Calculating BJT Base Current with Zener Diode

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SUMMARY

The BJT base current in a circuit with a Zener diode is determined primarily by the emitter current, following the relationship I_E = β I_B. The Zener diode acts as a shunt regulator, maintaining a constant voltage at the base as long as sufficient current flows through it. The ability of the Zener to hold a specific voltage, such as 5.6V, is contingent upon the resistor R1 and the emitter current, as well as the transistor's beta (β). If the emitter current exceeds a certain threshold, the voltage drop across R1 can reduce the output voltage.

PREREQUISITES
  • Understanding of BJT operation and parameters, including beta (β)
  • Knowledge of Zener diode characteristics and applications
  • Familiarity with voltage dividers and Thevenin's theorem
  • Basic circuit analysis skills, including Ohm's law
NEXT STEPS
  • Study the role of Zener diodes in voltage regulation
  • Learn about BJT biasing techniques and their impact on performance
  • Explore Thevenin's theorem in practical circuit applications
  • Investigate the effects of varying emitter current on BJT operation
USEFUL FOR

Electrical engineers, electronics students, and hobbyists interested in transistor circuits and voltage regulation techniques.

likephysics
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What is the BJT base current if the base is connected to a zener diode like in the circuit here - http://www.satcure-focus.com/tutor/page5.htm
With a voltage divider it's thevenin's equivalent. But with zener, should I consider it as a voltage source?
 
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likephysics said:
What is the BJT base current if the base is connected to a zener diode like in the circuit here - http://www.satcure-focus.com/tutor/page5.htm
With a voltage divider it's thevenin's equivalent. But with zener, should I consider it as a voltage source?

Base current depends on the emitter current you are drawing [itex]I_E=\beta I_B\;[/itex], it does not depend on the zener. Think of the zener as a shunt regulator. The zener draw enough current to keep the zener voltage at the base. Whether the zener can hold 5.6V depend on the R1 and emitter current and the beta of the transistor. If you draw too much current at the emitter and the base current is high enough that the R1 drop more voltage, then the output voltage will go down.

The zener is a shunt regulator.
 

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