BJT Biasing and Stability Questions and Answers

Q: What is thermal runaway?

Runaway is a closed positive feedback loop: more current makes more heat, more heat makes more current, and unless something in the circuit opposes it the device destroys itself. A brief overload is an overstress rather than a runaway, current falling would be self-limiting, and gain drift is a symptom of the same effects at a harmless level. Identifying the loop, rather than any single effect, is what makes the phenomenon predictable. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Q: What breaks the thermal runaway loop in an emitter-biased stage?

As collector current rises, the emitter resistor's drop rises and eats into the base-emitter forward bias, reducing the current - negative feedback acting against the thermal positive feedback. The collector resistor limits the extreme but does not regulate, leakage does not saturate, and the divider's resistance barely moves. A positive feedback loop is beaten by putting a stronger negative one around it, which is the general remedy. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which quantity must be kept below unity to guarantee thermal stability?

Stability requires the thermal loop gain - the product of the electrical and thermal sensitivities - to be less than one, exactly as with any feedback loop. The stability factor is a related but different measure and cannot be below unity anyway. The resistance ratio and the voltage fraction are design choices that influence the loop gain without being it. Reducing a thermal problem to a loop gain criterion is what lets it be checked rather than guessed at.

Q: Why is thermal coupling important when a diode is used for compensation?

Compensation depends on the two drops tracking, and they track only if the devices share a temperature, which means mounting the diode on the same heatsink or in the same package. Matched currents help the initial accuracy rather than the tracking, and neither breakdown voltage nor area is essential. Any scheme that relies on two components matching also relies on them experiencing the same conditions, and temperature is the condition most easily overlooked.

Q: What does a thermistor in the bias divider do?

A negative-coefficient thermistor in the lower divider arm falls in resistance as things warm, pulling the base voltage down and reducing the current. It does not alter the emitter resistance, beta rises rather than falls with temperature, and it does not limit current directly. Using a component whose imperfection is the wanted behaviour is a neat inversion, and it is why thermistors appear in bias networks at all. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: How is an emitter follower usually biased?

The standard arrangement is a base divider setting the base voltage and an emitter resistor to ground carrying the current, exactly as in a common-emitter stage but with the output taken from the emitter. Fixed bias would be as unstable here as anywhere. A grounded emitter is impossible in a follower, since the emitter is the output. Bias arrangements transfer between configurations, and only the terminal used for output changes. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

21.

Which three temperature effects all raise collector current?

A.Beta, the Early voltage and the saturation voltage of the device
B.Leakage, the thermal voltage and the base spreading resistance
C.The base-emitter drop, the transition frequency and beta
D.Beta, the base-emitter drop and leakage

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.

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22.

What is thermal runaway?

A.Current and temperature reinforcing one another to destruction
B.The junction temperature exceeding its rated maximum during a brief overload
C.The collector current falling away as the device warms beyond its rating
D.The gain of a stage drifting upward as the equipment reaches its working temperature

Answer: A

Runaway is a closed positive feedback loop: more current makes more heat, more heat makes more current, and unless something in the circuit opposes it the device destroys itself. A brief overload is an overstress rather than a runaway, current falling would be self-limiting, and gain drift is a symptom of the same effects at a harmless level. Identifying the loop, rather than any single effect, is what makes the phenomenon predictable. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

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23.

What breaks the thermal runaway loop in an emitter-biased stage?

A.The divider's Thevenin resistance rises as the components warm up
B.The rising emitter voltage reduces the forward bias
C.The collector resistor limits the current available from the supply rail
D.The base-emitter junction's leakage current saturates at high temperatures

Answer: B

As collector current rises, the emitter resistor's drop rises and eats into the base-emitter forward bias, reducing the current - negative feedback acting against the thermal positive feedback. The collector resistor limits the extreme but does not regulate, leakage does not saturate, and the divider's resistance barely moves. A positive feedback loop is beaten by putting a stronger negative one around it, which is the general remedy. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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24.

Which quantity must be kept below unity to guarantee thermal stability?

