Q: Bias analysis – E-MOSFET drain voltage at Q-point: For VDD = 30 V and a drain resistor RD = 8 kΩ, find the drain voltage at the Q point if the drain current is ID = 3 mA (assume source at ground and neglect channel-length modulation).

Introduction / Context: Finding node voltages at a MOSFET operating point is a basic step in biasing and verifying headroom for signal swing. With a known drain current and resistor, the drain voltage follows directly from Ohm’s law. Given Data / Assumptions: VDD = 30 V. RD = 8 kΩ. ID = 3 mA. Source at ground; ignore channel-length modulation so ID is set by the bias network. Concept / Approach: The drain resistor drops Vdrop = ID * RD. The drain node voltage is then VD = VDD − Vdrop. This is a straight substitution problem. Step-by-Step Solution: Compute drop: Vdrop = 0.003 A * 8000 Ω = 24 V.Compute node: VD = 30 V − 24 V = 6 V.Conclusion: Drain sits at approximately 6 V. Verification / Alternative check: Check power in RD: P = ID^2 * RD = (0.003)^2 * 8000 ≈ 0.072 W → a 1/4 W resistor is adequate with margin. Why Other Options Are Wrong: 10 V and 24 V result from arithmetic errors. 30 V would require zero current (no drop), not the stated 3 mA. 0 V is impossible with positive VDD and finite drop. Common Pitfalls: Forgetting to subtract the drop from VDD or mixing units (kΩ vs Ω) leads to wrong drain voltages. Final Answer: 6 V

Q: High-frequency amplifier topology: D-MOSFETs are often stacked in a cascode configuration to mitigate bandwidth loss caused primarily by which effect?

Introduction / Context: At high frequencies, parasitic capacitances limit gain-bandwidth. The cascode (a common-source feeding a common-gate stage) is a classic way to extend bandwidth by reducing the Miller effect and keeping the gain device's drain at a near-constant AC potential. Given Data / Assumptions: MOSFET intrinsic capacitances (Cgs, Cgd) are present. Voltage-gain stages suffer Miller multiplication of Cgd. Goal: broader bandwidth and improved stability. Concept / Approach: The effective input capacitance seen by a driving source contains Cgd multiplied by (1 − Av), the Miller effect. The cascode holds the drain of the lower device at AC ground (or small swing), dramatically reducing the Miller multiplication and the capacitive loading that otherwise reduces high-frequency gain. Step-by-Step Solution: Recognize dominant limitation: Cgd leads to large effective input capacitance.Insert common-gate upper device: its low input impedance at the source keeps the lower drain at small AC swing.Result: reduced capacitive reactance effects → higher bandwidth. Verification / Alternative check: AC simulation shows extended −3 dB frequency and flatter gain with cascode compared to single-device CS. Why Other Options Are Wrong: High input impedance is beneficial, not a loss mechanism. Low output impedance is typically improved by cascode for current source behavior, not the main bandwidth limit. Inductive reactance is usually not the dominant internal parasitic in MOSFET small-signal devices. Common Pitfalls: Ignoring layout parasitics; long leads and pads add extra C that undermines cascode benefits. Final Answer: capacitive reactance

Q: MOSFET terminal count – practical devices: How many terminals does a MOSFET present, considering that in discrete devices the body/substrate may be internally tied to the source?

Introduction / Context: Understanding how many terminals a MOSFET exposes guides correct biasing and measurement. While the MOS structure has gate, source, drain, and body (substrate), packaging and internal connections vary. Given Data / Assumptions: Discrete power MOSFETs often short the body to source internally. IC processes may bring the body out as a separate terminal in 4-terminal FETs. We count external, usable terminals. Concept / Approach: A MOSFET fundamentally has four regions: gate (G), drain (D), source (S), and body (B). In many discrete devices, B is internally tied to S → the package exposes three pins (G, D, S). Some specialty or IC devices provide a separate body connection → four terminals. Step-by-Step Solution: Identify intrinsic terminals: G, D, S, B.Check packaging: B tied to S → 3 leads; B separate → 4 leads.Hence: “3 or 4.” Verification / Alternative check: Datasheets show pinouts; many TO-220/TO-247 parts are 3-lead, while small-signal MOSFET arrays or IC processes sometimes present 4 terminals. Why Other Options Are Wrong: “2 or 3” ignores the body connection reality; two-terminal operation is not a MOSFET package convention. “Only 3” or “only 4” excludes common variants. “5” is not standard for a single MOSFET. Common Pitfalls: Assuming the body diode orientation without confirming body–source tie; incorrect assumptions can affect reverse conduction behavior. Final Answer: 3 or 4

Q: Transconductance curve of a JFET: Identify the quantities plotted to obtain the standard JFET “transconductance (transfer) characteristic” curve used in bias design and small-signal modeling.

