BJT Biasing and Stability Questions and Answers

Answer: C

Fixed bias achieves one plus beta, the worst possible value, because nothing in the circuit responds to a change in collector current. A leakage change of one microamp moves the collector current by a hundred microamps with a beta of a hundred. Unity is the ideal, achieved only with strong feedback. That the worst case is the arrangement with no feedback at all is not a coincidence: stability in bias, as elsewhere, comes from feedback. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: D

Taking the base resistor from the collector rather than the supply means a rise in collector current lowers the collector voltage and so reduces the base current, opposing the original change. Raising the emitter voltage is the emitter bias mechanism, and a fixed divider is the voltage divider arrangement. It is negative feedback in the bias loop, and the improvement over fixed bias comes entirely from that one changed connection. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: A

With the base fed through a single large resistor, the base voltage moves as the base current changes, so the emitter feedback is diluted. The gain reduction and the need for a bypass are consequences of the emitter resistor rather than limits on its stabilising action. The remedy is to fix the base voltage with a divider stiff enough that base current changes hardly move it, which is precisely what the fourth arrangement does. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: B

The voltage divider with an emitter resistor is the standard, because a stiff divider fixes the base voltage and the emitter resistor then fixes the emitter current almost independently of beta. Fixed bias is too unstable, collector feedback only partly effective, and a second supply is an expense. An arrangement that makes the operating point depend on resistor ratios rather than on device parameters is what makes production quantities possible.

Answer: C

With the base held at a fixed voltage, the emitter sits one junction drop below it and the emitter resistor then sets the current - a calculation containing no device parameter at all. The third option is the beta-dependent route the arrangement exists to avoid. That the answer can be written without beta appearing anywhere is the whole point, and it is what the stability factor of such a stage reflects. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Subtracting the junction drop before dividing:

\[ I_{E} = \frac{3 - 0.7}{1200} \approx 1.9 \ \mathrm{mA} \]

giving about 1.9 mA, and the collector current is within one per cent of it. 2.5 mA forgets the drop. Because no device parameter enters, this current is as reliable as the resistor and the divider, which is exactly the property the arrangement was chosen for.

Answer: A

Both resistors carry essentially the same current:

\[ \begin{aligned} V_{CE} &= 12 - 1.9\mathrm{m}(3300+1200) \\ &= 12 - 8.6 = 3.4 \ \mathrm{V} \end{aligned} \]

so about 3.4 volts - workable but low for a large swing, and a smaller collector resistor would centre the point better. Computing the quiescent voltage and then asking whether it leaves room to swing is the second half of any bias calculation.

Answer: B

A stiff divider is one whose Thevenin resistance is well below the impedance looking into the base, so base current changes barely move the base voltage - a tenth of it is the usual rule. Equal resistors and a half-supply tap are arbitrary choices, and a divider current below the base current would be hopelessly soft. Stiffness is always a comparison between a source impedance and what it drives, never an absolute value. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: C

Lower divider resistances waste more supply current and load the signal source, since the divider appears in parallel with the input. The gain and the stability factor both improve or stay the same, and the bypass capacitor depends on the emitter resistance rather than the divider. Stiffness is bought with current and input impedance, which is why the tenth-of-beta rule is a compromise rather than an ideal. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Beta rises, the base-emitter drop needed falls by about two millivolts per degree, and leakage roughly doubles every ten degrees - all three push the collector current up. That they conspire rather than cancel is the fact the whole topic rests on. The Early voltage, saturation voltage and transition frequency do change with temperature but affect other things. Where every mechanism pushes one way, positive feedback with heat is a real possibility rather than a curiosity.