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BJT transistors (NPN/PNP): key features and uses

BJT transistors (Bipolar Junction Transistors) are widely used for amplification (analog stages) and switching (drivers, interfaces, discrete logic). The main families are NPN and PNP, with different drive polarity and common configurations (common emitter, emitter follower, common base).

Typical applications

  • Load switching: relays, small motors, solenoids, LEDs and lamps, interfacing to industrial inputs.
  • Power stage driving: pre-drivers for MOSFETs/IGBTs, buffers, level shifting.
  • Amplification: audio, sensors, signal conditioning, oscillators and (model-dependent) RF stages.
  • Discrete logic: inverters, latches and simple timing circuits.

How to select a BJT

Start from the intended function (switch or amplifier) and operating conditions (voltage, current, frequency and dissipation).

  • Type: NPN or PNP, according to the schematic and drive polarity.
  • Vceo / Vces: maximum collector-emitter voltage. Keep margin for transients.
  • Ic: continuous (and if relevant, pulsed) collector current. Consider inrush current for the load.
  • Power dissipation (Pd): depends on package, mounting and ambient temperature. Heatsinking may be required.
  • Gain (hFE): important for amplification and for base-current sizing. In switching, BJTs are often used in saturation with margin.
  • Speed / frequency: fT, rise/fall times and saturation charge affect high-speed performance.
  • Package and pinout: TO-92, SOT-23, TO-220, DPAK, etc. Check E-B-C pin order and PCB compatibility.

Practical notes for switching

  • Base resistor: size it to ensure saturation with margin while staying within the drive limits.
  • Inductive loads: add a flyback diode across relays/motors to clamp voltage spikes.
  • Thermal: evaluate Vce(sat) and Ic; small packages can become thermally limiting.

FAQ

BJT or MOSFET to drive a load?
It depends on voltage/current and the drive source: BJTs are simple and robust but require base current; MOSFETs are voltage-driven and often dissipate less in switching, especially at higher currents.

Can I replace a BJT with an equivalent part?
Yes, if key parameters (Vce, Ic, Pd, hFE and package/pinout) match and the circuit function is the same. Also consider “low-noise”, “high-speed” or “power” variants when required.

BF178 Transistor NPN 115V 50mA 120MHz NOS

SKU: 210BF178
In Stock
Silicon planar transistors in TO-39 houses for high voltage video applications. Brand SGS ATES and SESCOSEM.
€0.89  VAT excluded

2N3055 NPN Transistor 60V 15A 115W TO-3

SKU: 2102N3055
In Stock

General-purpose transistor for high-current, medium-power applications. NPN junction , 60V voltage, 15A current, TO-3 case. Brand: CDIL

Product codes: 2N3055, 2000000057255, 005725

€2.42
 / €2.55
 VAT excluded

AC141K Germanium transistor NPN 32V 1,2A 0,5MHz NOS

SKU: 210AC141K
In Stock

Germanium transistor in houses X04. Brand SGS ATES.

€1.38  VAT excluded

BC109C Transistor NPN 25V 200mA 150MHz TO-18 CDIL

SKU: 210BC109C
Out of Stock

BJT transistor for small signals realized with the Silicon Planar process. Suitable for low noise audio amplifiers.

€0.00  VAT excluded

BCY56 NPN Transistor 45V 100mA 85MHz TO-18 Philips NOS

SKU: 210BCY56
In Stock

Low-noise BIP silicon NPN bipolar transistor for use in audio, switching and measurement devices.

€1.84  VAT excluded

BC172A Transistor NPN 25V 100mA TO-92 Gold Terminals ITT NOS

SKU: 210BC172A
In Stock

BJolar bipolar transistor for use in signal amplification. Ideal for Vintage amplifiers

€0.24  VAT excluded

BC321 Transistor PNP 30V 150mA 100MHz TO-92 NOS

SKU: 210BC321
In Stock

Bipolar silicon BJT transistor with PNP junction complementary to the BC318.

Similar transistors: BC213, BC257, BC307, BC557, 2SB725

€0.71  VAT excluded

BC327-25 PNP Transistor 45V 800mA 100MHz TO-92 CDIL

SKU: 210BC327-25
In Stock

Bipolar silicon BJT transistor with PNP junction complementary to the BC337.