Introduction / Context: For JFET biasing, the key device curve is the transfer (transconductance) characteristic, which shows how the control voltage VGS sets the drain current ID. Designers use it to select the Q-point and to estimate small-signal parameters. Given Data / Assumptions: JFET operating in its normal region with gate reverse-biased. Transfer curve taken at sufficiently high VDS to ensure saturation behavior. We seek the standard axes used. Concept / Approach: The JFET transfer characteristic plots ID as a function of VGS, often following a square-law-like relation between 0 and the pinch-off voltage VP. This is distinct from the output characteristic, which plots ID versus VDS at fixed VGS. Step-by-Step Solution: Hold VDS large enough for saturation.Sweep VGS and measure ID.Plot ID (vertical axis) versus VGS (horizontal axis) → transfer curve. Verification / Alternative check: Datasheets show both families: output characteristics (ID–VDS) and transfer characteristics (ID–VGS). The latter is used to derive gm as the slope dID/dVGS. Why Other Options Are Wrong: Options involving IC/VCE refer to BJTs, not JFETs. IS versus VDS is not a standard JFET characterization. ID × RDS is not a device characteristic axis pair. Common Pitfalls: Confusing the transfer curve with the output characteristic; they serve different purposes in design. Final Answer: ID versus VGS

Question 1

Device testing – n-channel D-MOSFET: An ohmmeter check shows these resistances (measured with device out of circuit): G–D = ∞, G–S = ∞, D–SS = ∞ or about 500 Ω depending on meter polarity, and D–S = about 500 Ω in both directions. Based on these rea…

Accepted Answer

Introduction / Context: Technicians often perform quick ohmmeter checks on MOSFETs removed from a circuit. For an n-channel enhancement/depletion MOSFET with body/substrate tied to source (SS), the gate is insulated (very high resistance), and an intrinsic body diode exists between drain and source. Understanding expected readings helps identify shorts and opens. Given Data / Assumptions: n-channel D-MOSFET, substrate shorted to source (SS). Measured out of circuit with a typical handheld ohmmeter. Readings: G–D = ∞, G–S = ∞, D–SS = ∞ or ~500 Ω depending on polarity, and D–S ≈ 500 Ω regardless of polarity. Concept / Approach: Ideal expectations: Gate to any terminal should read essentially infinite resistance (oxide insulation). Drain–Source should show a diode effect: low resistance one way (forward body diode) and very high the other. If D–S reads a similar low resistance in both directions, that suggests a bilateral conduction path, i.e., a short. Step-by-Step Solution: Check gate insulation: G–D = ∞ and G–S = ∞ → gate is not shorted.Check intrinsic diode: D–SS should read low in one polarity and ∞ in the other → this matches the body diode behavior.Evaluate D–S: reading ~500 Ω both directions indicates conduction both ways → inconsistent with a single diode and points to D–S short/fault. Verification / Alternative check: Reverse meter leads across D–S. A healthy device should show one low and one very high reading. Similar low readings both ways corroborate a shorted junction/structure. Why Other Options Are Wrong: Open G–D or open D–SS would not cause bilateral low D–S readings. “Nothing” ignores the symmetrical low D–S behavior. Gate oxide short would show low G–D or G–S, which we do not see. Common Pitfalls: Testing in-circuit can back-feed through other parts; always test with at least one lead lifted or the device removed. Final Answer: short D to S

Question 2

Field-effect transistor operation: The point at which a JFET channel just stops allowing further increase of controlled drain current with more negative VGS (i.e., the device can no longer control current) is called the:

Accepted Answer

Introduction / Context: In a JFET, the gate–channel junction is reverse-biased, and controlling the channel width with VGS regulates drain current ID. A key operating point is “pinch-off,” where the depletion region expands enough to restrict the channel such that increasing VDS no longer significantly increases ID at a fixed VGS. Given Data / Assumptions: Junction FET (JFET) device. Gate reverse-biased for control. We are naming the condition where the JFET ceases to control current growth in the usual linear sense. Concept / Approach: At pinch-off, the channel is constricted; the device enters the constant-current (saturation) region where ID ≈ IDSS * (1 − VGS/VP)^2 for an n-channel device (qualitative). The term “pinch-off” specifically refers to the VGS and VDS condition where channel cross-section effectively pinches. Step-by-Step Solution: Start from ohmic region at small VDS.Increase VDS at fixed VGS until the channel narrows enough → pinch-off.Beyond that, ID is roughly constant; further VDS increases drop across the pinch-off region. Verification / Alternative check: Look at the JFET output characteristics ID vs VDS at several VGS; you will see flat “saturation” plateaus beginning at pinch-off. Why Other Options Are Wrong: Breakdown is an excessive reverse-bias phenomenon. “Depletion region” is a structure, not the operating point name. “Saturation point” is a broader term; the JFET’s entry into saturation is defined by pinch-off. Common Pitfalls: Confusing MOSFET and JFET terminology; for JFETs, pinch-off defines entry to the constant-current region. Final Answer: pinch-off region

Question 3

JFET terminals – basic identification: The three external connections (“legs”) of a JFET are the drain, the gate, and the ______.

