Class problem help (Converter U-I)

[Razvan]

Hello everyone, I have this problem where i need to propose the supply/input voltage and the output current (that I want to obtain) and then to calculate and choose the correct components so my circuit can work.


So, I choose the supply to 5V and the output that i want to obtain to 20mA. From my calculations, I got R = 250ohms, and i choose it 270ohms (standard). But now, my teacher wants to calculate the values for the NPN transistor and choose one properly (I choosed bc107a, but i dont think it's good, because i didn't make any calculations). I also tried to do my best with the schematic in orcad.


Can i get a little help?

 

[Alec_t]

Welcome to EP!
If 20mA flows through R1, how much current would you expect to flow through R2 and what would be the expected voltage on the transistor collector?
Are all the transistor voltages and currents within the limits specified in the transistor's datasheet?

 

[Cannonball]

Hello Razvan,
Just a word of encourgement. Alec-t has given you a very good hint. Good Luck.

 

[Razvan]

Welcome to EP!
If 20mA flows through R1, how much current would you expect to flow through R2 and what would be the expected voltage on the transistor collector?
Are all the transistor voltages and currents within the limits specified in the transistor's datasheet?

That's the problem. I dont't have a particular transistor imposed, i need to choose one from the 'catalog' that checks my circuit... And about the R2 (witch i think is RL) I don't know how to choose (or calculate) it. I'm so confused.

 

[Razvan]

I got it now, so: i choosed my Vi=5V and the output current (Ie) that i wanted to obtain (≈20mA), because this circuit it's an U-I Converter. AND i got the Ie to 18.52mA, which is very ok and i'm happy.



Now i have calculate the VCE on the transistor (with formulas and stuff) and choose the correct transistor from the catalog (i put BC107a because it's the only transistor i know :banghead. Also, do you know what is the role of the diode in the whole circuit?

 

[Alec_t]

Also, do you know what is the role of the diode in the whole circuit?

It does nothing useful if V1 stays at 5V. It would prevent the transistor base-emitter junction being driven beyond its reverse voltage rating if V1 were negative.