# Kirchhoff's Voltage Law for PNP Transistor Circuit

In summary, the conversation is about a student struggling with applying KVL to PNP transistor bias circuits and the confusion caused by the polarities. The student is attempting to find IE in the given circuit and uses Thévenin's Theorem to simplify the circuit. The conversation also includes a diagram to help with understanding the polarity of the base-emitter junction.
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## Homework Statement

I seem to have no problem applying KVL to NPN transistor bias circuits, but a world of trouble getting my polarities straight on PNP transistor bias circuits. The +'s and -'s are driving me crazy.

The following circuit was presented in the "Voltage-Divider Biased PNP Transistor" section of my Electronics textbook. My task is to find IE. The book gives IE as:

$$I_{E}=\frac{-V_{TH}+V_{BE}}{R_E+R_{TH}/\beta _{DC}}$$

The circuit in question:

## Homework Equations

Kirchhoff's Voltage Law --> Sum of voltage rises + drops = 0

Voltage Divider Law --> $V_x=\left (\frac{R_x}{R_T} \right )E$

## The Attempt at a Solution

The first thing I did was redraw the circuit.

Then I used Thévenin's Theorem to get reduce the left-hand "window" to one voltage source and one resistance.

$$V_{TH}=V_{R2}=\left (\frac{R_2}{R_1+R_2} \right )\left ( -V_{CC} \right )$$

$$R_{TH}=\frac{R_1R_2}{R_1+R_2}$$

The Thévenized circuit is now:

Now comes the part that I always screw up; getting the polarities correct on my KVL equation...

$$V_{TH}-I_ER_E-V_{BE}-I_BR_{TH=0}$$

$V_{TH}-I_ER_E-V_{BE}-\left (\frac{I_E}{\beta } \right )\left ( R_{TH} \right )=0$ because $I_B=\left (\frac{I_E}{\beta } \right )$

$$I_E\left ( R_E+\frac{R_{TH}}{\beta } \right )=V_{TH}-V_{BE}$$

$$I_E=\frac{V_{TH}-V_{BE}}{R_E+R_{TH}/\beta }=\frac{\left (\frac{R_2}{R_1+R_2} \right )\left ( -V_{CC} \right )-0.7V}{R_E+R_{TH}/\beta }$$

I also drew this little diagram to help me with the PNP transistor because I tend to get confused about the polarity of the base-emitter junction:

I think my answer is the same as the book's but I'm not sure. They don't give VBE as a specific voltage level so I don't know if it's +0.7V or -0.7V.

Seeing the circuit drawn that way gives me vertigo! Flip your first diagram vertically to help with the intuition stuff. More poisitive supplies go toward the top of a circuit diagram, and more negative supplies go toward the bottom.

If I can get rid of the vertigo, I'll try to check your equations...

Seeing the circuit drawn that way gives me vertigo! Flip your first diagram vertically to help with the intuition stuff.

Exactly my thoughts. The very first circuit is upside down.

## 1. What is Kirchhoff's Voltage Law?

Kirchhoff's Voltage Law, also known as Kirchhoff's Second Law, states that the sum of all voltages in a closed circuit must equal zero. This law is based on the principle of conservation of energy and is a fundamental concept in circuit analysis.

## 2. How does Kirchhoff's Voltage Law apply to PNP Transistor Circuits?

Kirchhoff's Voltage Law applies to all types of circuits, including PNP Transistor Circuits. In these circuits, the law is used to analyze the voltage drops across the various components, such as the transistor, resistors, and power supply.

## 3. Why is Kirchhoff's Voltage Law important in circuit analysis?

Kirchhoff's Voltage Law is important in circuit analysis because it allows us to determine the voltage drops across different components in a circuit. This information is crucial in understanding how the circuit operates and can help us troubleshoot any problems that may arise.

## 4. How do you apply Kirchhoff's Voltage Law in a PNP Transistor Circuit?

To apply Kirchhoff's Voltage Law in a PNP Transistor Circuit, you need to first draw the circuit diagram and label all the components. Then, you can use the law to write equations for the voltage drops across each component. Finally, you can solve these equations to find the unknown voltages.

## 5. What are the limitations of Kirchhoff's Voltage Law?

Kirchhoff's Voltage Law has a few limitations, such as assuming ideal components and only being applicable to closed circuits. It also does not take into account the effects of non-linear components, such as diodes and transistors. Additionally, it does not account for electromagnetic interference in the circuit. Therefore, it should be used as a basic guideline and not as a definitive solution in complex circuits.

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