Accepted Answer

Introduction / Context: Correct terminal identification is essential when biasing and measuring field-effect transistors. A JFET has three functional terminals that define how the channel is controlled and how current flows. Given Data / Assumptions: Discrete JFET device (n- or p-channel). Standard three-terminal configuration. Common symbols and pinouts used. Concept / Approach: The JFET channel is the conduction path between the drain and source. The reverse-biased gate–channel junction modulates the channel width. The three terminals are named drain (D), source (S), and gate (G); “channel” is not a terminal. Step-by-Step Solution: Identify current path: drain ↔ source.Identify control: gate reverse-bias alters depletion region and channel width.Therefore, the third terminal besides drain and gate is the source. Verification / Alternative check: Schematic symbols label D, S, and G; datasheets provide pin numbering for specific packages. Why Other Options Are Wrong: “Channel” describes the region, not a terminal. “Substrate” is relevant in MOS technology; a discrete JFET presents three leads only. “Cathode” and “emitter” are diode/BJT terms, not JFET terminals. Common Pitfalls: Misidentifying source and drain can matter in devices with asymmetric pinch-off behavior; always consult the datasheet pinout. Final Answer: source

Question 4

JFET small-signal parameter: The ratio of a small change in output drain current to a small change in input gate-to-source voltage (at a defined operating point) is called:

Accepted Answer

Introduction / Context: Designers characterize FET amplifiers using small-signal parameters measured about a quiescent point. For JFETs, one key figure is how effectively gate-to-source voltage variations control drain current. Given Data / Assumptions: We consider small-signal incremental behavior around the Q-point. Output quantity is drain current ID; input control is VGS. Linearization applies for small perturbations. Concept / Approach: Transconductance, symbol gm, is defined as gm = dID/dVGS at the operating point, with units of siemens (A/V). “Siemens” is the unit, not the parameter. “Gain” is generic; “resistivity” is a material property. Step-by-Step Solution: Define parameter: gm = change in ID / change in VGS.Units check: amperes per volt → siemens.Apply in design: voltage gain of a common-source stage ≈ −gm * RD (simplified). Verification / Alternative check: Datasheets specify gm versus ID; bench measurements can extract gm from small AC sweeps around the bias point. Why Other Options Are Wrong: “Siemens” is the unit, not the parameter name. “Resistivity” is unrelated to the device small-signal transfer. “Gain” is ambiguous without specifying transconductance. Common Pitfalls: Confusing transconductance with output conductance (dID/dVDS) or with static ratios like ID/VGS; gm is a derivative at the bias point. Final Answer: transconductance

Question 5

D-MOSFET input behavior vs. frequency: How does the input impedance of a depletion-type MOSFET change as signal frequency varies, considering the gate's capacitive effects?

Accepted Answer

Introduction / Context: Although MOSFET gates are insulated (extremely high DC resistance), the input is not purely resistive. Intrinsic gate–source and gate–drain capacitances make the input impedance frequency-dependent in AC applications. Given Data / Assumptions: Depletion-type MOSFET (D-MOSFET) with intact gate oxide. Gate leakage is negligible; capacitances dominate AC input impedance. Small-signal conditions. Concept / Approach: Capacitive reactance is Xc = 1 / (2 * π * f * C). As frequency f increases, Xc falls, lowering input impedance; as frequency decreases, Xc rises, increasing input impedance. Thus, input impedance is higher at lower frequencies and lower at higher frequencies. Step-by-Step Solution: Model gate path as capacitors to source/drain.Compute reactance trend: increase f → decrease Xc → lower impedance.Conclude: decreasing frequency increases input impedance. Verification / Alternative check: Network analyzer or impedance meter measurements show input magnitude falling with frequency due to capacitive paths, matching Xc behavior. Why Other Options Are Wrong: Statements claiming constancy or increase with frequency contradict Xc behavior. “Independent of frequency” ignores the well-documented Cgs and Cgd effects. Common Pitfalls: Assuming near-infinite impedance at RF; at high f the capacitive path can significantly load preceding stages, especially with Miller multiplication in voltage-gain stages. Final Answer: As frequency decreases, input impedance increases.

Question 6

Common-source JFET amplifier characteristics: Which description best matches the typical properties of a properly biased common-source JFET voltage amplifier?

