BJT PNP Switch: Why Vee Must Reach Zero for On

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SUMMARY

In a BJT PNP switch, the base voltage does not need to reach zero to turn on the transistor; rather, it must be sufficiently lower than the emitter voltage (Vee) to achieve forward biasing. Specifically, if Vee is 5V, the base must be at least 4.3V (5V - 0.7V) to ensure the base-emitter junction is forward biased. The discussion also highlights a common misconception regarding the saturation voltage (VCE sat) of PNP transistors compared to NPN transistors, noting that PNP VCE sat is typically lower in voltage regulator applications.

PREREQUISITES
  • Understanding of BJT (Bipolar Junction Transistor) operation
  • Knowledge of forward biasing in semiconductor devices
  • Familiarity with voltage regulator circuits
  • Basic principles of transistor saturation voltage (VCE sat)
NEXT STEPS
  • Study the characteristics of PNP vs. NPN transistors in detail
  • Learn about the role of base-emitter voltage in BJT operation
  • Investigate the design and function of voltage regulators using BJTs
  • Explore simulation tools for testing BJT circuits, such as LTspice
USEFUL FOR

Electronics engineers, students studying semiconductor devices, and anyone involved in designing or troubleshooting BJT circuits will benefit from this discussion.

likephysics
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In a BJT PNP switch, why does the base voltage need to go to zero to turn it on.
If Vee is 5V, the base has to be 5-0.7v to make the BE fwd biased.
 
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"In a BJT PNP switch, why does the base voltage need to go to zero to turn it on." - It doesn't

"If Vee is 5V, the base has to be 5-0.7v to make the BE fwd biased" - Correct.
 
uart said:
"In a BJT PNP switch, why does the base voltage need to go to zero to turn it on." - It doesn't

"If Vee is 5V, the base has to be 5-0.7v to make the BE fwd biased" - Correct.

Great. I just simulated it, it works.
Did the same thing last week, damn thing didn't work. That's why I doubted my understanding.

The reason I started with PNP switch is bcoz I don't understand why the VCE sat of PNP pass transistor (in voltage regulator) is much less than NPN Vce sat.
 

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