Transistor load and saturation problem

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SUMMARY

The discussion focuses on solving a transistor load and saturation problem involving an NPN transistor. The required resistor value (RE) to achieve a current of 0.5 A through the load is calculated to be 5.2Ω. The maximum load resistor (RL) that can be used without saturating the transistor is determined to be 27.8Ω. To ensure the transistor enters saturation with a load resistor of 10Ω, the base voltage (VB) must exceed 13.9V.

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  • Understanding of NPN transistor operation
  • Knowledge of Ohm's Law
  • Familiarity with Kirchhoff's Voltage Law
  • Basic circuit analysis techniques
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Homework Statement


consider the following circuit:
attachment.php?attachmentid=58540&stc=1&d=1367880937.jpg


a) Determine the value of the resistor, R so that a current of 0.5 A flows through the load.
b) What is the maximum value of th load resistor?
c) If the load resistor is 10Ω, what value of VB will put the transistor into saturation?

Homework Equations



IC ≈ IE = VE/RE

IC ≈ (VB-0.6V)/RE

ICRL+VCE+VE=Vcc

The Attempt at a Solution


a) 0.5 ≈ (3.2 - 0.6)/RE
RE = 5.2Ω

b) (I am guessing that the VCE is 0.6 V here but I am not sure)
0.5RL+0.6+0.5 = 15
RL = 27.8Ω

c) From what I can tell, the transistor is an NPN and in that case, in order to saturate it, the base voltage must be greater than the collector or emitters'.

VC=ICRC
VC=(0.5)(27.8)=13.9V

VE=IERE
VE=(0.5)(5.2)=2.6V

So in order to be greater than both the VB must be greater than 13.9V

Parts b and c I was not so sure about, thanks for any input.
(Sorry that the image is not nicely sized, I'm not sure how to fix it.)
 

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a) is right.
b): where does the 0.5 come from? Fact: Ve + Vce + iRL = 15. You have Ve already (= iR). You can increase the load R until saturation, Vc = Vb. You know Vb so go for it.

c) Yes, saturation is when Vb = Vc. So let the load resistor = 10 ohms; then how high must Vb be to get Vc = Vb?
 

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