Accepted Answer

Introduction / Context: The common-source (CS) configuration is the JFET analog of the BJT common-emitter and the MOSFET common-source stage. It provides voltage gain with phase inversion and is widely used as the basic voltage-amplifying stage. Given Data / Assumptions: JFET operated in the saturation region at a suitable Q-point. Drain load resistance RD chosen for gain. Signal small enough for linear approximation. Concept / Approach: Voltage gain magnitude for a CS JFET is approximately |Av| ≈ gm * RD (neglecting ro). JFETs have high input impedance because the gate–channel junction is reverse-biased, so gate current is very small. With proper gm and RD, high voltage gain is achievable (often much greater than unity), unlike the source follower which has Av < 1. Step-by-Step Solution: Bias JFET so that it operates in saturation.Select RD to set gain: larger RD → larger |Av| within headroom limits.Recognize high input Z due to reverse-biased gate. Verification / Alternative check: AC analysis or measurements will show phase inversion and |Av| often in the tens, validating “very high voltage gain” relative to unity. Why Other Options Are Wrong: “Voltage gain less than 1” describes a source follower (common-drain). “No voltage gain” is false for CS. “Very low input impedance” contradicts JFET characteristics. Common Pitfalls: Ignoring output resistance ro and bypass capacitor choices, which affect actual gain and bandwidth. Final Answer: a high input impedance and a very high voltage gain

Question 7

Bias analysis – E-MOSFET drain voltage at Q-point: For VDD = 30 V and a drain resistor RD = 8 kΩ, find the drain voltage at the Q point if the drain current is ID = 3 mA (assume source at ground and neglect channel-length modulation).

Accepted Answer

Introduction / Context: Finding node voltages at a MOSFET operating point is a basic step in biasing and verifying headroom for signal swing. With a known drain current and resistor, the drain voltage follows directly from Ohm’s law. Given Data / Assumptions: VDD = 30 V. RD = 8 kΩ. ID = 3 mA. Source at ground; ignore channel-length modulation so ID is set by the bias network. Concept / Approach: The drain resistor drops Vdrop = ID * RD. The drain node voltage is then VD = VDD − Vdrop. This is a straight substitution problem. Step-by-Step Solution: Compute drop: Vdrop = 0.003 A * 8000 Ω = 24 V.Compute node: VD = 30 V − 24 V = 6 V.Conclusion: Drain sits at approximately 6 V. Verification / Alternative check: Check power in RD: P = ID^2 * RD = (0.003)^2 * 8000 ≈ 0.072 W → a 1/4 W resistor is adequate with margin. Why Other Options Are Wrong: 10 V and 24 V result from arithmetic errors. 30 V would require zero current (no drop), not the stated 3 mA. 0 V is impossible with positive VDD and finite drop. Common Pitfalls: Forgetting to subtract the drop from VDD or mixing units (kΩ vs Ω) leads to wrong drain voltages. Final Answer: 6 V

Question 8

High-frequency amplifier topology: D-MOSFETs are often stacked in a cascode configuration to mitigate bandwidth loss caused primarily by which effect?

Accepted Answer

Introduction / Context: At high frequencies, parasitic capacitances limit gain-bandwidth. The cascode (a common-source feeding a common-gate stage) is a classic way to extend bandwidth by reducing the Miller effect and keeping the gain device's drain at a near-constant AC potential. Given Data / Assumptions: MOSFET intrinsic capacitances (Cgs, Cgd) are present. Voltage-gain stages suffer Miller multiplication of Cgd. Goal: broader bandwidth and improved stability. Concept / Approach: The effective input capacitance seen by a driving source contains Cgd multiplied by (1 − Av), the Miller effect. The cascode holds the drain of the lower device at AC ground (or small swing), dramatically reducing the Miller multiplication and the capacitive loading that otherwise reduces high-frequency gain. Step-by-Step Solution: Recognize dominant limitation: Cgd leads to large effective input capacitance.Insert common-gate upper device: its low input impedance at the source keeps the lower drain at small AC swing.Result: reduced capacitive reactance effects → higher bandwidth. Verification / Alternative check: AC simulation shows extended −3 dB frequency and flatter gain with cascode compared to single-device CS. Why Other Options Are Wrong: High input impedance is beneficial, not a loss mechanism. Low output impedance is typically improved by cascode for current source behavior, not the main bandwidth limit. Inductive reactance is usually not the dominant internal parasitic in MOSFET small-signal devices. Common Pitfalls: Ignoring layout parasitics; long leads and pads add extra C that undermines cascode benefits. Final Answer: capacitive reactance

Question 9

MOSFET terminal count – practical devices: How many terminals does a MOSFET present, considering that in discrete devices the body/substrate may be internally tied to the source?

Accepted Answer

Introduction / Context: Understanding how many terminals a MOSFET exposes guides correct biasing and measurement. While the MOS structure has gate, source, drain, and body (substrate), packaging and internal connections vary. Given Data / Assumptions: Discrete power MOSFETs often short the body to source internally. IC processes may bring the body out as a separate terminal in 4-terminal FETs. We count external, usable terminals. Concept / Approach: A MOSFET fundamentally has four regions: gate (G), drain (D), source (S), and body (B). In many discrete devices, B is internally tied to S → the package exposes three pins (G, D, S). Some specialty or IC devices provide a separate body connection → four terminals. Step-by-Step Solution: Identify intrinsic terminals: G, D, S, B.Check packaging: B tied to S → 3 leads; B separate → 4 leads.Hence: “3 or 4.” Verification / Alternative check: Datasheets show pinouts; many TO-220/TO-247 parts are 3-lead, while small-signal MOSFET arrays or IC processes sometimes present 4 terminals. Why Other Options Are Wrong: “2 or 3” ignores the body connection reality; two-terminal operation is not a MOSFET package convention. “Only 3” or “only 4” excludes common variants. “5” is not standard for a single MOSFET. Common Pitfalls: Assuming the body diode orientation without confirming body–source tie; incorrect assumptions can affect reverse conduction behavior. Final Answer: 3 or 4

