Field Effect Transistors Questions and Answers

Q: Which family is less prone to thermal runaway in a power stage?

A MOSFET's on-resistance rises as it heats, so a hot device carries less of a shared current and the imbalance self-corrects - which is why MOSFETs parallel readily and bipolar devices need ballast resistors. The threshold does fall with temperature, working the other way, but the resistance effect dominates in the conducting state. A device whose imperfection acts as negative feedback is far easier to use in parallel.

Q: Why do MOSFETs parallel more readily than bipolar transistors?

The device taking more current heats, its resistance rises, and it takes less - negative feedback that equalises the sharing without any external component. Thresholds do not match especially well, a common gate drive is convenient but not the mechanism, and the safe operating area is a separate matter. Whether an imperfection acts with or against an imbalance decides whether devices can be paralleled at all. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Assertion (A): Assertion: A field effect transistor's input impedance is far higher than a bipolar transistor's. Reason (R): Reason: Its gate is either insulated or reverse biased and draws essentially no current. Select the correct answer from the options below.

Both are true and the second is the reason: with no gate current, the input impedance is set by whatever bias resistor the designer fits rather than by the device. That is why a field effect input stage can present megohms while a bipolar one presents kilohms. A specification the designer chooses directly is more valuable than one inherited from the device, and this is the clearest example in the subject. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Q: Consider the following statements about junction field effect transistors. 1. The channel conducts with zero gate voltage 2. The gate must never be forward biased 3. The transfer characteristic follows a square law Which of the statements given above is or are correct?

All three are correct, and between them they define the device: normally on, controlled by reverse bias, and square-law in its transfer. The first explains why self bias works, the second why the input impedance is high, and the third why the transconductance is modest. Three properties, three consequences, and the whole of the device's behaviour follows from them. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Match each configuration with its property. List I A. Common source B. Common gate C. Common drain List II P. Voltage gain with inversion Q. Wide bandwidth and low input impedance R. Unity gain and a high input impedance Select the correct match.

The pairing is A-P, B-Q, C-R, exactly parallel to common emitter, common base and common collector in the bipolar family. Common source inverts and gives the gain, common gate offers a low input impedance and a wide bandwidth because no inverting gain bridges its terminals, and common drain buffers at unity gain. That the correspondence holds property for property means every intuition about impedance reflection and the Miller effect carries straight across, which halves the work of learning the second family.

Q: Match each parameter with the device it belongs to. List I A. Pinch-off voltage B. Threshold voltage C. Current gain List II P. Junction field effect transistor Q. Enhancement MOSFET R. Bipolar transistor Select the correct match.

The pairing is A-P, B-Q, C-R. Each parameter presupposes a structure: a channel that can be pinched off, an inversion layer that appears above a threshold, or a base current that is amplified. Recognising which structure a parameter assumes is a quick way to place an unfamiliar specification in the right family.

51.

Which family is less prone to thermal runaway in a power stage?

A.Both are equally prone once properly heatsinked
B.Field effect, because its threshold voltage falls with temperature
C.Field effect, because on-resistance rises with temperature
D.Bipolar, because its saturation voltage falls with temperature

Answer: C

A MOSFET's on-resistance rises as it heats, so a hot device carries less of a shared current and the imbalance self-corrects - which is why MOSFETs parallel readily and bipolar devices need ballast resistors. The threshold does fall with temperature, working the other way, but the resistance effect dominates in the conducting state. A device whose imperfection acts as negative feedback is far easier to use in parallel.

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52.

Why do MOSFETs parallel more readily than bipolar transistors?

A.Their threshold voltages match closely between devices
B.Their gates can be driven from one common voltage source
C.Their safe operating areas overlap sufficiently to share
D.Current sharing is self-correcting through the on-resistance

Answer: D

The device taking more current heats, its resistance rises, and it takes less - negative feedback that equalises the sharing without any external component. Thresholds do not match especially well, a common gate drive is convenient but not the mechanism, and the safe operating area is a separate matter. Whether an imperfection acts with or against an imbalance decides whether devices can be paralleled at all. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

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53.

Assertion (A): Assertion: A field effect transistor's input impedance is far higher than a bipolar transistor's.

Reason (R): Reason: Its gate is either insulated or reverse biased and draws essentially no current.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: A

Both are true and the second is the reason: with no gate current, the input impedance is set by whatever bias resistor the designer fits rather than by the device. That is why a field effect input stage can present megohms while a bipolar one presents kilohms. A specification the designer chooses directly is more valuable than one inherited from the device, and this is the clearest example in the subject. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

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54.

