Understanding the Voltage Between the Emitter and Base of a Transistor When Forward Biased

When discussing the operation of a transistor, one fundamental aspect that comes into play is the voltage across the base (B) and emitter (E) terminals, especially when the transistor is forward biased. This voltage is crucial for understanding the transistor's behavior and ensuring a stable circuit operation. In this article, we will delve deep into the concept of the voltage drop between the emitter and base of a transistor when it is forward biased and explore why it cannot be zero even when the base is grounded.

Theoretical Background: Silicon Junction Transistor Voltage Drop

For a silicon junction transistor (BJT), the typical voltage drop between the base and emitter when the transistor is forward biased is approximately 0.7V DC. This voltage drop is a characteristic of the silicon diode junction that forms the base-emitter junction in the transistor. The intrinsic forward voltage of silicon diodes, including the base-emitter junction, is approximately 0.7V.

Formally, this can be expressed as: ( V_{BE} approx 0.7 , text{VDC} ).

Why Voltage Cannot Be Zero Even When Base is Grounded?

One common misconception arises when considering the scenario where the base-emitter junction is connected to a grounded node. It is important to understand that even if the base is grounded, the emitter voltage cannot be zero due to the inherent forward voltage drop across the base-emitter junction. This voltage drop must be maintained to ensure proper operation of the transistor.

Moreover, the base voltage in a forward-biased base-emitter junction is typically set by the supply voltage and the biasing network. If the base is grounded, the emitter will be at a voltage that is 0.7V lower than the collector-to-emitter voltage ( V_{CE} ) (assuming ( V_{CE} ) is positive). This is because the base-emitter junction requires a voltage greater than 0.7V to be forward-biased.

The Role of the Emitter Resistor in Ensuring Stable Circuit Operation

In a practical circuit, the emitter of the transistor is not grounded directly but is often connected to a biasing resistor. This setup ensures that the quasi-stable operating point (Q-point) of the transistor is independent of temperature fluctuations and power supply variations.

The emitter resistor, ( R_E ), plays a crucial role in stabilizing the base current ( I_B ), which in turn stabilizes the collector current ( I_C ). Without the emitter resistor, the quiescent current ( I_B ) would be temperature and power dissipated dependent, leading to instability in the Q-point.

The relationship can be mathematically expressed as:

[I_B frac{V_{CC} - V_{CE} - V_{BE}}{R_E}]

Where ( V_{CC} ) is the collector supply voltage, ( V_{CE} ) is the collector-emitter voltage, and ( V_{BE} ) is the base-emitter voltage (approximately 0.7V for a silicon transistor).

Illustrative Example: Common Emitter Configuration

Let's consider a simple common emitter transistor configuration to understand how the base-emitter voltage works in practice. Assume the following values for a typical configuration:

Collector supply voltage: ( V_{CC} 12V ) Collector-emitter voltage: ( V_{CE} 10V ) (in saturation mode) Emitter resistor: ( R_E 1kOmega ) Base-bias resistor: ( R_1 10kOmega ) Emitter-emitter voltage: ( V_E ) Base-emitter voltage: ( V_{BE} )

Given ( V_{CE} 10V ), and assuming the transistor is in saturation ( collector-emitter voltage is practically 0V), we need to calculate the base-emitter voltage.

The emitter voltage ( V_E ) is given by:[V_E V_{CC} - I_C cdot R_E - V_{CE}]

Rewriting it, we get:[I_C frac{V_{CC} - V_{CE} - V_E}{R_E}]

Since ( V_{BE} approx 0.7V ) and ( V_E V_B - V_{BE} ), where ( V_B ) is the base voltage, we can approximate ( V_E ) as:[V_E V_B - 0.7V]

Substituting ( V_E ) back into the equation:[I_C frac{12V - 10V - (V_B - 0.7V)}{1kOmega}][I_C frac{2.7V - V_B}{1kOmega}]

The base voltage ( V_B ) is determined by the biasing network. If the base is grounded, ( V_B 0V ), then:[V_E 0.7V][I_C frac{2.7V - 0V}{1kOmega} 2.7mA]

This calculation shows that despite the base being grounded, the emitter remains above ground at 0.7V due to the base-emitter junction's voltage drop.

Conclusion

In summary, the voltage between the emitter and base of a transistor when it is forward biased is a fundamental aspect of transistor operation. This voltage cannot be zero even if the base is grounded due to the inherent forward voltage drop across the base-emitter junction. This voltage drop, approximately 0.7V for a silicon transistor, is essential for the proper functioning of the transistor and maintaining the stability of the circuit's Q-point. The use of an emitter resistor ensures that this voltage drop is maintained, leading to a more stable and predictable circuit behavior.

Understanding these principles is crucial for designing and troubleshooting transistor-based circuits, and it forms the foundation of many electronic systems and devices.