BJT | Bipolar Junction Transistor Basics

The transistor is a three-terminal semiconductor device commonly used to amplify a signal or switch a circuit on and off.

Amplification is the process of taking a small signal and increasing the signal size. Transistors are used as switches in electric motor drives to control the voltage and current applied to motors. Transistors are capable of extremely fast switching with no moving parts.

There are two general types of transistors in use today: the bipolar transistor (often referred to as a bipolar junction transistor, or BJT) and the field-effect transistor (FET).

Another common use of transistors is as part of integrated circuits (ICs). As an example, a computer microprocessor chip may contain as many as 3.5 million transistors.

BJT | Bipolar Junction Transistor Basics

 In their most basic form, bipolar transistors essentially consist of a pair of PN junction diodes that are joined back to back as illustrated in Figure 1.

bjt | bipolar junction transistor basics
Fig. 1

It consists of three sections of semiconductors: an emitter (E), a base (B) and a collector (C). The base region is very thin, so a small current in this region can be used to control a larger current flowing between the base and collector regions

There are two types of standard BJT transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. NPN and PNP transistors operate in a similar manner, their biggest difference being the direction of current flow through the collector and emitter.

Bipolar transistors are so named because the controlled current must go through two types of semiconductor material, P and N. The BJT is a current amplifier in that a current flow from the base to the emitter results in a larger flow from the collector to the emitter.

BJT current amplification, bipolar junction transistor operation
Figure 2. BJT current amplification.

This, in effect, is current amplification, with the current gain known as the beta of the transistor. The circuit shown in Figure 2 illustrates the way in which a BJT is used as a current amplifier to amplify the small current signal from a photovoltaic sensor. The operation of the circuit can be summarized as follows:

• The transistor is connected to two different DC voltage sources: the supply voltage and the voltage generated by the photovoltaic sensor when exposed to light.

• These voltage supplies are connected so that the base-emitter junction is forward-biased and the collector-emitter junction is reverse-biased,

• Current in the base lead is called the base current, and current in the collector lead is called the collector current.

• It is called the common-emitter transistor configuration because both the base and collector circuits share the emitter lead as a common connection point.

• The amount of base current determines the amount of collector current.

• With no base current—that is, no light shining on the photovoltaic sensor—there is no collector current (normally off).

• A small increase in base current, generated by the photovoltaic sensor, results in a much larger increase in collector current; thus, the base current acts to control the amount of collector current.

• The current amplification factor, or gain, is the ratio of the collector current to the base current; in this case 100 mA divided by 2 mA, or 50.

Transistor as a Switch

When a transistor is used as a switch, it has only two operating states, on and off. Bipolar transistors cannot directly switch AC loads and they are not usually a good choice for switching higher voltages or currents.

In these cases a relay in conjunction with a low-power transistor is often used. The transistor switches current to the relay coil while the coil contacts switch current to the load.

BJT switching of an AC load, bipolar junction transistor working principle
Fig. 4. BJT switching of an AC load.

The circuit shown in Figure 4 illustrates the way in which a BJT is used to control an AC load. The operation of the circuit can be summarized as follows:

• A low-power transistor is used to switch the current for the relay’s coil.

• With the proximity switch open, no base or collector current flows, so the transistor is switched off. The relay coil will be de-energized and voltage to the load will be switched off by the normally open relay contacts.

• When the transistor is in the off state, the collector current is zero, the voltage drop across the collector and emitter is 12 V, and the voltage across the relay coil is 0 V.

• The proximity sensor switch, on closing, establishes a small base current that drives the collector fully on to the point where it is said to be saturated, as it cannot pass any more current.

• The relay coil is energized and its normally open contacts close to switch on the load.

• When the transistor is in the on state, collector current is at its maximum value and the voltage across the collector and emitter drops to near zero while that across the relay coil increases to approximately 12 V.

• The clamping diode prevents the induced voltage at turnoff from becoming high enough to damage the transistor.

Darlington Transistor

Darlington transistor as part of a touch-switch circuit.
Figure 4. Darlington transistor as part of a touch-switch circuit.

The Darlington transistor (often called a Darlington pair) is a semiconductor device that combines two bipolar transistors in a single device so that the current amplified by the first transistor is amplified further by the second. The overall current gain is equal to the two individual transistor gains multiplied together. Figure 4 shows the Darlington transistor as part of a resistance touch-switch circuit. The operation of the circuit can be summarized as follows:

• The Darlington pairs are packaged with three legs, like a single transistor.

• The base of transistor Q1 is connected to one of the electrodes of the touch switch.

• Placing your finger on the touch plate allows a small amount of current to pass through the skin and establish current flow through the base circuit of Q1 and drive it into saturation.

• The current amplified by Q1 is amplified further by Q2 to switch the LED on.


Like junction diodes, bipolar junction transistors are light-sensitive. Phototransistors are designed specifically to take advantage of this fact. The most common phototransistor is an NPN bipolar transistor with a light-sensitive collector-base PN junction.

When this junction is exposed to light it creates a control current flow that switches the transistor on. Photodiodes also can provide a similar function, but at a much lower gain.

Phototransistor employed as part of an optical isolator.
Fig. 5. Phototransistor employed as part of an optical isolator.

Figure 5 shows a phototransistor employed as part of an optical isolator found in a programmable logic controller (PLC) AC input module circuit. The operation of the circuit can be summarized as follows:

• When the push button is closed, 120 V AC is applied to the bridge rectifier through resistors R1 and R2.

• This produces a low-level DC voltage that is applied across the LED of the optical isolator.

• The zener diode (ZD) voltage rating sets the minimum level of voltage that can be detected.

• When light from the LED strikes the phototransistor, it switches into conduction and the status of the push button is communicated in logic, or low-level DC voltage, to the processor.

• The optical isolator not only separates the higher AC input voltage from the logic circuits but also prevents damage to the processor by line voltage transients.

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