Easy transistor question: NPN--the corresponding change?

In summary, the question involves finding the collector current of an npn transistor with a specified VBE at a given temperature, and then finding the corresponding collector current for a different VBE. The solution involves using the simplified Ebers-Moll equation and taking into account the saturation current and temperature. Ultimately, the ratio of the two collector currents can be found by dividing two exponential expressions.
  • #1
adamaero
109
1

Homework Statement


Given: an npn transistor having VBE = 0.76V for a collector current of 10mA at T=350 K (n=1) Find the corresponding collector current for VBE = 0.70V

2. Relevant questions
a) Wouldn't that temperature be way to high for the transistor to act normally?? I mean 350K is 170F.
b) What equations are relevant? Do I need to use diode equations? Which one, please. I have a very large book, and there are equations everywhere.

The Attempt at a Solution


I don't really understand the load line base-emitter junction characteristics...
I know this has something to do with it:
https://wiki.analog.com/_detail/uni...university:courses:electronics:text:chapter-9
 
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  • #2
adamaero said:

Homework Statement


Given: an npn transistor having VBE = 0.76V for a collector current of 10mA at T=350 K (n=1) Find the corresponding collector current for VBE = 0.70V

2. Relevant questions
a) Wouldn't that temperature be way to high for the transistor to act normally?? I mean 350K is 170F.
No.
b) What equations are relevant? Do I need to use diode equations? Which one, please. I have a very large book, and there are equations everywhere.

The Attempt at a Solution


I don't really understand the load line base-emitter junction characteristics...
I know this has something to do with it:
https://wiki.analog.com/_detail/university/courses/electronics/text/chptr9-f4.png?id=university:courses:electronics:text:chapter-9[/QUOTE]
No it doesn't.
There is a simple equation("simplified Ebers-Moll equation) relating emitter current to Vbe, saturation current, and T. It assumes Vce > Vbe but not excessively so (not in Early region). Saturation current is the base-emitter reverse-voltage leakage current. Limit this equation in comparing the same transistor in differing bias conditions, which is what you're doing here. The saturation current varies very widely from unit to unit and over temperature, but this can be ignored if all you're doing is varying the bias parameters (Vbe and iE etc.) You can also assume alpha = 1.
 
  • #3
adamaero said:
a) Wouldn't that temperature be way to high for the transistor to act normally?? I mean 350K is 170F.
That is a typical computer CPU temperature, and some transistors are rated for much higher temperatures.
 
  • #4
It is rather simple.
Based on the knowledge how Ic depends on VBE (exponential law) you simply can create two equations for Ic1=f(VBE1) and Ic2=f(VBE2)
As a next step, find the ratio Ic2/Ic1 and use your knowledge for dividing two exponential expressions. That`s all.
 

What is an NPN transistor?

An NPN transistor is a type of bipolar junction transistor (BJT) that has three layers of doped semiconductor material. The layers are made of two types of materials, either N-type or P-type, with the middle layer being a different type than the outer layers. This creates two P-N junctions within the transistor, which allows for amplification of electrical signals.

How does an NPN transistor work?

An NPN transistor works by controlling the flow of current from the collector terminal to the emitter terminal through the base terminal. When a small current is applied to the base, it allows a larger current to flow from the collector to the emitter. This amplification effect is what makes NPN transistors useful in electronic circuits.

What is the difference between an NPN and PNP transistor?

The main difference between an NPN and PNP transistor is the direction of current flow. In an NPN transistor, current flows from the collector to the emitter, while in a PNP transistor, current flows from the emitter to the collector. Additionally, the majority carriers in an NPN transistor are electrons, while in a PNP transistor, the majority carriers are holes.

What are the common applications of NPN transistors?

NPN transistors are commonly used in electronic circuits for amplification and switching purposes. They are also used in digital logic circuits, voltage regulators, and audio amplifiers. NPN transistors are also used in the construction of integrated circuits.

How do you test an NPN transistor?

To test an NPN transistor, you can use a multimeter to measure the resistance between the different terminals. The resistance between the base and emitter should be low, while the resistance between the base and collector should be high. Additionally, you can use a circuit to check if the transistor is amplifying or switching properly.

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