Question 10

Transconductance curve of a JFET: Identify the quantities plotted to obtain the standard JFET “transconductance (transfer) characteristic” curve used in bias design and small-signal modeling.

Accepted Answer

Introduction / Context: For JFET biasing, the key device curve is the transfer (transconductance) characteristic, which shows how the control voltage VGS sets the drain current ID. Designers use it to select the Q-point and to estimate small-signal parameters. Given Data / Assumptions: JFET operating in its normal region with gate reverse-biased. Transfer curve taken at sufficiently high VDS to ensure saturation behavior. We seek the standard axes used. Concept / Approach: The JFET transfer characteristic plots ID as a function of VGS, often following a square-law-like relation between 0 and the pinch-off voltage VP. This is distinct from the output characteristic, which plots ID versus VDS at fixed VGS. Step-by-Step Solution: Hold VDS large enough for saturation.Sweep VGS and measure ID.Plot ID (vertical axis) versus VGS (horizontal axis) → transfer curve. Verification / Alternative check: Datasheets show both families: output characteristics (ID–VDS) and transfer characteristics (ID–VGS). The latter is used to derive gm as the slope dID/dVGS. Why Other Options Are Wrong: Options involving IC/VCE refer to BJTs, not JFETs. IS versus VDS is not a standard JFET characterization. ID × RDS is not a device characteristic axis pair. Common Pitfalls: Confusing the transfer curve with the output characteristic; they serve different purposes in design. Final Answer: ID versus VGS

Question 11

Electrostatic protection for MOSFETs: when a MOSFET device is not installed in a circuit, by what means are its pins commonly kept at the same potential to prevent ESD damage?

Accepted Answer

Introduction / Context: MOSFETs (metal–oxide–semiconductor field-effect transistors) are extremely sensitive to electrostatic discharge because the gate is insulated by a very thin oxide. When the device is not in use, manufacturers and technicians must keep all leads at the same potential to prevent charge buildup and destructive discharge through the gate oxide. Given Data / Assumptions: A discrete MOSFET is being handled or stored off the PCB. The goal is to keep all terminals at an equipotential and shunt static safely. Common packaging/handling accessories include conductive foam, antistatic bags/foil, and wrist straps. Concept / Approach: To equalize potentials, the device pins should be shorted together through a slightly resistive, conductive medium. Conductive foam accomplishes this by contacting all leads simultaneously, bleeding off any charge and keeping the gate, source, and drain at essentially the same potential without allowing high surge currents. Nonconductive materials cannot perform this equalization. Step-by-Step Solution: Identify the ESD objective: keep all leads at nearly the same potential.Select a medium that is conductive enough to equalize charge but not a hard short with zero impedance.Conductive foam meets this need and is the industry-standard shipping medium for MOSFETs and ICs. Verification / Alternative check: New MOSFETs often arrive with their pins inserted into black conductive foam or shorting clips; measuring resistance through the foam shows a low, finite resistance path among pins, confirming equipotential behavior. Why Other Options Are Wrong: Shipping foil: Antistatic bags may protect against external fields but do not necessarily short all pins together. Nonconductive foam: Provides mechanical protection only; it does not equalize potentials. A wrist strap: Protects the human handler; it does not keep the device pins at the same potential. Common Pitfalls: Confusing “antistatic” (dissipative) packaging with “conductive” pin-to-pin shorting. Handling MOSFETs out of foam on an ungrounded surface, allowing differential charging of leads. Final Answer: conductive foam

Question 12

In enhancement-mode MOSFET (E-MOSFET) amplifier design, which biasing method is most commonly used in practice to set a stable operating point?

