2N2222A

The 2N2222A is one of the oldest and most ubiquitous NPN transistors, originally introduced by Motorola in the 1960s and now available from onsemi, Diotec, STMicroelectronics, and dozens of other manufacturers. The ‘A’ suffix version has a 40V VCEO rating (vs. 30V for the non-A 2N2222), making it suitable for 24V systems. With 600mA collector current, it sits between small-signal transistors like the BC547 (100mA) and power transistors like the TIP122 (5A). The 625mW power dissipation in TO-92 limits it to low-power applications without a heatsink. The 100-300 hFE range at 150mA provides substantial current gain, allowing a 5mA base current from a microcontroller GPIO to switch up to 500mA loads (with some saturation voltage). The 300MHz transition frequency makes it usable for RF applications up to approximately 30MHz (fT/10 as a practical rule). The complementary PNP transistor is the 2N2907A. Pin 1 is Emitter, Pin 2 is Base, Pin 3 is Collector (TO-92, flat face toward you, pins down).

The 2N2222A is an NPN bipolar junction transistor. Current flows from collector to emitter (conventional current direction) when base current is applied. The relationship IC = hFE × IB defines the current gain; at IC=150mA, hFE is typically 150-250, meaning IB=0.6-1.5mA produces IC=150mA. For switching applications, the transistor must be driven into saturation: IB must exceed IC/hFE by a factor of 2-10 (overdrive factor) to ensure VCE drops to VCE(sat) ≈ 0.3V. For a 150mA load, IB should be at least 3mA (IC/hFE = 1.5mA × 2 overdrive). For a 500mA load, VCE(sat) rises to 1.0V because hFE drops at high current; IB must be 50mA (10% of IC) for the datasheet-specified saturation voltage. The base-emitter junction is a forward-biased diode with VBE ≈ 0.7V; the base resistor value is RB = (Vdrive − VBE) / IB. For a 5V MCU driving a 150mA load: IB = 3mA, RB = (5V − 0.7V) / 3mA ≈ 1.5kΩ. The collector current cannot exceed 600mA continuously, and total power dissipation must stay below 625mW: P = VCE × IC + VBE × IB. At saturation: P ≈ 0.3V × 150mA = 45mW, well within limits.

2N2222A pin assignment (TO-92-3, flat face toward you, pins down): Pin 1 (left) = Emitter, Pin 2 (center) = Base, Pin 3 (right) = Collector. The A suffix version has tighter hFE grouping and higher 40V VCEO vs 30V for 2N2222. Complementary PNP: 2N2907A (-60V/-600mA). Metal-can TO-18 version: 2N2222A (same die in TO-18 with Pin 1=E, Pin 2=B, Pin 3=C from bottom view). SMD equivalent: MMBT2222A (SOT-23) for surface-mount designs.

• MCU Relay Driver: MCU GPIO → 1kΩ base resistor → 2N2222A base; collector to relay coil; emitter to GND; flyback diode across coil; IC=~100mA
• LED Switch: MCU → 2.2kΩ → base; collector to LED+resistor; emitter GND; switches up to 200mA LED strings
• Logic Inverter: Input → base resistor; output at collector with pull-up; inverts signal with current gain
• Emitter Follower: Input → base; output at emitter; voltage gain ≈ 1; current gain = hFE; low output impedance buffer
• Small Motor Drive: PWM → base resistor; collector to 12V motor; emitter GND; up to ~400mA with heatsink on tab

2N2222A is a 40V/600mA NPN general-purpose transistor in TO-92. The PN2222A provides the same specs with different pinout (EBC vs CBE for metal can). The MMBT2222A is the SOT-23 surface-mount equivalent. The BC547B offers higher gain (200-450 hFE) at lower 45V/100mA. The 2N3904 provides 40V/200mA at lower cost for small-signal applications. The MPS2222A from onsemi is the SOT-23 equivalent. The 2N4401 provides 40V/600mA with higher gain in TO-92. For higher power, TIP122G Darlington offers 100V/5A with 1000x hFE.