onsemi KST4123MTF

Overview

KST4123MTF from ON Semiconductor (formerly Fairchild) is an NPN epitaxial silicon general-purpose transistor in SOT-23 package, rated for 30 V V_CEO, 200 mA I_C, and 350 mW P_C; used in low-power switching and amplification circuits such as signal buffering in portable audio and sensor interface stages.

For engineers reviewing the KST4123MTF datasheet, pinout, applications, or equivalent options, key selection criteria include DC current gain (h_FE = 50–150 at I_C = 2–50 mA), saturation voltages (V_CE(sat) = 0.3 V, V_BE(sat) = 0.95 V @ I_C/I_B = 50/5 mA), and 250 MHz f_T for mid-frequency small-signal use.

Technical Context

The KST4123MTF operates as a discrete NPN bipolar junction transistor with fixed-emitter configuration and no integrated biasing or protection. Its junction-to-ambient thermal resistance of 357°C/W reflects SOT-23's limited power dissipation capability under natural convection.

It delivers verified DC current gain across two operating points: h_FE ≥ 50 at V_CE = 1 V, I_C = 2 mA and h_FE ≥ 25 at V_CE = 1 V, I_C = 50 mA, supporting both small-signal amplification and moderate-current switching roles.

Key Specifications

Pinout & Package

SOT-23 surface-mount plastic package with gull-wing leads; compact 2.90 mm × 1.30 mm footprint and 1.14 mm height; optimized for automated placement and reflow soldering in space-constrained PCBs.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN general-purpose transistor applications.

Compared with MMBT4123 and BC846B, the KST4123MTF offers a balanced combination of low noise (6 dB), moderate gain (50–150), and tight saturation voltage (0.3 V), making it optimal for cost-sensitive, space-constrained applications requiring predictable mid-frequency behavior without premium gain or voltage headroom.

Availability

KST4123MTF is available at Aetrix Electronics and suitable for portable audio preamps, microcontroller GPIO drivers, signal level shifting, and low-power sensor interfaces requiring stable component supply and consistent parametric performance across production batches.

Supply support for KST4123MTF includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

ON Semiconductor is a global semiconductor supplier delivering energy-efficient, intelligent power and sensing solutions for automotive, industrial, cloud, medical, and edge applications.

The KST4123MTF belongs to ON Semiconductor's general-purpose bipolar transistor family, designed specifically for cost-effective, reliable switching and amplification in consumer and industrial electronics where SOT-23 footprint and predictable DC/AC characteristics are essential.

FAQ

What is the maximum collector current rating for KST4123MTF?

The KST4123MTF has a maximum continuous collector current (I_C) rating of 200 mA at T_A = 25°C. This rating assumes proper PCB copper area and ambient conditions; derating is required above 25°C per its 357°C/W junction-to-ambient thermal resistance. Exceeding 200 mA risks thermal runaway or permanent degradation of the KST4123MTF device.

Does KST4123MTF support low-noise amplification?

Yes, the KST4123MTF specifies a noise figure of 6 dB at V_CE = 5 V, I_C = 100 µA, R_S = 1 kΩ, and 10 Hz–15.7 kHz bandwidth. This makes the KST4123MTF suitable for low-level analog signal amplification, such as microphone preamplifiers or sensor buffers, where preserving signal-to-noise ratio is critical.

What is the pin configuration of KST4123MTF in SOT-23?

The KST4123MTF uses standard SOT-23 pinout: Pin 1 is Base, Pin 2 is Emitter, and Pin 3 is Collector. This configuration is confirmed in the original Fairchild datasheet Rev. A2 and matches industry-standard SOT-23 orientation for NPN transistors. Correct orientation is essential to avoid reverse-bias damage during circuit operation.

Can KST4123MTF replace BC847 in existing designs?

The KST4123MTF is not a direct replacement for BC847 due to differences in h_FE range (KST4123MTF: 50–150 vs. BC847: 110–800) and noise performance (KST4123MTF specifies 6 dB NF; BC847 does not). While both are SOT-23 NPN transistors, circuit validation is required-especially in gain-critical or low-noise applications-before substituting KST4123MTF for BC847.

What is the typical V_CE(sat) of KST4123MTF under switching conditions?

The KST4123MTF exhibits a typical collector-emitter saturation voltage (V_CE(sat)) of 0.3 V when operated at I_C = 50 mA and I_B = 5 mA. This low saturation voltage reduces conduction losses and self-heating, making the KST4123MTF well-suited for efficient low-voltage switching in battery-powered systems.