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Here is Widlar Current source driving load:

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It gives almost constant current for wide range of load resistances:

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Here is one circuit involving BJT:

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You can see it is biased using one voltage and one current source. My question is, how I can use Widlar current source to bias this circuit? I mean, how should I connect it?

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  • \$\begingroup\$ I haven't the faintest idea what you are wanting. \$\endgroup\$ – Andy aka Nov 29 '15 at 0:27
  • \$\begingroup\$ I'm trying to use Widlar circuit instead of red circle in last image. I thought it's clear :) \$\endgroup\$ – hari Nov 29 '15 at 0:29
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    \$\begingroup\$ It should work as you show it- the compliance of the Widlar source is down to a tenth of a volt or so plus Iout * R1 (top schematic) and you have plenty of voltage at the emitter (nominally about 2V). Of course the Widlar source will be pretty bad if you try to use discrete transistors. \$\endgroup\$ – Spehro Pefhany Nov 29 '15 at 1:30
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    \$\begingroup\$ The Widlar current source relies on the matching of the base-emitter current/voltage and thermal characteristics of Q1 and Q2. While transistors made from one silicon die will match closely, this is not so true for discrete transistors. That is why the Widlar current source was designed for integrated circuit applications and not discrete circuits. \$\endgroup\$ – Barry Nov 29 '15 at 2:03
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    \$\begingroup\$ The transistors have to be matched closely and held very close to the same temperature. Difficult outside of an IC, but if you try it either use a pair in the same package or at least connect the two thermally. Usually for discrete designs we add some emitter resistors. \$\endgroup\$ – Spehro Pefhany Nov 29 '15 at 3:07
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Connect the collector of Q2 from the 1st circuit to the emitter of the NPN in the 2nd.

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