What Is a BJT?

A Bipolar Junction Transistor is a semiconductor device constructed from three semiconductor regions. These regions form two PN junctions and create three external terminals:

• Base (B)
• Collector (C)
• Emitter (E)

The base is the control terminal, while the collector and emitter form the main current path through the transistor.

The term bipolar refers to the fact that both electrons and holes participate in the conduction process.

BJT Construction

A BJT consists of three semiconductor regions arranged as either NPN or PNP.

NPN Structure

        N
   Collector
        │
        │
        P
      Base
        │
        │
        N
     Emitter

An NPN transistor has an N-type collector, a thin P-type base and an N-type emitter.

PNP Structure

        P
   Collector
        │
        │
        N
      Base
        │
        │
        P
     Emitter

A PNP transistor has a P-type collector, a thin N-type base and a P-type emitter.

The emitter is heavily doped so that it can inject charge carriers efficiently. The base is made relatively thin so that most injected carriers can cross it and reach the collector.

The Three BJT Terminals

Base

The base is the control terminal of a BJT. A relatively small base current can control a much larger collector current when the transistor is operating in its active region.

Collector

The collector collects charge carriers passing through the base region. It normally handles the largest current in a conventional BJT amplifier or switching circuit.

Emitter

The emitter injects charge carriers into the base region. The emitter is normally heavily doped compared with the base.

NPN and PNP Transistors

There are two fundamental BJT polarities:

The two types can perform similar functions, but their required voltage polarities and current directions are complementary.

NPN Transistor Operation

For an NPN transistor operating in its normal active region, the base-emitter junction is forward biased while the base-collector junction is reverse biased.

A small base current permits a much larger collector current to flow.

             +V
              │
             Load
              │
              C
              │
             NPN
              │
              E
              │
             GND

Control ─── B

Increasing the base drive generally increases collector current until the device reaches another operating region or a specified current limit.

PNP Transistor Operation

A PNP transistor operates with polarities opposite to those of an NPN transistor.

The emitter is normally at a more positive potential than the base when the transistor is forward biased in its active region.

             +V
              │
              E
             PNP
              │
              C
              │
             Load
              │
             GND

Control ─── B

PNP transistors are often used in complementary amplifier stages and high-side switching circuits.

The BJT Junctions

A BJT contains two PN junctions:

• Base-emitter junction
• Base-collector junction

The bias condition of these two junctions determines the transistor's operating region.

How a BJT Controls Current

One of the simplest ways to understand a BJT is to consider the relationship between base current and collector current.

For a transistor operating in its active region, collector current can be approximately expressed as:

IC ≈ β × IB

where:

• IC = collector current
• IB = base current
• β = transistor current gain

The relationship is an approximation. The actual gain varies with collector current, temperature, voltage and individual device characteristics.

Current Gain

BJT current gain is commonly represented by β or hFE.

For example, if a transistor has a current gain of 100 and is operating in a region where that approximation is applicable, a base current of 1 mA could correspond to approximately 100 mA of collector current.

In practical switching circuits, designers often avoid relying on the datasheet's maximum or typical hFE value and instead provide sufficient base drive to ensure reliable switching.

Collector Current and Emitter Current

The three terminal currents of a BJT are related by Kirchhoff's current law.

IE = IC + IB

where:

• IE = emitter current
• IC = collector current
• IB = base current

Because base current is normally much smaller than collector current in normal operation, emitter current is usually slightly greater than collector current.

Base-Emitter Voltage

The base-emitter junction behaves similarly to a semiconductor diode.

For a typical silicon BJT operating at ordinary currents, the forward base-emitter voltage is often around 0.6 to 0.7 V, although the actual value changes with current and temperature.

It should therefore not be treated as a fixed voltage.

As temperature increases, the base-emitter voltage required for a given current generally decreases.

BJT as a Switch

A BJT can be used as an electronic switch by operating it primarily between cutoff and saturation.

Cutoff

In cutoff, base drive is insufficient and the transistor conducts very little collector current. The transistor is considered OFF.

Saturation

In saturation, the transistor is strongly driven into conduction and the collector-emitter voltage becomes relatively low.

This makes BJTs useful for controlling:

• Relays
• LEDs
• Small lamps
• Buzzers
• Small motors
• Other electronic loads

BJT Switching Circuit

             +V
              │
             Load
              │
              C
          ┌───┤
Control ─R── B
          └───┤
              E
              │
             GND

The resistor between the control signal and base limits base current. Without appropriate current limiting, excessive base current can damage the transistor or the driving circuit.

BJT as an Amplifier

When a BJT is biased in its active region, changes in base current can produce larger changes in collector current.

This allows the transistor to amplify electrical signals.

Small AC Input
      │
      ▼
    Base
      │
     BJT
      │
      ▼
Larger AC Output

The additional output energy comes from the DC power supply. The transistor controls the transfer of that energy into the output circuit.

Common BJT Configurations

Three traditional BJT amplifier configurations are:

• Common emitter
• Common collector
• Common base

They differ according to which transistor terminal is common to the input and output circuits.

Common-Emitter Configuration

The common-emitter configuration is one of the most widely used BJT amplifier arrangements.

It can provide substantial voltage gain and current gain.

In a typical common-emitter voltage amplifier, the output voltage is inverted relative to the input voltage.

Common-emitter amplifiers are widely used in:

• Audio amplifiers
• Voltage amplifiers
• Signal conditioning
• Oscillators
• Driver stages

Common-Collector Configuration

The common-collector configuration is commonly called an emitter follower.

