A phototransistor is a switch that: A) completes a circuit when light strikes the emitter B) completes a circuit when light strikes the base C) varies current depending on the intensity of the light on the emitter D) completes a circuit when light strikes the collector
The Correct Answer and Explanation is:
Correct Answer: B) completes a circuit when light strikes the base
Explanation:
A phototransistor is a light-sensitive transistor that operates similarly to a regular bipolar junction transistor (BJT), but with one major difference: instead of using an electrical current at the base terminal to control the flow of current between the collector and emitter, it uses light.
In a standard BJT, current flows from the collector to the emitter when a small current is applied to the base. In a phototransistor, light plays the role of that base current. When light strikes the base region, it generates electron-hole pairs, allowing current to flow from the collector to the emitter. Thus, option B is correct — a phototransistor completes a circuit when light strikes the base.
Most phototransistors are two-terminal devices (collector and emitter only) with the base left exposed to light. The base is typically not electrically connected; instead, it is optically activated. When photons hit the base region, they provide the energy needed to allow electrons to move, effectively turning the transistor “on” and allowing current to flow.
The amount of current that flows through the phototransistor depends on the intensity of the light. So while the phototransistor can act like a switch, it can also function in an analog mode, where the output current varies proportionally with the amount of light. However, in the context of the question — focusing on the switching action — the most accurate statement is that the circuit is completed when light strikes the base.
To summarize:
- Emitter and collector are part of the current path.
- Base is light-sensitive and controls the switching.
- When light hits the base, it “turns on” the transistor.
- This enables current flow from collector to emitter, completing the circuit.
