbs until the device saturates. A capacitor fault would not be temperature selective in this way, and a wrong resistor value would misbias the stage from cold. That a symptom appears only under one condition is what identifies which mechanism is responsible.

Answer: A

Unit-to-unit spread with no other pattern points at a bias arrangement whose operating point follows beta, which varies three to one across a batch. Supply regulation and resistor tolerance would produce far smaller and more even variation, and coupling capacitors do not affect the quiescent point at all. Whether a symptom varies between units, with temperature, or with signal level, points at three quite different classes of cause. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

A correct operating point with a low gain means the direct conditions are right and only the signal path is wrong, so the emitter resistor is no longer bypassed and is degenerating the gain. A soft divider or a wrong emitter resistor would shift the quiescent point too, and a low supply would reduce the swing rather than the gain. Separating faults that move the bias from faults that only affect signals halves the search immediately. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: C

The emitter voltage divided by the emitter resistor gives the current at once, and the current is what the design actually specifies. Meter loading is a genuine but secondary consideration, stiffness needs two measurements to assess, and beta needs the base current as well. Measuring at the point where a single reading converts straight into the quantity of interest is worth a moment's thought before probing anything. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: D

Both are true and the second is exactly why the first holds: the expression for emitter current contains only a voltage and a resistance, with no device parameter in it. Beta enters only through the small base current loading the divider, which a stiff divider makes negligible. Arranging a design so that the least reliable parameter cancels out of the governing equation is the general technique this arrangement illustrates. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: A

Both hold and the second is the reason: with nothing in the circuit responding to a change in collector current, every drift in leakage or beta appears at the output multiplied rather than opposed. It remains perfectly usable in switching circuits, where the operating point is either extreme and its exact value does not matter. A specification's importance always depends on what the circuit is for. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.