BJT Biasing and Stability Questions and Answers

Q: What is the chief defect of fixed bias with a single base resistor?

The base current is set by the resistor and the supply, so the collector current is that current times beta, and beta varies three to one between devices and rises with temperature. It needs only one supply, gives full gain, and has a perfectly usable input impedance. An arrangement whose output depends linearly on the least reliable parameter in the device is the definition of an unstable bias, which is why fixed bias survives only in switching circuits where the operating point does not matter.

Q: What is the stability factor of a fixed bias arrangement?

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.

Q: Why is emitter bias with a single supply only partly effective?

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.

Q: Which bias arrangement is the most widely used in discrete amplifiers?

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.

Q: In a voltage divider bias, what fixes the emitter current?

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.

Q: A divider gives a base voltage of \( 3 \ \mathrm{V} \) with \( R_{E} = 1.2 \ \mathrm{k\Omega} \). The emitter current is about

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.

Q: That stage has \( R_{C} = 3.3 \ \mathrm{k\Omega} \) and \( V_{CC} = 12 \ \mathrm{V} \). Its collector-emitter voltage is about

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.

Q: What makes a bias divider stiff?

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.

Q: What is the cost of making the bias divider very stiff?

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.

11.

What is the chief defect of fixed bias with a single base resistor?

A.Its input impedance is too low to be driven
B.Its operating point depends directly on beta
C.It requires two separate supply voltages
D.It cannot provide any voltage gain at all

Answer: B

The base current is set by the resistor and the supply, so the collector current is that current times beta, and beta varies three to one between devices and rises with temperature. It needs only one supply, gives full gain, and has a perfectly usable input impedance. An arrangement whose output depends linearly on the least reliable parameter in the device is the definition of an unstable bias, which is why fixed bias survives only in switching circuits where the operating point does not matter.

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12.

What is the stability factor of a fixed bias arrangement?

A.Beta divided by two
B.Zero
C.One plus beta
D.One

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.

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13.

How does collector-to-base feedback bias improve stability?

A.Rising collector current raises the emitter voltage
B.The base voltage is fixed by a divider ratio
C.The base current is set by a constant current source
D.Rising collector current reduces the base drive

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.

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14.

Why is emitter bias with a single supply only partly effective?

A.The base voltage still depends on the base current
B.The emitter resistor reduces the available gain
C.The emitter resistor must be bypassed for signals
D.The collector resistor no longer sets the gain

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.

Discuss in Forum

15.

Which bias arrangement is the most widely used in discrete amplifiers?

A.Emitter bias from a separate negative supply
B.The voltage divider with an emitter resistor
C.Fixed bias with a single base resistor
D.Collector feedback with one resistor

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.

Discuss in Forum

16.

In a voltage divider bias, what fixes the emitter current?

A.The base current multiplied by one plus beta
B.The divider's Thevenin resistance and the supply together
C.The base voltage less the base-emitter drop, over the emitter resistor
D.The supply voltage divided by the collector resistor

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.

Discuss in Forum

17.

A divider gives a base voltage of \( 3 \ \mathrm{V} \) with \( R_{E} = 1.2 \ \mathrm{k\Omega} \). The emitter current is about

A.2.5 mA
B.3 mA
C.1.2 mA
D.1.9 mA

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.

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18.

That stage has \( R_{C} = 3.3 \ \mathrm{k\Omega} \) and \( V_{CC} = 12 \ \mathrm{V} \). Its collector-emitter voltage is about

A.3.4 V
B.5.6 V
C.8.6 V
D.1.5 V

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.

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19.

What makes a bias divider stiff?

A.Its current is smaller than the base current
B.Its resistance is small compared with the reflected emitter resistance
C.Its two resistors are equal in value
D.Its Thevenin voltage equals half the supply

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.

Discuss in Forum

20.

What is the cost of making the bias divider very stiff?

A.A higher stability factor for the arrangement
B.A larger emitter bypass capacitor being needed
C.Increased supply current and a lower input impedance
D.A reduced voltage gain in the stage

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.

Discuss in Forum

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