[GRe]

"Alex Pogossov" wrote in message
...

[...]

why do you use only one transistor. Using two would also increase the
output impedance....

----------- V+
| |
R LOAD
| |
| C
--------B Q1
| E
C |
Q2 B--------
E |
| Re
| |
----------- GND

Yes, and one most likely does not need -45V rail. Q2 can be a 5-cent
general purpose like BC547. Resistor R can be connected to +B rail, not ro
the load (common cathodes).


Like you say, with R to B+ you'll remove the error dV+/R from above circuit.
But i.s.o. R connected to B+ any V+ will do.

B+/V+ CC
| |
R LOAD
| |
| C
+-------B Q1
| E
C |
Q2 B-------+
E |
| Re
| |
+--------+-- GND


This circuit has two disadvantages:
1) It may oscillate at dozens of MHz with certain unlucky combination of
Q1 ans Q2;
2) Regulated current is temperature dependent -- drops as the temperature
rises.

I've always liked the simplicity and junk box available parts (maybe except
Q1) of this type of CCS.

1) If oscillation occurs it can usually be suppressed by a small cap between
Q2-C and Q1-E.

2) dI/dT is'nt much, only about -0.3%/°C. With suitable (thermal) placement
of Q2 and a typical room ambient temp. variation of +/-5°C you'll end up
with a current error of -/+1.5%, insignificant for tube electronics I guess.

Nevertheless dI/dT can be improved with little change.

V+ -+----+ CC
| | |
R2 R LOAD
| | |
| | C
| +------B Q1(n)
| | E
| E |
+---B Q2(p) |
| C |
R3 | Re
| | |
+----+-------+-- GND

Unfortunatly this circuit trades very low drift for not so high Ro, also
you'll need stabilized V+.



V+ -+----+-------+ CC
| | | |
R2 R R LOAD
| | | |
| | | C
| | +-------B Q1
| | | E
| C C |
+---B Q3 Q2 B-------+
| E E |
R3 | | Re
| +---+---+ |
| | |
| R4 |
| | |
+--------+------------+-- GND

Unfortunatly again, this one trades low drift and high Ro for the KISS
principle by yet another transistor.

Any thoughts for a CCS with high Ro and low drift, with junk box available
parts and KISSes like a dream?

Regards,
Gio Re.