[patrick-turner]

Alex Pogossov Aug 21 (21 hours ago)


Other recipients:


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.


Suppose you have this bottom schema used for CCS tail of tubed LTP.
And suppose you have the B+ rail = 0V and the GND rail at -50V.
Then suppose Q2base = -25Vdc, and all bypassed and stabilised.
Then Q2 emitter would be at -24.3V and this feeds Q1 base so Q1 emitter is at -25Vdc, same as Q2 base at -25V. So you have very little voltage between Q2 collector and Q1 emitter and hence very little current, but I guess there may be some, and I have not made the circuit so I dunno how well it works or whether or not it oscillates or behaves strangely when trying to be good.

I'll keep your inovations in mind, but the use of a single bjt with unbypassed Re and a stable Vb between base and bottom of Re is all anyone needs to get extremely high Rout, and effects of any non linearity in the Rout character are utterly negligible and completely swamped by inherent distortion in the 2 LTP tubes, even when they are good triodes with their RLa = more than 10Ra.

Happy soldering, Patrick Turner.