Let an opamp drive an emitter-follower (a darlington is not needed for your low output current).
The opamp should be non-inverting with a gain of one.
Take the negative feedback from the emitter of the emitter-follower then the output voltage will be the same as the input voltage.

can I use variable input voltage & will it give same output voltage as input voltage with increased current

MOD:
The TS posted to a very old necropost.
Created a new Thread .

Link to old Thread for reference.
https://forum.allaboutcircuits.com/threads/emitter-follower-as-current-booster.80487/

 

Yes. But the maximum current available will depend on the current and power-handling capabilities of the transistor used for the emitter-follower.
Welcome to AAC!

 

give same output voltage

As long as the emitter follower is inside the control loop
so that its inherent offset is mitigated.

Regards, Dana.

 

Note also that the current boost is in only one direction.
Thus an NPN transistor will boost the output source current and a PNP transistor will boost the output sink current.

 

can I use variable input voltage & will it give same output voltage as input voltage with increased current

MOD:
The TS posted to a very old necropost.
Created a new Thread .

Link to old Thread for reference.
https://forum.allaboutcircuits.com/threads/emitter-follower-as-current-booster.80487/

Hello,

There are two basic approaches, one with a control circuit and one with just a transistor or two.
The transistor is not magic however, if you expect to get 20 amps at 30 volts out of the emitter than you must have a power supply that can supply at least 20 amps and a little more than 30 volts. Surprisingly some people think you can input 5v to a transistor and magically get 10v out, even though there is no 10v supply on hand.

The simplest is the transistor alone. That will boost the current as long as the power supply can handle it. It will also drop a little voltage, like around 1v, so your output voltage will be less than your input voltage. This may not be a problem if you have a constant load and you can adjust the output by measuring the output and adjusting by hand. If you need good temperature stability then you have to use a control circuit though.
Second is the dual transistor. One drives the other, so you get a lot of current gain. The drawback is the output is now decreased by about 2v.

Using a control circuit with the transistor(s), the control circuit automatically adjusts the output so as to keep it constant. This helps several problems such as load change and temperature variations.

 

How much current and voltage do you need ?

Regards, Dana.