Current mirror using BJT application in a circuit

[shaswat]

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 transformer in order to isolate the output from the input section.

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 then its easy to get a small amount of current in uA which is 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.

 

[kripacharya]

I think you need to understand power supply specs.

When it is said that a power supply is 3.3v and 30mA current, it usually means that the voltage is fixed at 3.3v, and it can supply any current upto 30mA.
Meaning that if you take more than 30mA current, then there is no guarantee that the voltage will remain at 3.3v, and usually the voltage will drop down.
This is a constant-voltage power source.

On the other hand there is the constant current source. Your Current mirror bjt circuit is a simplistic example. Here, the current remains constant at 30mA, but the voltage on your collector will vary depending on the load, and a high enough resistance will saturate upto the main supply voltage, at which point the current will start decreasing.

Now if you want to have constant voltage AND constant current at the same time, then all you need is a fixed resistor load !!!
In your case, 3.3 v and 110ohm will be exact 30mA. But thats pretty useless.

So now you need to clarify your thoughts, and explain what you really want to do?

 

[shaswat]

I think you need to understand power supply specs.

When it is said that a power supply is 3.3v and 30mA current, it usually means that the voltage is fixed at 3.3v, and it can supply any current upto 30mA.
Meaning that if you take more than 30mA current, then there is no guarantee that the voltage will remain at 3.3v, and usually the voltage will drop down.
This is a constant-voltage power source.

On the other hand there is the constant current source. Your Current mirror bjt circuit is a simplistic example. Here, the current remains constant at 30mA, but the voltage on your collector will vary depending on the load, and a high enough resistance will saturate upto the main supply voltage, at which point the current will start decreasing.

Now if you want to have constant voltage AND constant current at the same time, then all you need is a fixed resistor load !!!
In your case, 3.3 v and 110ohm will be exact 30mA. But thats pretty useless.

So now you need to clarify your thoughts, and explain what you really want to do?

I need to make a power supply whose requirement is 3.3v with 30mA. Holding the output at 3.3V is not a big issue as I used a constant voltage source. But I need current in order to drive my device/IC. So can I get that amount of current using this configuratin. As collector current is beta times the base current. So if I choose a transistor having beta 100 and limit the base current to 0.3 then can I get my required current?
As transistor is a current amplifier is it possible to get the current? Correct me if I am wrong and boat on the wrong way

 

[keith1200rs]

I think, as kripacharya says, you are misunderstanding the power supply requirements of an IC/circuit. What IC are you trying to power?

Keith