MMBT4403 | datasheet | fairchild | transistor onsemi

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MMBT4403

The MMBT4403 is a small-signal PNP transistor in a compact SOT-23 package—perfect if you’re working with limited space. It’s basically the surface-mount version of the popular 2N4403. Ideal for tasks like switching signals, level shifting, or amplifying small currents, it comfortably handles up to 600mA and voltages around 40V. Great for driving LEDs, small relays, or other low-power loads. Paired with the NPN version (MMBT4401), you’ll often find it in consumer electronics, communication gear, or industrial boards. It’s affordable, responsive, and easy to use.

MMBT4403 Pinout

When you’re using the MMBT4403 transistor, remember it’s a PNP type, so your base voltage needs to be lower than the emitter to switch it on. Usually, you’ll connect the emitter to your positive power supply, the base through a resistor down to your control signal, and the collector to your load. Once the base voltage drops low enough, current flows smoothly from emitter to collector. Because it’s tiny, keep it away from high-power tasks, and always use a diode for protection if you’re driving inductive loads like relays.

MMBT4403 Equivalent PNP Transistor

If you’re replacing an MMBT4403 transistor, your best bets are MMBT2907A or BC857B—they’re almost identical in specs and usually drop right in. But keep in mind, BC857B can only handle around 100mA, which means it might struggle with bigger loads. While MMBT5401 or MMBTA92 handle higher voltages, their gain is lower, so they aren’t great choices if you need strong amplification. Always double-check your circuit’s voltage, current, and gain needs before swapping these transistors.

MMBT4403 Amplifier Circuit Schematic

The circuit you’re looking at uses an MMBT4403 transistor in a common-base amplifier setup. Here, the base is your input, emitter goes to ground, and collector connects through a 200Ω resistor to a -30V supply, acting as your output. This setup gives low input impedance and is great for high-frequency signals. It doesn’t amplify current much, but it provides strong voltage gain with a 180° phase shift. To get the best results, ensure a low-impedance input source, and consider adding a capacitor or buffer at the output to keep your signal stable.

MMBT4403 Switching Transistor Usage

The MMBT4403 transistor is great when you’re building simple low-side switches to control LEDs, relays, or buzzers. Just connect its emitter to your positive supply (e.g., +12V), and the collector to your load, then ground the other side of the load. Drive the base from your MCU through a resistor (1kΩ to 10kΩ is fine). When your MCU sends a low signal, the transistor switches on and powers the load; a high signal switches it off. Keep the current under 500mA, and add a diode if you’re controlling relays. It’s cheap, reliable, and perfect for small switching tasks.

MMBT4403 SMD Footprint Wiring

The MMBT4403 transistor comes in an easy-to-solder SOT-23 package. When looking at it from the top (with markings facing you), the L-shaped pin on the bottom-left is the base (pin 1). Clockwise from there are the emitter (pin 2) and collector (pin 3). Connect the base through a resistor (~1kΩ) to your control signal, emitter usually goes to your positive voltage, and collector connects to your load then ground. For inductive loads like relays or motors, put a diode across the collector to handle voltage spikes. Keep current under 600mA for safe operation.

MMBT4403 Base Resistor Calculation

When using the MMBT4403 transistor as a switch, the base resistor is key. Let’s say you’re driving a 200mA load from a 3.3V MCU. With a conservative gain (hFE) around 100, you’d want about 2mA at the base—double that for safety, making it 4mA. The resistor is then (3.3V – 0.7V) ÷ 4mA, around 650Ω. Practically, you can use anything from 560Ω to 680Ω. Avoid values too high (less current, poor switching) or too low (risk damaging your transistor or MCU).