Accepted Answer

Introduction / Context: Enhancement-mode MOSFETs require a gate-to-source voltage above a threshold to conduct. Establishing a predictable, thermally stable operating point (Q-point) is critical for linear amplification and repeatable performance across devices and temperature. Given Data / Assumptions: Device type: E-MOSFET (no conduction at VGS = 0 for n-channel). Goal: bias the gate at a defined DC potential relative to the source. Practical, component-efficient methods are preferred for discrete designs. Concept / Approach: The voltage-divider bias (two resistors forming a divider from the supply to ground with the tap feeding the gate, often combined with source degeneration) is widely used because it defines VGS reliably and provides negative feedback via the source resistor for stability. Other methods are less common or suited to special cases. Step-by-Step Solution: Choose a target drain current and VDS for the desired operating region.Select a source resistor to set a stable source voltage (self-bias).Use a gate voltage divider to set VG such that VGS = VG − VS meets the target above threshold.Bypass or partially bypass the source resistor to balance gain and stability. Verification / Alternative check: Circuit simulators and datasheet transfer curves confirm that a divider plus source resistor produces a robust operating point across device spreads and temperature variations. Why Other Options Are Wrong: Constant current: Typically defines drain current but still requires a defined VGS; less common for simple biasing. Drain-feedback: Used but less universal; loop gain and device spreads can complicate design. Zero biasing: Appropriate for depletion devices, not enhancement types which need VGS > Vth. Common Pitfalls: Ignoring the source resistor’s thermal feedback role in stabilizing operating current. Failing to consider gate leakage and divider impedance, leading to drift. Final Answer: voltage-divider

Question 13

For a JFET operated with VGS = 0 V (for example, an n-channel JFET at room temperature), which description best characterizes its operating state?

Accepted Answer

Introduction / Context: Junction field-effect transistors (JFETs) are depletion-mode devices. With the gate-source junction reverse biased or at zero bias, the channel conducts and the drain current is set largely by geometry and doping (IDSS at VGS = 0). Understanding these regions is crucial for analog biasing and switching applications. Given Data / Assumptions: VGS = 0 V. Typical n-channel JFET behavior (signs invert for p-channel). Standard region naming: “saturation” for the constant-current region beyond pinch-off in FET terminology. Concept / Approach: At VGS = 0 V, the JFET channel is fully open (no additional depletion from negative gate bias). With sufficient VDS, the device enters its constant-current region where ID ≈ IDSS. This region is called “saturation” for FETs (distinct from BJT saturation) and corresponds to current limited primarily by VGS, not VDS. Step-by-Step Solution: Set VGS = 0 V → channel at its widest for the device.Increase VDS past the pinch-off point → ID plateaus near IDSS.Operating description: saturated (constant-current region). Verification / Alternative check: Consult transfer curves ID vs. VGS: at VGS = 0, ID = IDSS. Output characteristics show flat ID vs. VDS beyond pinch-off, confirming the “saturated” label in FET nomenclature. Why Other Options Are Wrong: An analog device: True but not a state/region. An open switch: Incorrect; conduction is maximum, not open. Cut off: Occurs when VGS is sufficiently negative (≈ VGS(off)), not at 0 V. Common Pitfalls: Confusing FET saturation with BJT saturation (they refer to different behaviors). Assuming VGS = 0 means no conduction; for depletion devices it implies maximum conduction. Final Answer: saturated

Question 14

In field-effect devices, when an applied input voltage modulates the conductive channel’s resistance (and therefore current), what is this phenomenon called?

Accepted Answer

Introduction / Context: The defining feature of JFETs and MOSFETs is that an electric field, applied via a control terminal (gate), modulates the charge carriers in a semiconductor channel. This electric-field control changes the channel resistance and current without requiring significant input current at the gate. Given Data / Assumptions: Input signal appears at the gate (JFET or MOSFET). The gate modulates channel width/charge density. We seek the correct term for this mechanism. Concept / Approach: Applying a gate voltage establishes an electric field across the gate dielectric (MOSFET) or depletion region (JFET). This field attracts or repels carriers, changing channel conductivity. The generic name for this control principle is the “field effect,” which gives FETs their name. Step-by-Step Solution: Apply a gate voltage relative to source.The resulting electric field alters carrier density in the channel.Channel resistance and drain current change accordingly.Therefore, the action is termed the field effect. Verification / Alternative check: Device equations (transfer characteristics ID vs. VGS) directly relate current to gate-controlled fields; empirical datasheets confirm near-zero gate current in MOSFETs, distinguishing field control from current injection control (as in BJTs). Why Other Options Are Wrong: Saturization: Nonstandard term; saturation in FETs refers to the constant-current region, not the control mechanism. Polarization: Refers to dielectric dipole alignment; not the channel control principle. Cutoff: A region where current is minimal due to strong reverse gate bias; not the general effect. Common Pitfalls: Confusing BJT current-controlled behavior with FET field-controlled behavior. Assuming gate current modulation is required; in MOSFETs the gate current is ideally zero. Final Answer: field effect

Question 15

Which JFET amplifier configuration is best for buffering a high-resistance signal source into a low-resistance load (i.e., high input impedance and low output impedance)?

