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What role R3 plays in this circuit?

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samy555

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Hello
I read in a book
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01.jpg
02.jpg
My question is: what is the function of the R3? What if I took it away from the circuit?
Thank you
 

The Z-diode needs a certain current Iz for providing the desired voltage drop Vz.
This current must not be determined by the base current of Q2 only; instead it is fixed using a proper resistor R3.
The voltage drop across R3 controls the collector current of transistor Q2.
 
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