BJT Biasing and Stability Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

Answer: B

In practice the direct bias is arranged exactly as for a common-emitter stage and the base is then bypassed to ground with a capacitor, so it is common for signals but biased normally. Tying the base to the supply would forward bias the collector junction, and a grounded emitter leaves nothing to drive. Separating what a node does for direct current from what it does for signals is the key to reading any of these circuits. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.