The second graph is not aplicable in my case, I think. There is no 10V

Since you intend to saturate the transistor, you should assume a low CTR and drive the LED appropriately. If you want to minimize dissipation in the LED, you need to determine how low LED current can be to still saturate the output transistor.

Circuit simulator results often don't mirror reality. If you want more meaningful results, you have to change simulation parameters. For such a simple circuit, I wouldn't bother. You should be able to do the design on your own.

 

I know in my case the 4N35 doesn't allow the Q4 to saturate at 0.2V which is a normal value.

As far as I know you can never saturate the lower transistor in a darlington configuration as it will allways remain an emmiter follower, so ~0.65Vce at best. In other words, even if you short out collector to base, the B-E diode will remain.

 

Since you intend to saturate the transistor, you should assume a low CTR and drive the LED appropriately. If you want to minimize dissipation in the LED, you need to determine how low LED current can be to still saturate the output transistor.

Circuit simulator results often don't mirror reality. If you want more meaningful results, you have to change simulation parameters. For such a simple circuit, I wouldn't bother. You should be able to do the design on your own.

Thanks a lot for your suggestions!
I've done de calculations taking into account a \(V_{CE(OK3)}=0.2V\) and a \(V_{BE(Q4)}=0.66V\) and everithing makes sense!

 

As far as I know you can never saturate the lower transistor in a darlington configuration as it will allways remain an emmiter follower, so ~0.65Vce at best. In other words, even if you short out collector to base, the B-E diode will remain.

Unless you feed the collector of the first transistor directly from the supply, possibly with a current limiting resistor.

 

Unless you feed the collector of the first transistor directly from the supply, possibly with a current limiting resistor.

Yes then you could have low Vce in Q4, but that is not a darlington arrangement anymore.

 

Is it not known as a split darlington?

 

Never heard of it, and google images don´t seem to have either. Maybe by some other name?
Edit: well except for this one http://www.sourcewell.com/en/product_detail.php?id=276

 

http://www.bucek.name/pdf/6n138,139.pdf