Size of Resistors

I'm a digital disginer who is having to some some pesky analog stuff.


I've been desiging some circuits using pen and paper and a little with
spice. As far as I can tell the designs theoretically work.

I've designed a a constant current source using npn and pnp
transistors. My only concern is that im using resistors in the 1-10ohm
range.

I need to do this because I need a high output (around 250mA) and only
have a small supply (+/- 5v)

If I build this am i likely to encounter problems. Will the tolerance
of the resistors make it difficult to control. If so does anyone have
any solutions.

Thanks in advance

G

The spice netlist is below if anybody is interested

This circuit provided a constant current through the load LED (D1) when
Vi1 is greater than approx 3v
The value of the current is dependent on R2 and R5

V1 V+ 0 SINE(0 5 100K)
R1 V+ N005 1k
Q2 N003 N001 V+ 0 PNP
Q3 N001 N001 V+ 0 PNP
Q4 i N001 V+ 0 PNP
Q1 V+ N005 N003 0 NPN
Q5 N005 N005 0 0 NPN
Q6 N003 N007 0 0 NPN
Q7 N001 N003 N007 0 NPN
R2 N007 0 10
R4 V+ N006 1k
Q8 N004 N002 V+ 0 PNP
Q9 N002 N002 V+ 0 PNP
Q10 i N002 V+ 0 PNP
Q11 V+ N006 N004 0 NPN
Q12 N006 N006 0 0 NPN
Q13 N004 N008 0 0 NPN
Q14 N002 N004 N008 0 NPN
R5 N008 0 10
D1 i 0 QTLP690C

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