Accepted Answer

Introduction / Context: Many sensor and audio front ends require a buffer stage that minimally loads the source but can drive a lower-impedance load. The classic FET solution is the source follower, which offers very high input impedance and low output impedance with near-unity voltage gain. Given Data / Assumptions: Signal source has high resistance (needs minimal loading). Load is relatively low resistance (needs drive). Candidate topologies: common-source, common-gate, common-drain (source follower), cascode. Concept / Approach: The common-drain (source follower) presents gate input impedance dominated by gate leakage and bias network (typically megaohms), while the source output follows the gate voltage with a small offset and low output impedance (approximately 1/gm in parallel with bias resistances). This makes it ideal as a buffer. Step-by-Step Solution: Identify need: high Zin and low Zout.Evaluate topologies: common-source has high Zin but relatively high Zout; common-gate has low Zin; cascode improves bandwidth/early voltage but not Zout like a follower.Choose common-drain (source follower) for buffering. Verification / Alternative check: Small-signal models show follower output resistance near 1/gm, often tens to hundreds of ohms, while input resistance is set by gate leakage and bias resistors, commonly megaohms—exactly the desired transformation. Why Other Options Are Wrong: Common-source: Provides gain but higher output impedance; not the best buffer. Common-gate: Low input impedance; unsuitable for a high-resistance source. Cascode: Great for bandwidth and gain stability, but not primarily a buffer topology. Common Pitfalls: Confusing “source follower” with BJT “emitter follower” names; functions are analogous but device physics differ. Overlooking biasing; improper bias can raise distortion or limit swing. Final Answer: common-drain (source follower)

Question 16

For a field-effect transistor, if the drain current change is △ID = 1 mA in response to a gate-to-source voltage change of △VGS = 1 V, what is the device transconductance gm?

Accepted Answer

Introduction / Context: Transconductance (gm) quantifies how effectively a FET converts an input voltage variation at the gate into an output current change at the drain. It is a key small-signal parameter for gain and bandwidth calculations in analog design. Given Data / Assumptions: △ID = 1 mA. △VGS = 1 V. Definition: gm = △ID / △VGS, with units of siemens (A/V). Concept / Approach: Apply the definition directly. Ensure consistent units: milliamperes per volt yields millisiemens. Step-by-Step Solution: gm = △ID / △VGS.Substitute: gm = 1 mA / 1 V = 1 mA/V.Convert units: 1 mA/V = 1 × 10^-3 A/V = 1 mS. Verification / Alternative check: Dimensional check: A/V is siemens (S). Milliampere per volt corresponds to millisiemens, confirming the numerical result. Why Other Options Are Wrong: 1 kS: Off by 10^6; unrealistically large for discrete FETs. 1 kΩ and 1 mΩ: These are resistance units, not transconductance. Common Pitfalls: Mixing up transconductance (S) with resistance (Ω) or conductance (also S but of a resistor, not a transfer parameter). Forgetting to convert milliamps to amps when required. Final Answer: 1 mS

Question 17

In the constant-current (pinch-off) region of an n-channel JFET, how does the drain current ID vary as the gate-to-source voltage VGS changes?

Accepted Answer

Introduction / Context: In an n-channel JFET, the gate controls channel depletion. The constant-current (often called saturation) region describes operation where ID is largely independent of VDS but strongly dependent on VGS. Understanding how ID responds to VGS is essential for biasing and gain control. Given Data / Assumptions: Device: n-channel JFET. Operating region: constant-current region (beyond pinch-off in VDS). Question: qualitative dependence of ID on VGS. Concept / Approach: Making VGS more positive (less negative) widens the channel and increases carrier density, causing ID to rise toward IDSS. Making VGS more negative shrinks the channel and reduces ID, reaching near-zero at VGS(off). Thus ID is a monotonic function of VGS in this region. Step-by-Step Solution: Start at some VGS < 0 with ID set accordingly.Increase VGS (toward 0): channel depletion reduces → ID increases.Decrease VGS (more negative): depletion increases → ID decreases.Therefore, the correct qualitative statement is: as VGS increases, ID increases. Verification / Alternative check: Transfer curves (ID vs. VGS) from datasheets always show rising ID as VGS increases from VGS(off) toward 0 V for n-channel JFETs, confirming the relationship. Why Other Options Are Wrong: As VGS decreases, ID increases: Opposite of actual behavior. As VGS decreases, ID remains constant / As VGS increases, ID remains constant: ID is not constant with VGS; only weakly dependent on VDS in this region. Common Pitfalls: Confusing VGS dependence with VDS dependence; “constant-current region” refers to the latter. Mixing p-channel and n-channel sign conventions. Final Answer: As VGS increases, ID increases.

Question 18

When an input signal reduces the effective channel width/charge in a field-effect transistor, what is this process called?

