GROL Semiconductor and Tube Components 2 — Questions and Answers
Question 1: What is the primary purpose of the screen grid in a tetrode vacuum tube?
- To emit secondary electrons toward the collector
- To reduce interelectrode capacitance between control grid and plate (Correct answer)
- To regulate the cathode emission current
- To provide a reference voltage for the control grid
Correct answer: To reduce interelectrode capacitance between control grid and plate
The screen grid is held at a positive DC voltage and electrostatically shields the control grid from the plate, greatly reducing Miller capacitance and improving high-frequency gain.
Question 2: The amplification factor (μ) of a vacuum tube is defined as:
- The ratio of transconductance to plate resistance
- The ratio of plate current change to control grid voltage change
- The ratio of plate resistance to transconductance
- The magnitude of plate voltage change per unit grid voltage change needed to keep plate current constant (Correct answer)
Correct answer: The magnitude of plate voltage change per unit grid voltage change needed to keep plate current constant
μ equals ΔVp/ΔVg (with opposite sign) at constant plate current and also equals the product of transconductance (gm) and plate resistance (rp).
Question 3: In an NPN bipolar junction transistor operating in the active (linear) region, the junction biasing is:
- Both emitter-base and collector-base junctions reverse biased
- Both emitter-base and collector-base junctions forward biased
- Emitter-base junction forward biased and collector-base junction reverse biased (Correct answer)
- Emitter-base junction reverse biased and collector-base junction forward biased
Correct answer: Emitter-base junction forward biased and collector-base junction reverse biased
In the active region the forward-biased emitter-base junction injects minority carriers into the base, while the reverse-biased collector-base junction sweeps them into the collector.
Question 4: The DC current gain parameter hFE of a BJT is defined as the ratio of:
- Emitter current to base current
- Emitter current to collector current
- Collector current to base current (Correct answer)
- Base current to emitter current
Correct answer: Collector current to base current
hFE (also called β) equals IC/IB and typically ranges from about 20 to 500 for silicon transistors; it quantifies how much the base current is amplified at the collector.
Question 5: A reflex klystron differs from a two-cavity klystron in that it uses:
- A traveling-wave helix to slow the RF signal
- A magnetron cavity for broadband resonance
- A single resonant cavity with a repeller electrode that returns velocity-modulated electrons for a second pass through the gap (Correct answer)
- Multiple drift tubes for staged velocity modulation
Correct answer: A single resonant cavity with a repeller electrode that returns velocity-modulated electrons for a second pass through the gap
The reflex klystron uses one resonant cavity; a negative-voltage repeller reflects the velocity-modulated beam back through the same gap, producing oscillation without a second cavity.
Question 6: The suppressor grid of a pentode vacuum tube is held at cathode potential in order to:
- Accelerate secondary electrons from the plate toward the screen grid
- Repel secondary electrons emitted from the plate back to the plate rather than allowing them to reach the screen grid (Correct answer)
- Provide a fixed negative bias that reduces overall plate current
- Shield the control grid from variations in plate voltage
Correct answer: Repel secondary electrons emitted from the plate back to the plate rather than allowing them to reach the screen grid
Secondary electrons emitted from the plate would be attracted to the positive screen grid in a tetrode, causing the kink in the characteristic curve; the suppressor's near-cathode potential repels them back to the plate.
Question 7: A common-emitter BJT amplifier stage produces an output voltage signal that is:
- In phase with the input signal (0° phase shift)
- 90° out of phase with the input signal
- 45° out of phase with the input signal
- 180° out of phase with the input signal (inverted) (Correct answer)
Correct answer: 180° out of phase with the input signal (inverted)
When input base-emitter voltage increases, collector current increases, increasing the voltage drop across the collector load resistor and therefore decreasing the collector output voltage — a phase inversion.
GROL Semiconductor and Tube Components 2
What is the primary purpose of the screen grid in a tetrode vacuum tube?