Largest resistor value such that BJT remains in active mode

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SUMMARY

The discussion centers on determining the largest resistor value for a BJT to remain in active mode, given a transistor with a current gain (β) of 30. The key inequality derived is Vc - Vb > 0.4V, which is crucial for maintaining the transistor in active mode, as saturation occurs when Vce reaches 0.2V. The participants clarify that for a silicon transistor, the base-emitter voltage (Vbe) is largely constant, influencing the voltage relationship between the collector and base.

PREREQUISITES
  • Understanding of BJT operation and active mode conditions
  • Knowledge of transistor parameters, specifically β (beta)
  • Familiarity with voltage relationships in BJTs, including Vce and Vbe
  • Basic circuit analysis skills
NEXT STEPS
  • Research the implications of Vce saturation in BJTs
  • Learn about the role of β in transistor biasing and stability
  • Explore the characteristics of silicon transistors, particularly Vbe
  • Investigate resistor sizing for BJT biasing in active mode
USEFUL FOR

Electrical engineering students, circuit designers, and anyone involved in transistor circuit analysis and design.

theBEAST
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Homework Statement


We are given that the transistor has β = 30. The BJT is shown below (along with the solution). Every value in the figure is given with the exception of the equation I circled in red:
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The Attempt at a Solution


What I don't get is how they got the inequality Vc - Vb > 0.4V? Where does this come from? I thought saturation occurs when Vce reaches 0.2V, so shouldn't our inequality be Ve - Vc > 0.2V?
 
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Assume that the magnitude of the base-emitter voltage for the BJT is largely constant. What value might you choose?
 
theBEAST said:
I thought saturation occurs when Vce reaches 0.2V?
You are probably right. So at saturation what would be the voltage between C and B? Work it out for a silicon transistor.
 

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