# Current Amplification

Discussion in 'The Projects Forum' started by Alienite, Oct 15, 2016.

1. ### Alienite Thread Starter New Member

Oct 15, 2016
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0
Hello, I need to amplify a low current (30mA) voltage signal to a higher current without changing the voltage. The device requires about 700mA for optimum performance. The input voltage is 5v.

I understand that I may use some NPN transistor but I've no idea which one. Also I've read that transistor is an active device, so i'll need an additional power source for it?

2. ### crutschow Expert

Mar 14, 2008
13,496
3,373
A BJT NPN emitter follower circuit should work for you provided you can tolerate about a 0.7V drop from the input to the output voltage (in other words, with a 5V input, the output will be about 4.3V, albeit at the high current you want).
Yes, you will need a power supply of a least 5VDC.

If you can't tolerate that much drop, then you will need a more elaborate circuit, such as an NPN coupled to a PNP.

Is this just for a switching action output, or does the input vary in voltage between 0V to 5V and you want the output to follow?

3. ### Alienite Thread Starter New Member

Oct 15, 2016
3
0
At this stage I dont think 0.7V drop will affect the performance. And the input will remain constant all the time.
I've attached a circuit, kindly verify if it's correct.
Also, u mentioned a BJT NPN emitter follower; kindly tell the name of this device that I can tell to the shop guy.

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4. ### Alienite Thread Starter New Member

Oct 15, 2016
3
0
I've done some data search and I've learnt that I need a transistor with peak current more than the value that my device needs, that is more than 700mA, and with a gain high enough for the input current to cross 700mA. And then i'll have to add a resistor with the base to compensate for the additional current that the transistor will produce. The thing is, I'm all confused with choosing the right component and the calculation. Can u help me understand this part?

5. ### crutschow Expert

Mar 14, 2008
13,496
3,373
If you can put the load on the high-side, as you show in your diagram, than an N-MOSFET would be better, since a BJT should have at least 70mA base current for 700mA collector current.
That would deliver nearly the full 5V to the load.
Where do you purchase electronic parts so I can suggest a device to use.

Aug 27, 2015
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