Help needed for constant current source design.

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rrbadrike

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Joined 2010
I want to design constant current source for audio amp.but i dont know how to calculate value of resistor for required current.l have attached constant current source circuit here.So anyone please give me formula & calculation for this.
 

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DUG

DUG

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Joined 2003
To get you in the ballpark let's make some assumptions

Hfe of transistors is >= 100 at operating current

left transistor we will call Q1 the other Q2

current for Q2 will be close to (35-1.2) / R2

find the Vbe for Q2 at that current

I will be Vbe / R1

This will get you within a couple of % of what you need.

one more assumption: Q1 can handle the power dissipation and the SOA for your requirements

If Hfe is not that high you will have to look at current paths and do some adding and subtracting

hope that helps
 
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