Which function is performed by the pass element in a three-terminal regulator?

• A. To deliver load current with a controlled drop to maintain regulation.
• B. To provide current limiting.
• C. To sense the output voltage.
• D. To generate the reference voltage.

Answer: (A)

The pass element is the series device between input and output that the regulator uses to control how much current is allowed to the load. By adjusting its conduction, it creates just enough voltage drop from input to output so the output stays at the desired level even as input voltage or load current changes. The regulator’s error amplifier and reference set this control: when output tends to rise, the pass element is driven to reduce current (increasing drop); when output tends to fall, it’s driven to allow more current (reducing drop).

Sensing the output voltage is handled by the feedback pathway feeding the error amplifier, not by the pass transistor itself. Generating the reference voltage comes from the regulator’s internal reference circuit, which is separate from the pass element. Current limiting is typically a protective feature that may be implemented in conjunction with the pass element, but the primary role of the pass element is to deliver load current with a controlled drop to maintain regulation.