Calculating NPN Transistor Vin: Exploring the Minimum Input Voltage Requirement

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In summary: To clarify, the summary is: In summary, the conversation discusses the minimum input voltage for a transistor to begin to turn on. The question asks for the minimum voltage for which the transistor will begin to turn on and the equations used are not specified. The attempt at a solution includes questioning why the answer is 0.7V and considering the voltage dropped across a 100k resistor. The expert responds by explaining that the input voltage may be defined as Vbe, not including the 100k resistor, but it could also include Vbe and Vrb. Additionally, the expert mentions that Vbe is temperature dependent. The conversation concludes by discussing the region near the origin and the drop across a 100k resistor.
  • #1
TsAmE
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Homework Statement



What is the minimum input voltage for which the transistor will begin to turn on?

Homework Equations



None.

The Attempt at a Solution



I don't understand why the answer was 0.7V. I understand that yes by the time you reach the base there will be 0.7V left which will get dropped across the diode, but wouldn't Vin = 0.7 + voltage dropped across 100k?
 
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  • #2
Forgot to add the diagram
 

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  • #3
TsAmE said:

Homework Statement



What is the minimum input voltage for which the transistor will begin to turn on?

Homework Equations



None.

The Attempt at a Solution



I don't understand why the answer was 0.7V. I understand that yes by the time you reach the base there will be 0.7V left which will get dropped across the diode, but wouldn't Vin = 0.7 + voltage dropped across 100k?

That's a very good question. They must be defining "Vin" as Vbe, not including the 100k resistor. But you are correct, you could just as easily call Vin the input voltage to the 100k resistor, so it would include Vbe and Vrb.

BTW, you also know that the transistor's Vbe is temperature dependent, right? So it's not generally just 0.7V or 0.6V, although for many problems we just assume a set Vbe value.
 
  • #4
It is in the wording of the question, too.

for which the transistor will begin to turn on

This means before there is any current flowing in the resistor and therefore before there is any voltage across it. Once current begins to flow, then the resistor limits the amount of current, but before then, the resistor has no effect.

In this diagram (below), which I produced yesterday from LTSpice for another thread, we are talking about the region near the origin (550mV, 0 µA).
The drop across a 100 K resistor with even 1 µA flowing is 0.1 Volt and you can see that the current at 580 mV is a lot less than 1 µA.

[PLAIN]http://dl.dropbox.com/u/4222062/Base%20emitter%20curve.PNG

Base current of a 2N2222 NPN transistor vs base-emitter voltage
 
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  • #5
Good point, vk6kro!
 

Related to Calculating NPN Transistor Vin: Exploring the Minimum Input Voltage Requirement

1. What is the minimum input voltage requirement for an NPN transistor?

The minimum input voltage requirement for an NPN transistor depends on the specific transistor model and its characteristics. Generally, it can range from 0.6V to 0.7V for silicon-based transistors.

2. How is the minimum input voltage requirement calculated for an NPN transistor?

The minimum input voltage requirement for an NPN transistor is calculated by subtracting the forward voltage drop across the base-emitter junction from the minimum required voltage for proper transistor operation, typically around 0.2-0.3V.

3. Can the minimum input voltage requirement vary for different applications?

Yes, the minimum input voltage requirement for an NPN transistor can vary depending on the specific application and its operating conditions. It is important to consider factors such as temperature, current, and voltage fluctuations when determining the minimum input voltage requirement.

4. Is it possible to operate an NPN transistor with a lower input voltage than the minimum requirement?

It is not recommended to operate an NPN transistor with a lower input voltage than the minimum requirement, as it can lead to unreliable and unpredictable performance. It is best to choose a transistor with a suitable minimum input voltage requirement for your desired application.

5. How does the minimum input voltage requirement affect the performance of an NPN transistor?

The minimum input voltage requirement is a critical factor in the performance of an NPN transistor. If the input voltage is too low, the transistor may not turn on fully, resulting in reduced gain and potential distortion in the output signal. It is important to select a transistor with a suitable minimum input voltage requirement for optimal performance.

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