[GRe]

"Alex Pogossov" wrote in message
...

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+.

Alex:
Once we are on this topic, a minor modification -- recycling of Q1 base
current -- can improve Ro, particularly its patr associated with Early
effect and nonlinearity of the collector junction capacitance.

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

In this circuit, Q1 base current variations get almost fully (because of
high Q2 beta) injected into Q1 emitter, and, because usually differential
emitter resistance of Q1 is much smaller than Re, and because of Q1 beta is
also high, this injected current passes to Q1 collector and almost
completely cancels Q1 collector current variations otherwise caused by beta
modulation and non-linear capacitance.

Brilliant!

Maybe except for 1 remaining flaw.
Although Q1/Q2 Vbe drift as a function of ambient temperature is
compensated, Vbe drift of Q1 as a function of temperature (rise) caused by
device dissipation is not. As long as the load is more or less constant,
like in application for a LTP, this is no problem.
For application with a varying load close thermal coupling between Q1 and Q2
is necessary as well as a low Rth.j-c for Q1.

Regards,
Gio Re