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I want to design a circuit having output voltage of 3.3 V and 30mA current. As I am newbie in the electronics field, I see lots of circuit using buck topology, using IC's and using transformers.

Now as I heard something great in my BOOK i.e. current mirror configuration using BJT. As far as explained in my book, it is a circuit consisting of two matching transistors with the collector of one connected to the bases of both,thus producing the same collector current in each transistor.

Now in order to get the collector current in mA, only a small amount of uA current is required. By providing a little amount of base current can I get the sufficient amount of current from the collector end which s furthur used in my application??

If using this (current mirror configuration) with a capacitor power supply(using 0.15uF cap which gives upto 11mA currnt) then its easy to get a small amount of current in uA which is furthur supplied to the base terminal.

If not then what is the reason as I really stuck at that place.I also searched it on internet but didn't get a good response.

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    \$\begingroup\$ No offense, but it is a mess in your head. Better forget for a while all these buck topologies and start with the fundamentals - ohms law, how diodes and transistors work, how inductors and capacitors works. And current and voltage sources and whats the difference. Then try to work on some simple (really simple) schematics and read some (many) books. \$\endgroup\$ – johnfound Oct 10 '13 at 6:13
  • \$\begingroup\$ @johnfound I did and also do the same. But I just want to clear my doubts. Is it possible to use current mirror configuration like this?? \$\endgroup\$ – SSS Oct 10 '13 at 6:16
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    \$\begingroup\$ No. And don't ask me why. \$\endgroup\$ – johnfound Oct 10 '13 at 6:24
  • \$\begingroup\$ What are you trying to achieve? You have latched on to an idea (that sounds inappropriate) and we (the readers) have little idea what you really want functionally. You say 3.3V and 30mA current but what is this used for? \$\endgroup\$ – Andy aka Oct 10 '13 at 7:18
  • \$\begingroup\$ @Andyaka Can I use this power for driving our controllers or LED's or any load that operates smoothly at 30mA. \$\endgroup\$ – SSS Oct 10 '13 at 7:47
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Now in order to get the collector current in mA, only a small amount of uA current is required

In one model of transistor operation, the base current controls the collector current. I'm concerned that you might be thinking that the base-current somehow creates collector current - it doesn't. You could think of it as the base current operating a valve that lets through collector current. All the current has to come (ultimately) from your power source.

using 0.15uF cap which gives upto 11mA currnt

Capacitors of a certain capacitance don't really have a specific current associated with them. There may be a maximum current they can safely deliver but that is likely to be a lot more than 11 mA for all but the tiniest capacitors. In essentially DC applications, the current flowing out of a capacitor depends on the voltage to which the capacitor has been charged, the resistance of the load and the current decays exponentially over time as the stored energy is release and the voltage drops.

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  • \$\begingroup\$ Ohh Ahhh uhhh!!! Nothing to say as I was totally wrong. Thnxx for the ans. \$\endgroup\$ – SSS Oct 10 '13 at 9:18
  • \$\begingroup\$ Ok Now I got that how this configuration is responsible for the designing of a IC. \$\endgroup\$ – SSS Oct 10 '13 at 9:19

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