Voltage Drop & RE in NPN Transistors

In summary, the voltage drop across the emitter in an NPN transistor is not necessarily equal to 0. The purpose of RE, which is found immediately after the emitter, is to allow the transistor to accommodate two opposite states and regulate the power supply voltage. When the base current is high, the collector-emitter junction has a lower voltage and the resistors bear most of the power supply voltage. When the base current is low, the collector-emitter junction has a higher voltage and the resistors do not receive as much voltage. This allows the transistor to switch between two different states, resulting in a balance between the situations where big X + small Y = constant and small X + big Y = constant.
  • #1
sanado
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Is the voltage drop across the emitter in an NPN transistor equal to 0?
What is the purpose of RE (found immediately after the emitter?)
 
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  • #2
I think you're picturing the collector-emitter junction, and one or more resistors in the collector and/or emitter paths, spanning the range between the positive and negative terminals of a power supply. The transistor needs to accommodate two opposite states. If the base current goes high, the transistor conducts, the collector-emitter junction doesn't have much voltage across it, and then the resistors bear most of the power supply voltage. If the base current goes low, the collector-emitter junction stops conducting and takes up a lot more of the power supply voltage, and then the resistors won't don't get the voltage. You're expecting to go back and forth between the situation big X + small Y = constant and the situation small X + big Y= constant.
 
  • #3


I can provide an explanation for the concept of voltage drop and RE in NPN transistors. In an NPN transistor, the voltage drop across the emitter is not necessarily equal to 0. This is because the emitter is the source of current for the transistor and it has a resistance, which can cause a voltage drop. However, the exact value of the voltage drop across the emitter will depend on the specific circuit and operating conditions.

The purpose of RE, which is a resistor found immediately after the emitter, is to stabilize the operating point of the transistor and prevent thermal runaway. This resistor helps to limit the current through the transistor, ensuring that it operates within its safe operating range. It also helps to provide negative feedback, which can improve the stability and linearity of the transistor's performance.

In summary, the voltage drop across the emitter in an NPN transistor is not always equal to 0 and can vary depending on the circuit and conditions. The purpose of RE is to stabilize the transistor's operating point and improve its performance.
 

1. What is voltage drop in NPN transistors?

Voltage drop in NPN transistors refers to the decrease in voltage across the collector-emitter junction when an NPN transistor is in its active region. This drop occurs due to the voltage divider effect between the base-emitter and collector-emitter junctions.

2. How does voltage drop affect the performance of NPN transistors?

Voltage drop can affect the performance of NPN transistors in several ways. It can decrease the gain of the transistor, increase power dissipation, and limit the maximum collector current. Additionally, voltage drop can cause thermal runaway and lead to failure of the transistor.

3. What is the relationship between voltage drop and base current in NPN transistors?

The voltage drop across the collector-emitter junction is directly proportional to the base current in NPN transistors. This means that as the base current increases, the voltage drop also increases, which in turn affects the transistor's performance.

4. How can I calculate the voltage drop in NPN transistors?

The voltage drop in NPN transistors can be calculated using Ohm's law, where VCE = IC * RC, with VCE being the voltage drop, IC being the collector current, and RC being the collector resistor. Alternatively, it can also be calculated using the transistor's datasheet, which provides the maximum voltage drop at a given collector current.

5. How can I reduce the voltage drop in NPN transistors?

The voltage drop in NPN transistors can be reduced by using a lower collector resistor value, choosing a transistor with a lower VCE(sat) value, or using a Darlington pair configuration. Additionally, proper heat sinking and thermal management can also help reduce the voltage drop and prevent thermal runaway in the transistor.

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