Grade 10 electricity – Transistors Quiz
1. What are the three terminals of a bipolar junction transistor (BJT)?
A BJT has three terminals called Base, Emitter and Collector which control and carry currents in the device.
2. Which of the following describes the layer arrangement of an NPN transistor?
An NPN transistor has two n-type semiconductor regions separated by a p-type region, written as N-P-N.
3. In an NPN transistor, electrons primarily flow from which terminal to which terminal when forward biased?
In an NPN device under normal forward-active operation, electrons are injected from the emitter and collected at the collector.
4. What is the main purpose of a transistor used as an amplifier?
As an amplifier, a transistor makes a small input signal larger at its output while keeping the same waveform shape.
5. Which quantity controls the collector-emitter current in a bipolar junction transistor?
In a BJT the base current controls the larger collector-emitter current; the collector current is approximately beta times the base current.
6. What is the unit of the current gain (β) of a transistor?
Beta is a ratio of collector current to base current, so it is a dimensionless number with no unit.
7. When a transistor operates as a closed switch (fully ON), which region is it in?
Saturation is the state when both junctions are forward biased and the transistor conducts maximum current behaving like a closed switch.
8. When a transistor is acting as an open switch (fully OFF), which state is it in?
Cutoff means the base current is essentially zero so the transistor does not conduct between collector and emitter, acting like an open switch.
9. Which type of transistor is controlled mainly by input current rather than input voltage?
A BJT is a current-controlled device: the base current controls the collector-emitter current, unlike a MOSFET which is voltage-controlled.
10. Which transistor is primarily controlled by the voltage applied to its gate?
A MOSFET is a voltage-controlled device where the gate voltage modulates the current between drain and source.
11. The base-emitter junction of a BJT behaves like which component when forward biased?
The base-emitter junction is a p-n junction and behaves like a diode when forward biased, allowing current to flow.
12. If base current is increased while Vce is kept constant, what happens to collector current in a BJT (in active region)?
In the active region collector current is approximately β times the base current, so increasing base current increases collector current.
13. On the transistor symbol, which way does the emitter arrow point for a PNP transistor?
For PNP symbols the emitter arrow points toward the base, indicating conventional current flows into the emitter.
14. Which is a common everyday application of a transistor acting as a switch?
Transistors are widely used as electronic switches, for example to switch LEDs or motors in microcontroller projects.
15. If a transistor has a current gain (β) of 100 and the base current is 20 microamperes, what is the collector current?
Collector current ≈ β × base current = 100 × 20 μA = 2000 μA = 2 mA.
16. Which biasing of the base-emitter and base-collector junctions places a BJT in the forward-active region (used for amplification)?
Forward-active region requires the base-emitter junction forward biased to inject carriers and base-collector reverse biased to collect them for amplification.
17. Why is a heat sink sometimes attached to a power transistor?
Power transistors dissipate heat; a heat sink improves cooling to keep temperature safe and maintain reliable operation.
18. When testing a transistor with an ordinary multimeter, what is a simple method to check if it is working?
Using a multimeter diode check to verify the two p-n junctions behave like diodes is a common quick test for BJTs.
19. In an emitter follower (common-collector) circuit, how does the output voltage relate to the input?
An emitter follower provides a buffered output that tracks the input minus the base-emitter voltage drop (~0.6–0.7 V for silicon).
20. What is the main advantage of using a Darlington pair configuration of two transistors?
A Darlington pair connects two transistors to multiply their gains, producing a much larger overall current gain.
21. What happens when a transistor is driven into the breakdown region due to excessive voltage?
Breakdown means destructive avalanche of carriers due to excessive voltage, which can irreversibly damage the transistor.
22. Why is a resistor often placed in series with the base of a switching transistor?
A base resistor prevents excessive base current that could damage the transistor or the driving circuit, controlling the switch behavior.
23. Which symbol name is commonly used to represent the DC current gain of a transistor in datasheets (often seen as hFE)?
hFE is another notation for the transistor's DC current gain, equivalent to β, the ratio of collector current to base current.
24. In a common-emitter amplifier, how does the output signal relate in phase to the input signal?
A common-emitter amplifier produces an amplified output voltage that is inverted relative to the input signal.
25. Which set of terminals belongs to a field-effect transistor (FET)?
FETs use Gate, Drain and Source terminals; BJTs use Base, Emitter and Collector, so Gate/Drain/Source identify a FET.