EveryCircuit
Contact
Reviews
Home
ASMcoder
modified 2 years ago

Need help with NPN

0
8
104
01:31:38
I know how to find all the parameters of the circuit, provided that I know the current I of the collector, but I do not know how to find it. There is a way to approximate the calculation given that the voltage drop between the base and the collector is +- 0.7V, but this is not always accurate, does anyone know a better way, or how to find out the exact voltage drop?
published 2 years ago
kyo28
2 years ago
I didn't understand.... You mean how to discount the .7V drop in the simulation? I know u can edit the resistance.... But IRL it is described in the datasheet and can be different from manufacturer to manufacturer
ASMcoder
2 years ago
When I do the calculations assuming a voltage drop of 0.7 volts it is not accurate. The voltage drop varies depending on the voltage applied to the base, it can be more or slightly less (based on experiments in the simulator). But this approximation allows me to calculate the equivalent replacement of a transistor by a resistor (I*R=Voltage Drop), and the results are consistent with what I see in the simulator. Now I am no longer sure that it is correct to try to calculate the parameters of the circuit, provided that the voltage at the base is higher than the voltage at the emitter. ChatGPT pointed out that this is not the correct use of the transistor - which means there is no point in trying to calculate the circuit in this mode. On the other hand, in the simulator, this does not cause "obvious" breakdowns.
pip
2 years ago
you are driving an unlimited current through the base of your transistor into the emitter and only limiting is due to emitter resistor you would destroy the transistor. you need a base resistor base resistor is derived from collector current divided by beta stick a 100k resistor between base and v source and try your calculations again you Will be much closer
pip
2 years ago
pick a collector voltage let's say half your supply voltage and you want 5ma to pass through your transistor and let's say you want 1 volt on your emitter you divide 1 volt by your collector 5ma and that's your emitter resistor value eg 200R base current will be 5ma divided by transistor hfe=100 that's 50ua so your base resistor would be in your eg 30v divided by 50ua so 600k now I've not included our 1v emitter voltage or the 0.7v vbe so I pick 560k approx would be close enough to get you going in right direction
pip
2 years ago
I forgot to mention your collector resistor is half your supply voltage divided by 5ma as example in my previous comments so 1k with 10v supply
pip
2 years ago
your emitter resistor will now pass collector current plus base current roughly 5ma + 50ua
ASMcoder
2 years ago
pip, Thank you. The language barrier was overcome, and I was still able to make out what you were trying to tell)
pip
2 years ago
your welcome glad I helped a little.

EveryCircuit is an easy to use, highly interactive circuit simulator and schematic capture tool. Real-time circuit simulation, interactivity, and dynamic visualization make it a must have application for professionals and academia. EveryCircuit user community has collaboratively created the largest searchable library of circuit designs. EveryCircuit app runs online in popular browsers and on mobile phones and tablets, enabling you to capture design ideas and learn electronics on the go.

Copyright © 2026 by MuseMaze, Inc.     Terms of use     Privacy policy