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I am simulating a constant current source using PSpice

enter image description here

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The only difference is the PNP transistor i used. and Their specification seems to be similar.

Below is the link for the two transistors.

BC859B Datasheet

MMBT3906LT1G Datasheet

What is the difference between the two transistors that makes one circuit work, and the other circuit don't?

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    \$\begingroup\$ -1 for incomprehensible schematics. If you had drawn your schematic more clearly you would have likely seen the obvious errors for yourself. \$\endgroup\$ – The Photon Mar 10 '17 at 6:23
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enter image description here

THis is from the datasheet. Your schematic needs signal names and you should check the connections.

enter image description here

With V+ = 5V and 2V across this R3 and same V on R1 the current is constant thru the collector when terminated. The 2nd transistor is optional for ratio current amplification. (mirror)

I can explain further if needed.

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You've got over 5.38 volts across Emitter_Base in that first circuit.

The symbol is broken. Diodes don't need 5.38 volts to be forward-biased.

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  • \$\begingroup\$ Going for short and sweet but failing to answer the question again \$\endgroup\$ – Voltage Spike Mar 10 '17 at 20:34

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