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LM334 Iset/Ibias question

I originally posted this in sci.electronics.basics, but perhaps should have posted it here.

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I recently looked at the general schematic for the LM334 on National's datasheet and with a quick sweep of my arms came up with a design Iset/Ibias of 8, not 16 as they show on page 5. I'm off by a factor of two.

My logic went like this. 1/2 of the I from V+ flows via Q6 to the R rail. 1/4 via Q4 and 1/4 via Q5. Q5's 1/4*I flows via Q1 to the R rail, too. So now up to 3/4*I into the R rail. Q4's 1/4*I passes through two paths. The Ic(Q2)=Ic(Q1)/2... but Ic(Q1)=1/4*I, so that is 1/8*I, leaving the other 1/8*I for Q3's Vbe conduction, which also flows to the R rail. So the R rail gets 7/8*I and the V- picks up

1/8*I. Multiplying through by 8 to get rid of the divisor, I see a factor of 8 for Iset/Ibias... not 16.

Can someone do a quick description about how to arrive at something more like 16 from the schematic? I'm missing a clue (or two.)

To be handy, here's the sheet:

formatting link

Thanks in advance, Jon

Reply to
Jon Kirwan
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They do not give any device geometries so you can't really calulate its innards. What counts is the characteristics table on page 2 and that says that the ratio is internally fixed at around 14-18, depending which current range you are in.

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Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

Okay. So that's the problem. I thought they might just add a few more emitters to get that point across in the schematic, if they wanted to. But I guess they don't do that.

Thanks, Jon

Reply to
Jon Kirwan

Yeah, it's an over-simplified schematic, not much more than a sketch. The next generations of chips are one step worse, no schematic :-(

--
Regards, Joerg

http://www.analogconsultants.com/

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