Bipolar junction transistors (BJTs) are often called current-controlled devices because their output current is controlled by the input current. The reason for this is due to the physical structure of the BJT. BJTs are made up of two types of semiconductor material: n-type and p-type. The n-type material has an excess of free electrons, while the p-type material has a shortage of free electrons. When these two materials are joined together, a p-n junction is formed.

How a BJT Works

When a voltage is applied to the base-emitter junction, the energy from the voltage causes electrons to flow from the n-type emitter region to the p-type base region. This process is called forward biasing. The forward bias causes the depletion layer at the p-n junction to shrink, and the current flowing through the junction increases.

The amount of current that flows through the collector-emitter junction is controlled by the amount of current that flows through the base-emitter junction. This is because the base current controls the size of the depletion layer at the collector-emitter junction. A larger base current creates a smaller depletion layer, which allows more current to flow through the collector-emitter junction.

The relationship between the base current and the collector current is linear over a certain range. This means that the collector current will increase proportionally with the base current, up to a point. After this point, the collector current will start to saturate and will not increase any further, even if the base current is increased.

Benefits of Using BJTs as Current-Controlled Devices

There are several benefits to using BJTs as current-controlled devices, including:

• Linear relationship between base current and collector current: This makes BJTs easy to design and use in electronic circuits.
• High gain: BJTs can provide a high level of gain, which means that a small change in the base current can cause a large change in the collector current.
• Fast switching speeds: BJTs can switch from one state to another very quickly, which makes them ideal for use in high-speed electronic circuits.

Applications of BJTs

BJTs are used in a wide variety of electronic devices, including:

• Amplifiers: BJTs are used in amplifiers to increase the amplitude of a signal.
• Switches: BJTs are used in switches to turn circuits on and off.
• Transistors: BJTs are used in transistors to control the flow of current.

Conclusion

BJTs are versatile electronic devices that can be used in a variety of applications. Their ability to be used as current-controlled devices makes them ideal for use in electronic circuits where a small change in the input current can cause a large change in the output current.

FAQs

• Q: What is the difference between a BJT and a JFET?
  A: BJTs are current-controlled devices, while JFETs are voltage-controlled devices. This means that the output current of a BJT is controlled by the input current, while the output current of a JFET is controlled by the input voltage.
• Q: What are the advantages of using BJTs over JFETs?
  A: BJTs have higher gain and faster switching speeds than JFETs. However, JFETs are less noisy and have a lower input capacitance than BJTs.
• Q: What are some applications of BJTs?
  A: BJTs are used in a wide variety of electronic devices, including amplifiers, switches, and transistors.
• Q: How do you select the right BJT for a particular application?
  A: The selection of the right BJT for a particular application depends on several factors, including the desired gain, switching speed, and noise performance.
• Q: What are some common problems that can occur with BJTs?
  A: Some common problems that can occur with BJTs include thermal runaway, latch-up, and second breakdown.