Bipolar Junction Transistor(BJT) Characteristics


BJT Characteristics

The common characteristics of BJT are,

At Common Base Configuration,
  • Low Input Impedance
  • High Output Impedance
  • Zero(0) degree phase shift
  • High Voltage Gain Low Current Gain
  • Low Power Gain

At Common Emitter Configuration,
  • Medium Input Impedance
  • High Output Impedance
  • 180-degree phase shift
  • Medium Voltage Gain
  • Medium Current Gain
  • Very High Power gain

At Common Collector Configuration,
  • High Input Impedance
  • Low output impedance
  • Zero(0) degree phase shift
  • Low Voltage Gain
  • High Current Gain
  • Medium Power gain

General Current-Voltage or IV Characteristics of BJT,

The general IV characteristics of BJT are divided into five parts,
  1. Cutoff Region
  2. Active Region
  3. Quasi Saturation Region
  4. Hard saturation Region
  5. Breakdown Region


Cutoff Region

The cutoff region is that in which the BJT provides a very high resistance and it does not conduct any current. The cutoff region happens when there is no voltage in the base terminal or the applied voltage is lower than the threshold voltage of that BJT. We can say it acts as an open circuit.


Active Region

In this region the transistor conducts current. And the increasing or decreasing of the flow of current happens linearly. The collector current Ic increases linearly with the increase of base voltage or base current. In this region, the dynamic resistance offered by the transistor is more and the maximum power dissipation also happens in this region. Sometimes the active region is also called the ohmic region.


Quasi Saturation Region

This region occurs when the BJT operated with a very high-frequency signal. In this region, both junctions are forward biased. As the transistor does not goes into a deep saturation region so the changes in flowing current happen. Generally, this region exists due to the lightly doped drift layer. In this region, the BJT offers low resistance than the active region so the power loss happens very low. Instant turn on or instant turn off is possible for this region.


Hard Saturation Region

The hard saturation region is also known as the deep saturation region. In this region, the BJT will act s a completely closed switch. As the resistance offered by the transistor is minimum so the power dissipation is also very less. The BJT goes from the active region to the saturation region when the base voltage or base current is increased. In simple words, saturation is that where the flow of current through the transistor is maximum and it cannot be increased by increasing the base voltage. In this region, the transistor takes more time to turn off so it is only suitable for low-frequency applications. In this region also, both junctions are forward biased. In the saturation region, the collector current is not proportional to the base current as the base current control the collector current.


Breakdown Region

The Breakdown region is where the transistor conducts current even when the base current or base voltage is removed. It happens when a large voltage is applied to the transistor above its capacity. 


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Bipolar Junction Transistor(BJT) Characteristics Bipolar Junction Transistor(BJT) Characteristics Reviewed by Author on 6/10/2022 Rating: 5