The black box is the circuit I was thinking about implementing. The inputs are on the right, it's the resistance and voltage applied to the input. Paul Falstad didn't put how it does, but what it does is that the output current is linearly dependent on the voltage applied and also proportional to the current drawn by the other input. So it's Output current = V_in*I_in*k.

Now I saw that I made a mistake in the title. It's supposed to be I_out=f(V_in,I_in), but as I_out = I_in, the title made it look confusing. I forgot that second relation.

Ah, thanks for the explanation. It's an expression I've never heard in an engineering context, though the notion of the issues associated with having to sleep with a blanket that's too short is in the broader cultural awareness, but I think I've really only come across it as a visual joke in old movie clips, never used as a metaphor for not being able to get everything you want. As you say, we have words like "trade-offs" and "compromises" to fill those needs. That's one of the nice things about the English language, what we don't have, we will gladly steal from any other language without shame or apology.

Not being enough about "trade-off" being the same as "trade", "compromise" is the same word as "agreement". Man, what a project of language is Portuguese?! It's missing words!
In Portuguese we stole a lot of words. A foreigner would be here and understand a lot of things just because there is a lot of companies trying to "be cool" using English words. Companies are using so much English words alone in their culture that workers have a rage against this English-Portuguese dialect.

 

So it's Output current = V_in*I_in*k.

So what are the values of input and output V and I that you want this "box" to work with?
Have to know that if you want to build a real circuit.

 

So what are the values of V and I that you want this "box" to work with?
Have to know that if you want to build a real circuit.

I'm already satisfied, because it was just curiosity. But I imagine 5 V and 100 mA would be good values.