Which device is used in circuits to control a larger current by using a smaller current for operation?

• A. Diode
• B. Transformer
• C. Resistor
• D. Transistor

Answer: (D)

Transistors act as current amplifiers, letting a small input current control a much larger output current in the circuit. In a bipolar transistor, the collector current is roughly the current gain (β) times the base current, so a tiny base current can regulate a much larger collector-emitter current to drive a load. This makes the transistor ideal for switching and amplification: a small control signal from a microcontroller or sensor can turn on or drive a high-current load like a motor or LED string.

Diodes don’t provide amplification; they simply conduct in one direction with a fixed forward drop. Resistors are passive and merely limit or drop voltage and current without enabling a separate, amplified control of a larger current. Transformers transfer energy between circuits and can change current and voltage, but they rely on AC operation and mutual induction, not a small control current directly governing a larger current in the same way a transistor does.