Transistor (BJT)

A bipolar junction transistor (BJT) is a semiconductor device used to amplify or switch low-power signals in electronic circuits. Typically operated in the active region, it allows a small input current or voltage at the base to control a larger current flow between collector and emitter. They're often used in amplifiers, radios, and signal processing applications.

Fig.: Small signal transistors (BJT) in different housings (Author: Benedikt Seidl)

Fig.: Electronic symbol of a NPN transistor

Fig.: Electronic symbol of a PNP transistor

Applications

Shifting logic levels (e.g., 3.3V MCU to 5V device).
Switching loads: Controlling LEDs, relays, buzzers, or other peripherals.
Amplifying weak sensor signals before feeding into ADC.
Driving speakers or motors in H-bridge-like circuits.
Driving loads with MCU PWM output (e.g., dimming LEDs).

Features/Specifications

A BJT is a type of transistor that uses both electrons and holes (two types of charge carriers) to conduct current. It has three terminals: Emitter, Base, and Collector. BJTs come in two types: NPN and PNP, and they amplify current by using a small base current to control a larger current flowing from collector to emitter. The main difference between NPN and PNP BJTs is the direction of current flow and the required voltage polarity:

Type	Description	Common devices
NPN	Current flows from collector to emitter when a small positive voltage is applied to the base (relative to the emitter).	BC547, BC548, 2N3904, 2N2222, S8050, BC550
PNP	Current flows from emitter to collector when a small negative voltage is applied to the base (relative to the emitter).	BC557, BC558, BC556, 2N3906, 2N2907, S8550

Note: Since there are so many different types of BJTs with a wide range of characteristics, we will use here the two widely used types BC547 and BC557.

	BC547 Spec	BC557 Spec
Transistor Type	NPN	PNP
Collector-Emitter Voltage (V_CEO)	45 V	-45 V
Collector-Base Voltage (V_CBO)	50 V	-50 V
Emitter-Base Voltage (V_EBO)	6 V	-5 V
Collector Current (I_C)	100 mA	-100 mA
Power Dissipation (P_tot)	500 mW
DC Current Gain (h_FE)	110 to 800
Transition Frequency (f_T)	150 MHz
Operating Temperature	-65°C to +150°C

📄 BC547 (NPN) datasheet (45 kB)
📄 BC557 (PNP) datasheet (50 kB)

Connections

Fig.: Terminal diagram of a BJT transistor in a TO-92 package

Pin No	BC547 (NPN)	BC557 (PNP)
1	Collector	Emitter
2	Base	Base
3	Emitter	Collector

Used Components

Setup & Programming

As a basic setup, both an NPN and a PNP transistor are connected to a microcontroller, and a HIGH or LOW signal is alternately applied to the base. This results in a blinking effect with two LEDs due to the different behaviors of the transistors.

Arduino Uno

#define PIN_NPN 2
#define PIN_PNP 3

void setup() {
   pinMode(PIN_NPN, OUTPUT);
   pinMode(PIN_PNP, OUTPUT);
}

void loop() {
  digitalWrite(PIN_NPN, HIGH);
  digitalWrite(PIN_PNP, HIGH);
  delay(500);
  digitalWrite(PIN_NPN, LOW);
  digitalWrite(PIN_PNP, LOW);
  delay(500);
}

ESP32

#define PIN_NPN 23
#define PIN_PNP 21

void setup() {
   pinMode(PIN_NPN, OUTPUT);
   pinMode(PIN_PNP, OUTPUT);
}

void loop() {
  digitalWrite(PIN_NPN, HIGH);
  digitalWrite(PIN_PNP, HIGH);
  delay(500);
  digitalWrite(PIN_NPN, LOW);
  digitalWrite(PIN_PNP, LOW);
  delay(500);
}

Further information

Last edited by Christian Grieger on 2025-05-12

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Applications
Features/Specifications
Connections
Used Components
Setup & Programming
Further information