It generally has:

• High input impedance
• Low output impedance
• Voltage gain close to unity
• Useful current gain

Because of its buffering capability, the emitter follower is useful for driving lower-impedance loads from higher-impedance signal sources.

Common-Base Configuration

In the common-base configuration, the base is the common terminal between the input and output circuits.

This configuration has relatively low input impedance and can provide useful voltage gain and high-frequency performance.

It is sometimes used in RF and other high-frequency amplifier circuits.

BJT Biasing

A BJT amplifier requires an appropriate DC operating point before an AC signal can be amplified correctly.

This process is called biasing.

The bias network establishes the transistor's quiescent operating point, often called the Q-point.

Good biasing helps keep the transistor within the desired operating region and reduces unwanted distortion.

Why Biasing Is Important

Without appropriate biasing, an amplifier transistor may spend too much time in cutoff or saturation.

This can cause:

• Signal clipping
• Distortion
• Incorrect gain
• Unstable operation
• Excessive current

The bias design also needs to account for transistor gain variations and temperature changes.

Emitter Resistor

An emitter resistor is frequently used in BJT amplifier circuits to improve bias stability.

As emitter current increases, the voltage across the emitter resistor also increases. This produces negative feedback that can help oppose changes in collector current.

Emitter resistors are therefore important in many practical transistor amplifiers.

Thermal Behavior

BJTs can exhibit thermal instability under certain bias conditions.

As temperature increases, transistor characteristics change and collector current can increase. The resulting additional power dissipation can raise the temperature further.

This effect is one reason practical BJT circuits use appropriate bias networks, emitter resistors and thermal management.

BJT Power Dissipation

When a BJT simultaneously has significant collector-emitter voltage and collector current, it dissipates power.

A simplified instantaneous relationship is:

P ≈ VCE × IC

The average power must remain within the transistor's thermal and electrical limits.

Power BJTs may require heatsinks or other thermal-management methods.

Complementary BJTs

NPN and PNP transistors are often used together as complementary devices.

This arrangement is especially common in push-pull and Class AB audio amplifier output stages.

A complementary pair allows one transistor to handle one direction of load current while the other handles the opposite direction.

Small-Signal BJTs

Small-signal BJTs are designed primarily for low-power amplification and switching.

Typical applications include:

• Audio preamplifiers
• Sensor circuits
• Signal amplifiers
• Oscillators
• Current sources
• Switching circuits

Power BJTs

Power BJTs are designed to withstand higher current and power levels than small-signal devices.

They have historically been widely used in:

• Audio power amplifiers
• Linear power supplies
• Motor controllers
• Power switching circuits
• Industrial electronics

Modern applications frequently use MOSFETs or IGBTs instead of BJTs for high-efficiency switching, but power BJTs remain important in repair work and many established circuit designs.

High-Frequency BJTs

Some BJTs are designed specifically for high-frequency operation.

Important characteristics include transition frequency, parasitic capacitances, gain at the intended frequency and switching speed.

These devices can be used in:

• RF amplifiers
• Oscillators
• Radio circuits
• High-frequency signal processing

BJT Current Gain and Temperature

BJT gain is not constant. The value specified in a datasheet can vary with collector current, collector-emitter voltage, temperature and individual device characteristics.

For this reason, a circuit should not normally depend on one exact β value for reliable operation.

Bias networks that provide feedback or reduce sensitivity to β variations are commonly preferred in practical designs.

BJT Testing With a Multimeter

A conventional BJT can often be tested with a digital multimeter using diode test mode.

The base-emitter and base-collector junctions behave similarly to semiconductor diode junctions.

For a typical NPN transistor, the base should show a forward diode reading to the emitter and collector when the meter polarity is appropriate.

For a PNP transistor, the required meter polarity is reversed.

A transistor showing a near-short circuit between collector and emitter in both directions may be damaged.

BJT Faults

Common BJT failures include:

• Collector-emitter short circuit
• Base-emitter short circuit
• Base-collector short circuit
• Open junction
• Excessive leakage
• Reduced gain
• Thermal damage

When a transistor fails in a power circuit, the original cause should also be investigated before installing a replacement.

BJT Replacement

When replacing a BJT, do not select a replacement based only on the physical package or transistor number.

Important specifications include:

• Transistor polarity
• Collector-emitter voltage rating
• Collector current rating
• Power dissipation
• Current gain
• Transition frequency
• Switching characteristics
• Package
• Pin configuration
• Safe Operating Area

The manufacturer's datasheet should always be checked before substituting a transistor.

Common BJT Applications

BJT Advantages

• High transconductance for a given current
• Good analog linearity in suitable circuits
• Useful current gain
• Widely available
• Many low-cost devices exist
• Excellent for many small-signal amplifier applications

BJT Limitations

• Requires base current
• Gain varies with operating conditions
• Can suffer from thermal instability
• Stored charge can limit switching speed
• Requires appropriate biasing for linear amplification

Key Points

• A BJT has base, collector and emitter terminals.
• The two main BJT types are NPN and PNP.
• Both electrons and holes participate in BJT operation.
• Base current controls collector current in normal active operation.
• BJTs can operate as switches or amplifiers.
• Cutoff corresponds to the OFF state and saturation to the strongly ON state.
• Common amplifier configurations are common-emitter, common-collector and common-base.
• Proper biasing is essential for linear amplification.
• BJT gain varies with operating conditions.
• Thermal management is important in power BJTs.
• The datasheet should be consulted when selecting or replacing a BJT.