onsemi BC858ALT1 PNP General-Purpose Amplifier: Datasheet, Pinout, and Application Circuits
The BC858ALT1 from onsemi is a widely used PNP bipolar junction transistor (BJT) designed for general-purpose amplification and switching applications. Housed in a compact SOT-23 package, it is the complementary PNP type to the popular NPN BC848 series, making it an ideal building block for complementary output stages in audio amplifiers, signal processing, and various control circuits.
This article provides a detailed overview of its key specifications, pinout configuration, and practical application circuits.
Datasheet Overview and Key Specifications
The BC858ALT1 is characterized by its low current and low voltage operation, making it suitable for driver stages and small-signal amplification in portable and battery-operated devices.
Some of the most critical absolute maximum ratings and electrical characteristics from its datasheet include:
Collector-Emitter Voltage (VCEO): -30 V. This defines the maximum voltage that can be applied across the collector and emitter terminals with the base open.
Collector-Base Voltage (VCBO): -30 V.
Emitter-Base Voltage (VEBO): -5 V.
Continuous Collector Current (IC): -100 mA. The maximum continuous current the collector can handle.
Total Power Dissipation (Ptot): 250 mW at 25°C. This is the maximum power the device can dissipate without exceeding its operating temperature.
DC Current Gain (hFE): Typically ranges from 125 to 800 at IC = -2 mA and VCE = -5 V, grouped into gain categories (e.g., A, B, C).
Transition Frequency (fT): Minimum 100 MHz. This indicates its good high-frequency performance, suitable for amplifying mid-range RF and audio signals.
These specifications highlight the transistor's role as a robust and versatile component for managing small signals and power levels.
Pinout Configuration
Correctly identifying the pinout is crucial for circuit design. The BC858ALT1 comes in a SOT-23 surface-mount package. The pinout, when viewed from the top with the flat side facing up, is as follows:
Pin 1 (Emitter): This is the input for the majority charge carriers (holes in a PNP) and is typically connected to the positive supply rail or ground, depending on the circuit configuration.
Pin 2 (Base): The control terminal. A small current flowing out of the base controls a larger current flowing out of the collector.
Pin 3 (Collector): The output terminal, which collects the charge carriers. It is usually connected to the load.
Warning: Always consult the manufacturer's datasheet for your specific lot code, as pinouts can sometimes vary between manufacturers or similar part numbers.
Application Circuits
The BC858ALT1 is incredibly flexible. Here are two common circuit configurations:
1. Common-Emitter Amplifier
This is the most fundamental amplifier circuit, providing voltage gain. A small AC input signal at the base (through a coupling capacitor) is amplified, and an inverted, larger output signal is taken from the collector.
Biasing: Resistors R1 and R2 form a voltage divider to set the correct base bias for linear operation.
Gain: The voltage gain is approximately equal to - (RC / RE), where RC is the collector resistor and RE is the emitter resistor.
Applications: Audio pre-amplifiers, signal processing stages, and sensor interfaces.
2. PNP Switch (High-Side Switch)
The BC858ALT1 is excellent for switching loads such as LEDs, relays, or motors. In this configuration, the transistor operates in either cutoff (off) or saturation (on) mode.
Operation: When the control input (e.g., from a microcontroller) is pulled low (0V), current flows out of the base, turning the transistor ON. This allows current to flow from the positive supply (VCC), through the load, and into the collector and out the emitter to ground. A base resistor (RB) is critical to limit the base current.
Applications: Controlling power to peripherals, LED drivers, and relay drivers.
ICGOODFIND: The onsemi BC858ALT1 stands out as a fundamental and highly reliable PNP transistor for designers. Its excellent gain linearity, compact SOT-23 package, and complementary pairing with the BC848 series make it a go-to component for a vast array of analog circuits, from audio amplification to digital switching and beyond. Always cross-reference the official datasheet for critical design decisions.
Keywords: PNP Transistor, General-Purpose Amplifier, SOT-23 Package, Current Gain (hFE), Switching Application.
