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VA voltage for NPN (or equivalently Lamba for MOSFET-N)

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gianca

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Hello,
Sorry if this topic was covered already.
I need a transistor (preferentially an NPN) for an current source generator. It seems that on the datasheets such VA [Volt] doesn't appear. Is there a way to calculate it from the other parameters?
I have to choose a NPN for a current source with the output impedance in the range of 100 Mohm.
For clarity VA is the negative voltage which the IC-VCE curves tends to go on the left side crossing the X-axis. The output dynamic resistance of the NPN is given by VA/ICpol.

Thank you very much for your help.
G.
 

From single NPN (or PNP) transistors you can't expect Early voltages larger than of the order of |VA| = 100V , so your required output impedance could be achieved only for currents ≦ 102V/108Ω = 1µA with a single transistor. If you need larger currents with such high output impedance, you need a current mirror, probably with some feedback, see this snippet from H. Camenzinds book:
Camenzind_Curr.sources.png
 

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