Accepted Answer

Introduction / Context: FET operation can be described by two complementary mechanisms: enhancement (increasing channel carriers) and depletion (reducing channel carriers). Recognizing which mechanism is in play helps in understanding transfer curves and biasing strategies for JFETs and MOSFETs. Given Data / Assumptions: Input voltage causes the conductive channel to become narrower (fewer carriers). We seek the correct terminology for this behavior. Concept / Approach: When the gate voltage drives the channel toward fewer carriers (for n-channel, a more negative VGS), the channel cross-section shrinks and resistance increases; this is depletion. Conversely, when the gate voltage attracts additional carriers and enlarges the channel (or creates an inversion channel in MOSFETs), that is enhancement. Step-by-Step Solution: Observe that the channel size decreases under the applied input.Link decreased carriers/width to increased depletion region.Name the mechanism accordingly: depletion. Verification / Alternative check: JFETs are depletion-mode by nature: applying reverse gate bias depletes the channel until cutoff at VGS(off). MOSFETs may operate in depletion or enhancement modes depending on structure; the language consistently uses “depletion” for reduced channel carriers. Why Other Options Are Wrong: Enhancement: The opposite effect—adding carriers and increasing channel conduction. Substrate connecting: A layout/connection detail, not a conduction mechanism. Gate charge: Refers to charging the gate capacitances, not the channel depletion process. Common Pitfalls: Confusing depletion in JFETs (reverse bias) with enhancement in E-MOSFETs (positive VGS for n-channel). Assuming “gate charge” dictates conduction; it mainly affects switching speed, not the steady-state mechanism naming. Final Answer: depletion

Question 19

Dual-gate D-MOSFETs often exhibit a lower overall input capacitance. What is the usual connection of the two gates that leads to this reduced effective capacitance?

Accepted Answer

Introduction / Context: Dual-gate MOSFETs are popular in RF front-ends and mixers because they allow gain control and improved isolation. A practical benefit is reduced effective input capacitance seen by the preceding stage, which helps extend bandwidth and ease matching. Given Data / Assumptions: Device: dual-gate depletion MOSFET (D-MOSFET). Question: why the overall input capacitance is lower. Key concept: series combination of gate capacitances. Concept / Approach: When two capacitances are connected in series, the net capacitance is lower than either individual capacitance. In dual-gate MOSFETs, the two gate structures can behave like series-connected capacitors from the input to the channel, thereby reducing the effective input capacitance and improving high-frequency performance. Step-by-Step Solution: Model each gate with its gate-to-channel capacitance.Recognize that the cascading of two gates places these capacitances effectively in series from the RF input’s perspective.Series combination yields C_total less than either individual C, lowering input loading. Verification / Alternative check: RF small-signal models reflect reduced Cin and improved reverse isolation (feedback capacitance partitioning), consistent with measured S-parameters of dual-gate devices. Why Other Options Are Wrong: In parallel: Would increase, not decrease, effective input capacitance. With separate insulation / with separate inputs: True physical attributes but not the electrical reason for lower effective capacitance. Common Pitfalls: Assuming lower capacitance is due only to process differences; topology (series effect) is the key factor. Ignoring how reduced Cin affects input matching at high frequencies. Final Answer: in series

Question 20

JFET pinch-off behavior (n-channel): At what condition does an n-channel JFET reach pinch-off, meaning further increases in drain-to-source voltage no longer increase the drain current (ID)?

Accepted Answer

Introduction / Context: An n-channel Junction Field-Effect Transistor (JFET) is a depletion-mode device whose channel conductivity is controlled by the reverse-bias on its gate-to-channel PN junction. A key operating point is the pinch-off condition, where the channel is sufficiently depleted so that increasing the drain-to-source voltage (VDS) does not significantly increase the drain current (ID). This question checks your grasp of the JFET output characteristics and the definition of pinch-off in the saturation (constant-current) region. Given Data / Assumptions: An n-channel JFET operating with gate-to-source voltage VGS fixed (often at or below 0 V). Device modeled under typical small-signal conditions, ignoring channel-length modulation for the ideal definition. Temperature and process variations neglected for clarity. Concept / Approach: For a given VGS, as VDS rises, the reverse bias along the channel increases and the channel narrows. At the pinch-off voltage Vp (for that VGS), the channel is just pinched near the drain end and the device enters the so-called saturation or constant-current region: ID remains approximately constant even if VDS increases further. Step-by-Step Solution: Fix VGS and slowly increase VDS from a small value.As VDS increases, the depletion region widens more near the drain than the source.At VDS = Vp (for the chosen VGS), the channel just pinches off at the drain end.Beyond this point, ID ≈ constant; further increases in VDS yield little to no increase in ID (ideal model). Verification / Alternative check: Inspect a JFET family of output curves (ID vs. VDS for several VGS). Each curve shows a knee where it flattens: that knee corresponds to the onset of pinch-off (saturation region). Measuring ID beyond the knee shows minimal change with VDS. Why Other Options Are Wrong: VGS-based definitions (options b and d) describe cutoff or transconductance behavior, not the VDS pinch-off onset. VDG (option c) is not the standard defining parameter for the pinch-off condition in output characteristics. ID at threshold (option e) confuses MOSFET terminology with JFET behavior. Common Pitfalls: Confusing pinch-off (constant-current region) with cutoff (ID ≈ 0). Ignoring channel-length modulation, which causes a slight upward slope in real devices but does not change the basic definition. Final Answer: the value of VDS at which further increases in VDS will cause no further increase in ID

Field Effect Transistors (FET) Questions