What Is The Appropriate Ratio Of R1 And R2 In Emitter-degenerated BJT Biasing?

Introduction

Emitter-degenerated BJT biasing is a widely used technique in electronic circuit design to stabilize the operating point of a bipolar junction transistor (BJT). This method involves adding a resistor in series with the emitter of the transistor, which helps to reduce the effect of temperature changes on the transistor's operating point. In this article, we will discuss the appropriate ratio of R1 and R2 in emitter-degenerated BJT biasing.

Understanding Emitter-Degenerated BJT Biasing

Emitter-degenerated BJT biasing involves adding a resistor, typically denoted as R1, in series with the emitter of the transistor. This resistor is used to reduce the effect of temperature changes on the transistor's operating point. The value of R1 is chosen such that it provides a stable voltage at the emitter, which is independent of the temperature changes.

The Role of R2 in Emitter-Degenerated BJT Biasing

In emitter-degenerated BJT biasing, R2 is used as a voltage divider to provide a stable voltage at the base of the transistor. The value of R2 is chosen such that it provides a stable voltage at the base, which is independent of the temperature changes.

Calculating the Appropriate Ratio of R1 and R2

To calculate the appropriate ratio of R1 and R2, we need to consider the following factors:

• The voltage at the emitter, V_P
• The voltage at the base, V_B
• The voltage at the collector, V_C
• The current through the transistor, I_C

Assuming that the base current, I_B, is negligible, the emitter potential, V_P, can be calculated using the following equation:

$$V_P = V_X - V_{BE}$$

where V_X is the voltage at the emitter, and V_BE is the base-emitter voltage.

The voltage at the emitter, V_X, can be calculated using the following equation:

$$V_X = \frac{R_2}{R_1 + R_2} \cdot V_{CC}$$

where V_CC is the supply voltage.

Deriving the Ratio of R1 and R2

To derive the ratio of R1 and R2, we can start by substituting the expression for V_X into the equation for V_P:

$$V_P = \frac{R_2}{R_1 + R_2} \cdot V_{CC} - V_{BE}$$

We can then rearrange this equation to solve for the ratio of R1 and R2:

$$\frac{R_2}{R_1 + R_2} = \frac{V_P + V_{BE}}{V_{CC}}$$

Simplifying this equation, we get:

$$\frac{R_2}{R_1} = \frac{V_P + V_{BE}}{V_{CC}} - 1$$

Choosing the Appropriate Ratio of R1 and R2

The ratio of R1 and R2 can be chosen based on the desired operating point of the transistor. A higher ratio of R1 to R2 will result in a higher voltage at the emitter, which can be useful for applications where a high voltage is required.

However, a higher ratio of R1 to R2 can also result in a higher current through the transistor, which can be undesirable in some applications. Therefore, the ratio of R1 and R2 should be chosen carefully based on the specific requirements of the application.

Example Calculation

Let's consider an example calculation to illustrate the process of choosing the ratio of R1 and R2.

Suppose we want to design a circuit that requires a voltage at the emitter of 5V, and the supply voltage is 15V. We also assume that the base-emitter voltage is 0.7V.

Using the equation derived above, we can calculate the ratio of R1 and R2 as follows:

$$\frac{R_2}{R_1} = \frac{5 + 0.7}{15} - 1$$

Simplifying this equation, we get:

$$\frac{R_2}{R_1} = 0.33$$

Therefore, the ratio of R1 and R2 should be approximately 1:3.

Conclusion

In conclusion, the ratio of R1 and R2 in emitter-degenerated BJT biasing is an important parameter that affects the operating point of the transistor. By choosing the appropriate ratio of R1 and R2, we can design a circuit that meets the specific requirements of the application.

In this article, we have discussed the process of calculating the ratio of R1 and R2, and have provided an example calculation to illustrate the process. We have also highlighted the importance of choosing the appropriate ratio of R1 and R2 based on the specific requirements of the application.

References

• "The Art of Electronics" by Paul Horowitz and Winfield Hill
• "Electronic Devices and Circuit Theory" by Robert L. Boylestad and Louis Nashelsky
• "BJT Biasing Techniques" by Texas Instruments

Further Reading

• "Emitter-Degenerated BJT Biasing" by Analog Devices
• "BJT Biasing Techniques" by National Semiconductor
• "Emitter-Degenerated BJT Biasing" by Maxim Integrated

Q: What is emitter-degenerated BJT biasing?

A: Emitter-degenerated BJT biasing is a technique used to stabilize the operating point of a bipolar junction transistor (BJT). It involves adding a resistor in series with the emitter of the transistor to reduce the effect of temperature changes on the transistor's operating point.

Q: What is the purpose of R2 in emitter-degenerated BJT biasing?

A: R2 is used as a voltage divider to provide a stable voltage at the base of the transistor. The value of R2 is chosen such that it provides a stable voltage at the base, which is independent of the temperature changes.

Q: How do I calculate the ratio of R1 and R2 in emitter-degenerated BJT biasing?

A: To calculate the ratio of R1 and R2, you can use the following equation:

$$\frac{R_2}{R_1} = \frac{V_P + V_{BE}}{V_{CC}} - 1$$

where V_P is the voltage at the emitter, V_BE is the base-emitter voltage, and V_CC is the supply voltage.

Q: What is the significance of the base-emitter voltage (V_BE) in emitter-degenerated BJT biasing?

A: The base-emitter voltage (V_BE) is the voltage drop across the base-emitter junction of the transistor. It is typically around 0.7V for silicon transistors and 0.3V for germanium transistors.

Q: How do I choose the value of R1 and R2 in emitter-degenerated BJT biasing?

A: The values of R1 and R2 should be chosen based on the desired operating point of the transistor. A higher ratio of R1 to R2 will result in a higher voltage at the emitter, which can be useful for applications where a high voltage is required.

Q: What are the advantages of emitter-degenerated BJT biasing?

A: The advantages of emitter-degenerated BJT biasing include:

• Improved stability of the operating point
• Reduced effect of temperature changes on the transistor's operating point
• Improved linearity of the transistor's output

Q: What are the disadvantages of emitter-degenerated BJT biasing?

A: The disadvantages of emitter-degenerated BJT biasing include:

• Increased complexity of the circuit
• Increased number of components required
• Potential for increased power consumption

Q: Can I use emitter-degenerated BJT biasing with other types of transistors?

A: Emitter-degenerated BJT biasing is typically used with bipolar junction transistors (BJTs). However, it can also be used with other types of transistors, such as field-effect transistors (FETs), although the design may need to be modified accordingly.

Q: How do I troubleshoot emitter-degenerated BJT biasing circuits?

A: Troubleshooting emitter-degenerated BJT biasing circuits can be challenging due to the complex interactions between the various components. However, some common issues to look out for include:

• Incorrect values of R1 and R2
• Incorrect values of V_P and V_BE
• Incorrect values of V_CC
• Faulty components or connections

Conclusion

In conclusion, emitter-degenerated BJT biasing is a complex technique that requires careful consideration of the various components and their interactions. By understanding the principles and applications of emitter-degenerated BJT biasing, you can design and troubleshoot circuits that meet the specific requirements of your application.

References

• "The Art of Electronics" by Paul Horowitz and Winfield Hill
• "Electronic Devices and Circuit Theory" by Robert L. Boylestad and Louis Nashelsky
• "BJT Biasing Techniques" by Texas Instruments

Further Reading

• "Emitter-Degenerated BJT Biasing" by Analog Devices
• "BJT Biasing Techniques" by National Semiconductor
• "Emitter-Degenerated BJT Biasing" by Maxim Integrated