Easy transistor question: NPN--the corresponding change?

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Discussion Overview

The discussion revolves around the behavior of an NPN transistor at elevated temperatures, specifically examining the relationship between the base-emitter voltage (VBE) and the collector current (Ic). Participants explore relevant equations and concepts related to transistor operation, including the effects of temperature on performance and the application of the Ebers-Moll model.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a temperature of 350K is too high for normal transistor operation, suggesting it may exceed typical limits.
  • Another participant asserts that 350K is a common operating temperature for some transistors, particularly in applications like CPUs.
  • There is mention of the simplified Ebers-Moll equation, which relates emitter current to VBE, saturation current, and temperature, with the assumption that VCE is greater than VBE.
  • A participant proposes creating two equations for collector current based on different VBE values and suggests using the ratio of these currents to analyze the relationship.
  • Concerns are raised about the variability of saturation current across different transistors and temperatures, although this variability may be ignored when comparing bias conditions.

Areas of Agreement / Disagreement

Participants express differing views on the suitability of 350K for transistor operation, with some asserting it is acceptable while others question it. The discussion includes multiple approaches to solving the problem, indicating no consensus on a single method or conclusion.

Contextual Notes

Participants reference the need for specific equations and the implications of varying bias conditions, but there is no resolution on which equations are most appropriate for the given scenario.

adamaero
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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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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.
 
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.
 
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.
 

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