Electronics > Beginners
BJT Current Miror
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Dat Tran:
Anyone help me to explain this circuit, I don't know the role of R4 resistor? In my lessons, I didn't see it!
This circuit is in TIDUE56 application note of Ti

http://www.ti.com/lit/ug/tidue56/tidue56.pdf

Thank in advanced!

JS:
R4 is there to bias Q1, if all the current would be going to the bases it could easily saturate them or go out of control, the circuit would be highly dependent on Q1 HFE which is never good.

Then you set a working point for Q8 setting it's emitter current, and Q2-Q7 will follow as all they bases sit at the same voltage and have equal emitter resistors. That's a rough explanation, better analysis is required if you want to know exactly how it does it, mainly around R3 and Q1, but there you have a kick start.

JS
Dat Tran:
Thanks JS, when i study at my school, I don't see anything same that, I learn about Current mirror not include R4 so I really trouble when i see it! but now, it is ok. thanks JS again!
JS:
In theory isn't needed but in practice it's better if it's there. Text book circuits are usually not good practice but oversimplified ones, easier to get the head around but not what will end up in a product.

While designing a product two or three major things have to be considered that usually are not considered in text books examples. Reliability, component tolerances and temperature ranges. In a text book you consider a single fixed value, that will change with time, temperature and between components. When you design a product and you need to manufacture thousands of them and all of them should last a long time, you need not to be worried about the tolerances in components or temperature variations making it work different than intended.

As you go along you learn this things, add to your circuits books more and more components to do the same thing that you did with two components on first year. One thing that's changing the game is semiconductor manufacturers producing ICs for each application, containing all this considerations where you just need to connect a few external components and get the job done. When you get into a niche application you need to work as hard as usual with less experience as the common products you made were all integrated.

JS
Dat Tran:
Yeah, You are right! I'm a newbie in electronic and less experiment!
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