Capacitance multiplier: which Darlington to choose?

[Walter66]

Did change R1 for higher values of 2K7, 6K2 and 12K. Uce rises to 7.2V, 12V respective 22V, Ube remained stable, maybe 0.3V less but that could be reading errors.
Changed value for the the 560K parallel to the cap to 100K, 820K and put it out of work. No changes for the circuit at all.
It seems it has no meaning for the circuit at all.

How does this thing manages to go as low as Uce=1.7V with R1=510 Ohm when only one transistor is open? Those two transistors means one drives the other, the driving transistor isn't open, it couldn't start the other one.

I have to build a simple amplifier circuit with this Darlington using my tube PSU on a breadboard for further investigation.

 

[abbey road d enfer]

How does this thing manages to go as low as Uce=1.7V with R1=510 Ohm when only one transistor is open? Those two transistors means one drives the other, the driving transistor isn't open, it couldn't start the other one.

Analyze what the 200 ohm resistor does.
What load have you put when measuring voltages.

 

[Walter66]

Analyze what the 200 ohm resistor does.
What load have you put when measuring voltages.

Those internal resistors in a Darlington IC have the purpose to compensate for temperature drift.
The 200 Ohm between emitter and base of the power transistor makes for a more stable and not temperature drifting working point. The idle current can be choosen between 0.7V and 1.1V and behaves nearly linear. This avoids distortions in the output signal.

The circuit actually runs with a load of 40mA. In the real amp it should be 60mA.

 

[abbey road d enfer]

Those internal resistors in a Darlington IC have the purpose to compensate for temperature drift.
The 200 Ohm between emitter and base of the power transistor makes for a more stable and not temperature drifting working point. The idle current can be choosen between 0.7V and 1.1V and behaves nearly linear. This avoids distortions in the output signal.

Partially wrong analysis. In a PSU, THD is not a concern (within limits), particularly when the load does not vary much.
BTW, idle current is measured in amps (or milliamps).

The circuit actually runs with a load of 40mA. In the real amp it should be 60mA.

Do you mean that the figures you published were without load?
Without load, the Vbe is not enough to turn on the transistors. Current flows throuh the 2.5k and 200r resistors.

 

[Walter66]

All measurements done with a 40mA load.
Current through R1 (6.95V)=2.6mA
Ube=0.68V, Uce= 7.6V, R1=2K7

 

[abbey road d enfer]

All measurements done with a 40mA load.
Current through R1 (6.95V)=2.6mA
Ube=0.68V, Uce= 7.6V, R1=2K7

With 2.6mA in R1, Vbe should be about 1.3V.
Did you measure the resistance between B and E?

 

[Walter66]

Ubase= 286.9V, Uemitter=286.2V. The total voltages differ from different AC supply voltages over the day, but Ube still remains constant @ 0.7V.
How can I measure the resistance of this Darlington, please? I think it can only be calculated?

 

[abbey road d enfer]

I suggest you use this method that uses a 9V battery, a 2.7k resistor and a multimeter.

 

[Walter66]

Thank you, will do!
But for now: Merry Christmas!