Similar Transistor: BC638, BC640, 2SB1116, 2SB1116A, 2SB143

€0.08
 / €0.10
 VAT excluded

BC327-25 PNP Transistor 45V 800mA 100MHz TO-92 Diotec

SKU: 210BC327-25N-DI
In Stock

Bipolar silicon BJT transistor with PNP junction complementary to the BC337.

Similar Transistor: BC638, BC640, 2SB1116, 2SB1116A, 2SB143

€0.12
 / €0.16
 VAT excluded

BC327-40 PNP Transistor 45V 800mA 100MHz TO-92 SEMTECH

SKU: 210BC327-40
Out of Stock

Bipolar silicon BJT transistor with PNP junction complementary to the BC337.

Similar Transistor: BC638, BC640, 2SB1116, 2SB1116A, 2SB143

€0.10
 / €0.12
 VAT excluded

BC337-25 NPN Transistor 45V 800mA 200MHz TO-92 Diotec

SKU: 210BC337-25
In Stock

Bipolar silicon BJT transistor with NPN junction complementary to BC327.

Similar Transistor: BC637, BC639, 2SD1616, 2SD1616A, 2SD2181

€0.23
 / €0.30
 VAT excluded

BC369 PNP Transistors 20V 2A 60MHz TO-92

SKU: 210BC369
In Stock

Bipolar silicon BJT transistor with NPN junction complementary to the BC368.

Similar transistors: BC327, BC328, BC636, 2SB909, 2SB1116, NTE294.

€0.60  VAT excluded

BC516 Darlington Transistor PNP 30V 400mA 220MHz TO-92

SKU: 210BC516
In Stock

The BC516 is a silicon PNP Darlington transistor suitable for use in amplification applications.

Similar Transistor: MPSA75

€0.17
 / €0.21
 VAT excluded

BC517 Darlington Transistor NPN 30V 400mA 220MHz TO-92

SKU: 210BC517
Out of Stock

The BC517 is a silicon NPN Darlington transistor suitable for use in amplification applications.

Similar Transistor: MPSA25

€0.30
 / €0.36
 VAT excluded

BC546B NPN transistor 65V 100mA 300MHz TO-92

SKU: 210BC546B
In Stock

The BC546 is a general purpose silicon NPN transistor. Its complementary transistor is the BC556.

Similar Transistors: 2SC2240, 2SC2459, 2SC2674, 2SC2675, 2SC3378

€0.08
 / €0.10
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210BC557C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€0.09
 / €0.10
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210TIP31C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€0.48
 / €0.60
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210BDX53C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€1.26
 / €1.58
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210BDX54C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€0.86
 / €0.91
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210BDX33C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€0.65
 / €0.81
 VAT excluded

BC557C PBP transistor 45V 100mA 150MHz TO-92

SKU: 210BDX34C
In Stock

The BC557C is a PNP silicon transistor for general uses. Its complementary transistor is BC547.

Similar transistors: BC212, BC257, BC307, 2SB725, 2SA1137

€0.56
 / €0.70
 VAT excluded

TIP2955 Transistor BJT PNP 15A 60V 90W TO218 STMicroelectronics

SKU: 210TIP2955
In Stock

15A, 60V, 90W silicon PNP power transistor in TO-218 case, suitable for audio output stages, linear regulators and switching circuits.

€1.23
 / €1.36
 VAT excluded

TIP3055 Transistor BJT NPN 15A 60V 90W TO218 STMicroelectronics

SKU: 210TIP3055-NR
In Stock

15 A, 60 V, 90 W silicon NPN power transistor in TO-218 case, for audio amplifiers, linear power supplies and driving DC loads.

€0.68
 / €0.90
 VAT excluded

MJE15030G Transistor NPN 150V 8A 50W case TO-220 ON Semiconductor

SKU: 210MJE15030
Out of Stock

The MJE15030G is a 150V silicon NPN plastic case built power transistor designed for use as a high frequency driver in audio amplifiers. Its complementary transistor is the MJE15031G .

NPN junction , Vceo 150V, Ic 8A, Case TO-220. Brand: ON Semiconductor

€1.22
 / €1.28
 VAT excluded

MJE15031G Transistor PNP 150V 8A 50W case TO-220 ON Semiconductor

SKU: 210MJE15031
Out of Stock

The MJE15031G is a 150V silicon PNP-type plastic power transistor designed for use as a high frequency driver in audio amplifiers. Its complementary transistor is the MJE15030G .