Assertion (A): Assertion: A field effect transistor gives a lower voltage gain than a bipolar device at the same current.

Reason (R): Reason: Its gate draws no current.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, but R is not the correct explanation of A
C.Both A and R are true, and R is the correct explanation of A
D.A is true but R is false

Answer: B

Both statements are true but the second does not explain the first: the lower gain follows from the square-law transfer characteristic being less steep than an exponential one, while the absence of gate current explains the high input impedance instead. Two consequences of the same device, arising from two different aspects of it, and attaching each to its own cause is exactly what this item tests.

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55.

Assertion (A): Assertion: An enhancement-mode MOSFET can be self biased with a source resistor alone.

Reason (R): Reason: A source resistor makes the gate negative with respect to the source.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: C

The reason is true and is exactly why the assertion is false: an enhancement device needs the gate positive of the source, so the polarity self bias produces would keep it turned off. Self bias suits depletion devices only. A mechanism that is correct in itself can still be the wrong mechanism for the device at hand, and checking the required polarity first avoids the mistake.

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56.

Consider the following statements about junction field effect transistors.

1. The channel conducts with zero gate voltage

2. The gate must never be forward biased

3. The transfer characteristic follows a square law

Which of the statements given above is or are correct?

A.1 and 2 only
B.2 and 3 only
C.1 and 3 only
D.1, 2 and 3

Answer: D

All three are correct, and between them they define the device: normally on, controlled by reverse bias, and square-law in its transfer. The first explains why self bias works, the second why the input impedance is high, and the third why the transconductance is modest. Three properties, three consequences, and the whole of the device's behaviour follows from them. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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57.

Consider the following statements about MOSFETs.

1. The gate is insulated by an oxide layer

2. Enhancement types have no channel at zero gate voltage

3. The gate draws a small leakage current through the oxide

Which of the statements given above is or are correct?

A.1 and 2 only
B.1 and 3 only
C.2 and 3 only
D.1, 2 and 3

Answer: A

The first two are correct and the third is essentially false: the oxide is an insulator and the gate current is immeasurably small, which is both the device's strength and the reason static charge accumulates on it. A junction field effect transistor's gate does leak a little, since it is a reverse-biased junction, and confusing the two families on this point is the trap. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

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58.

Match each configuration with its property.

List I

A. Common source

B. Common gate

C. Common drain

List II

P. Voltage gain with inversion

Q. Wide bandwidth and low input impedance

R. Unity gain and a high input impedance

Select the correct match.

A.A-P, B-R, C-Q
B.A-P, B-Q, C-R
C.A-Q, B-P, C-R
D.A-R, B-Q, C-P

Answer: B

The pairing is A-P, B-Q, C-R, exactly parallel to common emitter, common base and common collector in the bipolar family. Common source inverts and gives the gain, common gate offers a low input impedance and a wide bandwidth because no inverting gain bridges its terminals, and common drain buffers at unity gain. That the correspondence holds property for property means every intuition about impedance reflection and the Miller effect carries straight across, which halves the work of learning the second family.

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59.

Match each parameter with the device it belongs to.

List I

A. Pinch-off voltage

B. Threshold voltage

C. Current gain

List II

P. Junction field effect transistor

Q. Enhancement MOSFET

R. Bipolar transistor

Select the correct match.

A.A-R, B-Q, C-P
B.A-P, B-R, C-Q
C.A-P, B-Q, C-R
D.A-Q, B-P, C-R

Answer: C

The pairing is A-P, B-Q, C-R. Each parameter presupposes a structure: a channel that can be pinched off, an inversion layer that appears above a threshold, or a base current that is amplified. Recognising which structure a parameter assumes is a quick way to place an unfamiliar specification in the right family.

Discuss in Forum

60.

Consider the following pairs of property and the family it favours.

Very high input impedance - Field effect

High transconductance per milliamp - Bipolar

Easy paralleling in power stages - Field effect

Freedom from static damage - Field effect

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

The first three are correctly paired and the fourth is not, so three match. Static damage is a field effect weakness rather than a strength, and follows directly from the insulated gate that gives the family its high input impedance. That the same structural feature produces the greatest advantage and the greatest fragility is worth holding together.

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