The Effect of Input Emitter Voltage on Output Collector Voltage in a Common Base Configuration

Understanding the behavior of Bipolar Junction Transistors (BJTs) in different configurations is fundamental in the field of electronics. The common base (CB) configuration of a BJT is particularly intriguing due to its unique characteristics, especially when it comes to the relationship between input and output voltages. In this article, we explore how an increase in the input emitter-base voltage (VEB) affects the output collector-base voltage (VCB) in a BJT operating in a common base configuration.

Introduction to BJT Configurations

BJTs can be configured in three main ways: common emitter (CE), common collector (CC), and common base (CB). Each configuration has distinct characteristics and is used for different applications. The common base configuration is notable for its high input impedance and low output impedance, making it suitable for certain types of amplifiers.

The Role of the Emitter-Base Voltage in BJT Configurations

In a common base configuration, the emitter-base voltage (VEB) is a critical parameter that influences the operation of the BJT. When VEB is increased, it typically leads to an increase in the emitter current (IE) due to the exponential relationship defined by the diode equation. In a BJT, the collector current (IC) is approximately equal to the emitter current (IE) under the assumption of negligible base current.

Output Voltage Behavior in Common Base Configuration

The output voltage in a common base configuration (VCB) is influenced by the collector current and the load connected at the collector. If the collector is connected to a load, such as a resistor, the output voltage across the load will depend on the collector current. As the collector current (IC) increases, the voltage drop across the load resistor also increases, causing the collector voltage (VC) to drop. This relationship is expressed by the equation: ( V_C V_{CC} - I_C cdot R_L ), where ( V_{CC} ) is the supply voltage and ( R_L ) is the load resistance.

Summary

Increasing the emitter-base voltage (VEB) in a BJT operating in a common base configuration leads to an increase in the emitter current (IE), which in turn results in an increase in the collector current (IC). If the collector current (IC) increases, the collector voltage (VC) generally decreases due to the voltage drop across the load resistor. Therefore, increasing the input voltage does not typically lead to an increase in the output voltage; instead, it usually results in a decrease in the collector voltage due to increased current flow through the load.

Differences Between PNP and NPN Transistors

The behavior of a BJT in a common base configuration can vary based on whether it is a PNP or an NPN transistor. A more detailed analysis would require a specific circuit diagram. For a more straightforward answer, a BJT in a common base configuration typically exhibits a non-inverting voltage gain.

Common Base Configuration: Non-Inverting Amplifier

For a BJT in a common base configuration, the output voltage (VCB) increases when the input voltage (VEB) is increased. This is because the less forward-biased emitter-base junction results in a decreased collector current, which, in turn, reduces the voltage drop across the load resistor. This makes the collector node voltage increase towards the supply voltage (VCC), contributing to an increase in the output voltage.

Further Reading

For a more detailed mathematical treatment of the common base configuration, refer to the following resources:

These resources provide a comprehensive understanding of the voltage gain and other key parameters in common base configurations.

By exploring these configurations and their behaviors, engineers and students can better understand the fundamental principles of BJT operation.