PNP junction , Vceo 150V, Ic 8A, Case TO-220. Brand: ON Semiconductor

€1.24
 / €1.30
 VAT excluded

Transistor 2SC1330 NPN Vcb 50V 0.1A 125 MHz TO92

SKU: 2102SC1330
In Stock

Transistor 2SC1330 NPN Vcb 50V 0,1A 125 MHz TO92.

€2.20  VAT excluded

NPN Transistor 2N3904 40V 200mA 625mW TO92 preformed terminals

SKU: 2102N3904
In Stock

Transistor bipolare NPN da 40 V e 200 mA in contenitore TO-92 con terminali preformati, ideale per commutazione di piccoli carichi e amplificazione di segnali fino a 100 MHz.

€0.15
 / €0.16
 VAT excluded

2N4400 Bipolar Transistor BJT NPN 60V 0.6A 0.31W TO-92

SKU: 2102N4400
In Stock

2N4400 Transistor bipolare BJT NPN 60V 0,6A 0,31W TO-92

€1.50  VAT excluded

NPN transistor BC237B TO‑92 45 V 100 mA

SKU: 210BC237B
In Stock

Transistor NPN BC237B in encapsulazione TO‑92, 45 V, 100 mA, 350 mW – ideale per amplificatori audio, stadi driver e circuiti a basso rumore.

Codici prodotto: BC237, BC237B, 2000000059822, 005982

€0.38
 / €0.40
 VAT excluded

PNP BF479 silicon transistor 25 V TO-50 RF/VHF NOS

SKU: 210BF479
In Stock

Transistor PNP silicio BF479 in package TO-50, tensione collettore-emettitore -25 V, corrente collettore -50 mA, adatto per stadi RF/VHF fino a circa 800 MHz. Componente NOS (New Old Stock).

€0.82  VAT excluded

NPN Transistor BF680 RF/VHF 25V 300mA TO-50 SGS-ATES NOS

SKU: 210BF680
In Stock

Transistor NPN BF680 SGS-ATES in package TO-50, ideale per stadi RF/VHF. Componente NOS (New Old Stock), perfetto per riparazioni e apparecchi vintage.

€2.08  VAT excluded

BD138 60V 1500mA SOT32 TO126 Power PNP Bipolar Transistor

SKU: 210BD138
In Stock

Transistor bipolare PNP da 60 V e 1,5 A in contenitore SOT-32 equivalente TO-126, ideale per amplificatori audio, stadi driver e sostituzioni in apparecchiature esistenti.

€0.77
 / €0.81
 VAT excluded

NPN Transistor BD139 STMicroelectronics 80V 1.5A SOT32 TO126

SKU: 210BD139
In Stock

80V 1.5A NPN bipolar transistor in SOT-32 package, pre-selected for gain, ideal for audio amplifiers, driver stages and high voltage complementary circuits.

€0.54
 / €0.57
 VAT excluded

PNP Transistor BD140 STMicroelectronics 80V 1.5A SOT32 TO126

SKU: 210BD140
In Stock

Transistor bipolare PNP da 80 V e 1,5 A in contenitore SOT-32, preselezionato in guadagno, ideale per amplificatori audio, stadi driver e circuiti complementari ad alta tensione.

€0.63
 / €0.66
 VAT excluded

Darlington NPN Transistor BD681 STMicroelectronics 100V 4A 40W

SKU: 210BD681
In Stock

Darlington NPN di potenza da 100 V e 4 A con guadagno minimo 750 e diodo integrato, ideale per pilotare relè, motori e carichi con correnti di comando ridotte.

€1.18
 / €1.24
 VAT excluded

PNP Darlington Transistor BD682 STMicroelectronics 100V 4A 40W

SKU: 210BD682
In Stock

Transistor Darlington PNP da 100 V e 4 A con guadagno minimo 750 e diodo di protezione integrato, per commutazione sul lato positivo e stadi push-pull complementari.

€1.04
 / €1.10
 VAT excluded

BJT transistors (NPN/PNP) for switching and amplification in analog and digital circuits. Selection by Vce, Ic, power dissipation, gain (hFE) and package.

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