A.The ratio of emitter resistance to collector resistance in the stage
B.The fraction of the supply voltage appearing across the device
C.The loop gain of the thermal feedback
D.The stability factor of the bias arrangement as designed

Answer: C

Stability requires the thermal loop gain - the product of the electrical and thermal sensitivities - to be less than one, exactly as with any feedback loop. The stability factor is a related but different measure and cannot be below unity anyway. The resistance ratio and the voltage fraction are design choices that influence the loop gain without being it. Reducing a thermal problem to a loop gain criterion is what lets it be checked rather than guessed at.

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25.

Why is thermal stability a greater concern in power stages?

A.Their current gain is inherently lower than small-signal devices
B.Their collector-emitter breakdown voltages are usually much smaller
C.Their operating points sit closer to the cut-off end of the load line
D.They dissipate enough to heat their own junctions

Answer: D

A small-signal stage dissipating milliwatts barely warms itself, whereas a power stage's own dissipation raises its junction temperature substantially and closes the loop. Gain, breakdown voltage and operating point are all different in power devices but none of them is the reason. A feedback loop only matters when it is actually closed, and self-heating is what closes this one.

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26.

How does a diode in the bias network compensate for temperature?

A.Its drop falls with temperature like the transistor's
B.Its leakage current rises to cancel the transistor's own leakage
C.Its forward resistance falls and stiffens the divider as things warm
D.Its capacitance changes to compensate the shift in operating point

Answer: A

A diode's forward drop falls at the same two millivolts per degree as the transistor's, so placing it in the bias path makes the base voltage track the required drop. Leakage cancellation, resistance changes and capacitance play no part. Compensating a temperature-dependent parameter with an identical device at the same temperature is a technique that recurs throughout analogue design, and it works only because the two really do match. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

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27.

Why is thermal coupling important when a diode is used for compensation?

A.The diode must have the same junction area as the transistor being biased
B.Both devices must be at the same temperature
C.The diode must be biased at the same current as the transistor's base
D.The diode's breakdown voltage must exceed the supply voltage in use

Answer: B

Compensation depends on the two drops tracking, and they track only if the devices share a temperature, which means mounting the diode on the same heatsink or in the same package. Matched currents help the initial accuracy rather than the tracking, and neither breakdown voltage nor area is essential. Any scheme that relies on two components matching also relies on them experiencing the same conditions, and temperature is the condition most easily overlooked.

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28.

What does a thermistor in the bias divider do?

A.Compensates the fall in the transistor's current gain at low temperatures
B.Limits the collector current directly once a threshold is exceeded
C.Reduces the base voltage as temperature rises
D.Raises the emitter resistance to increase the local feedback available

Answer: C

A negative-coefficient thermistor in the lower divider arm falls in resistance as things warm, pulling the base voltage down and reducing the current. It does not alter the emitter resistance, beta rises rather than falls with temperature, and it does not limit current directly. Using a component whose imperfection is the wanted behaviour is a neat inversion, and it is why thermistors appear in bias networks at all. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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29.

How is an emitter follower usually biased?

A.A single resistor from the supply to the base with no emitter component
B.A resistor from the collector to the base with the emitter grounded directly
C.Two supplies of opposite polarity feeding base and emitter separately
D.A divider on the base and a resistor to ground at the emitter

Answer: D

The standard arrangement is a base divider setting the base voltage and an emitter resistor to ground carrying the current, exactly as in a common-emitter stage but with the output taken from the emitter. Fixed bias would be as unstable here as anywhere. A grounded emitter is impossible in a follower, since the emitter is the output. Bias arrangements transfer between configurations, and only the terminal used for output changes. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

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30.

Why does an emitter follower's bias tolerate a softer divider?

A.Its input impedance is very high
B.Its collector current is much smaller than a common-emitter stage's
C.Its operating point has no effect on the voltage gain achieved
D.Its emitter resistor is always larger than the collector resistor

Answer: A

Because the emitter load is reflected to the base multiplied by beta, the impedance the divider must be stiff against is very high, so a softer divider still qualifies. The collector current is whatever the design chooses, the gain really is insensitive but that is not the reason, and the resistance comparison is incidental. Stiffness is relative to what is being driven, so a higher impedance load relaxes the requirement. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

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