Infineon BCR133S: A Low-Drop Emitter-Follower Transistor Optimized for Linear Voltage Regulation

In the realm of power management, linear voltage regulators remain a cornerstone for applications demanding clean, low-noise power supplies. While monolithic IC regulators are prevalent, discrete solutions offer unparalleled flexibility for custom designs. The Infineon BCR133S stands out as a specialized component engineered precisely for this role: a low-drop emitter-follower transistor that serves as an efficient and robust pass element in linear regulator circuits.

The core function of any linear regulator is to maintain a constant output voltage despite variations in input voltage or load current. This is achieved by dissipating excess power across a series pass element. The BCR133S is not a standard bipolar transistor; it is an integrated device that combines an NPN transistor with a base-emitter resistor network. This integrated bias resistor transistor (BRT) simplifies circuit design by reducing the external component count, enhancing reliability, and saving board space.

A key advantage of the BCR133S is its low saturation voltage, which translates directly into a low dropout performance. In a regulator configuration, the dropout voltage is the minimum voltage required across the regulator to maintain regulation. A lower dropout voltage means the regulator can continue to function correctly even when the input voltage is only slightly higher than the output voltage. This is critical for battery-powered applications where maximizing usable energy is paramount, as it allows the system to operate until the battery is nearly depleted.

Furthermore, the integrated base-emitter resistor provides a defined discharge path for the base charge, which improves switching behavior and noise immunity. This makes the BCR133S more robust against parasitic oscillations and less susceptible to external noise compared to a discrete transistor solution. Its high current gain (hFE) also minimizes the drive current required from the error amplifier (e.g., an op-amp or a shunt reference like a Zener diode), leading to a more efficient control loop.

Typical applications extend beyond classic voltage regulators. The BCR133S is exceptionally well-suited for:

Post-regulation for switching power supplies to filter out residual noise.

Constant current sources for LED driving or battery charging.

Interface buffering between high-impedance control ICs and higher-current loads.

ICGOODFIND Summary: The Infineon BCR133S is a highly specialized, space-efficient component that excels as a pass transistor in low-dropout linear voltage regulators and constant current sources. Its integrated bias resistors simplify design, enhance stability, and enable efficient operation from input voltages very close to the desired output level, making it an ideal choice for power-sensitive and noise-averse applications.

Keywords: Low-Dropout, Linear Voltage Regulation, Emitter-Follower, Integrated Bias Resistor Transistor, Saturation Voltage