[SOLVED] Problem while implementing a Widlar Current Mirror

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adi0803199

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I have designed a Widlar Current Mirror for a current of 10µA. The design specification is as follows ,
Vcc = 3.0V
R1 = 2.2 kΩ
Re2 = 12kΩ
Transistors : matched BC547 with hfe = 261
The problem that i have is when I connect a load resistance of 2.2kΩ , I get a emitter current of Ie2 = 6.166 µA and collecter current Ic2 = 75.318 µA. (transistor Q2)
I don't know how to debug this problem.Any help to solve this problem.......


I referred this website to design my circuit :
https://en.wikipedia.org/wiki/Widlar_current_source
 
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I get a emitter current of Ie2 = 6.166 µA and collecter current Ic2 = 75.318 µA
Click to expand...
Sounds simply impossible. Either the circuit or the measurement is incorrect.
 

Is the current in the primary side (Q1, R1) correct without any load?
How does it change with load?

Your current measurements sound very weird. Please mention the node voltages.
 

Possibilities:
  • Something connected the wrong way around.
  • Transistor overheated and damaged while soldering.
  • Really messy soldering, causing leakage current through burnt flux residue or whatever.
 
I found the nodal voltages at the emitter and collecter of transistor Q2 which was
Ve2 = 74 mV ,
Ie2=Ve2/Re= 274mV/12k =6.166 uA
Vbe1= Vc1 =Vb2 = 0.62 V
Vc2 = 2.834 V,
Vload =Vcc -Vc2 =0.1656 V
Iload= Vload/Rload = 0.1656 V/2.2k
=75.318 uA.
 
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Sorry I didn't get your point...In a Widlar current source,
Vbe1 = Vbe2+Iload*Re2.
So Vbe of Q1 and Q2 can never be the same unless Iload =0 A.
 

Quite obviously, Vbe matching means equal Vbe for equal Ic. It's also a prerequisite for predictable behaviour of the Widlar current source.
 
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