# Amplify current without changing voltage

Discussion in 'General Electronics Chat' started by rwp289, Oct 21, 2010.

1. ### rwp289 Thread Starter New Member

Sep 30, 2010
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I need to amplify a low current (50mA) voltage signal to a higher current without changing the voltage. The voltage output operates a meter that requires around 700mA to move the needle. The voltage range is from 2.3 to 12.8 DC.

I have read that this can be done by connecting the output to the Base of a NPN transistor. But I have also read that when the B is biased enough to allow for current to flow, it connects the Collector voltage to the Emitter, not the Base voltage.

Any help you can provide is aprreciated.

2. ### SgtWookie Expert

Jul 17, 2007
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Using a transistor like that will result in an offset of perhaps .7v to .8v of the voltage on the base.

You might use a power opamp, such as an L2720 or L2722, in a voltage follower configuration; output wired to the inverting (-) input, and your signal to the noninverting (+) input.

3. ### rwp289 Thread Starter New Member

Sep 30, 2010
11
0

That was quick. And a quick look at a voltage follower circuit seems to be the ideal way.

To be clear for future projects, as for the transistor option; If I were to hook up the output signal to the base, I would get the base voltage at the emitter, minus the .7v to .8v?

4. ### SgtWookie Expert

Jul 17, 2007
22,183
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Maybe not ideal, but it works, and is very inexpensive to implement with those opamps.

Very roughly, yes. The current through the base is amplified by the gain of the transistor. However, the gain is generally not linear across the range of input current, and Vbe will vary a good bit over temperature and current. However, there is no feedback.

With the opamp voltage follower/buffer, the inverting input tries to make the output do what the noninverting input is doing; there is positive feedback. There will be a slight difference between the input and output due to the input offset; generally a few mV's. There are ways to adjust the offset, but unless your application requires a great deal of accuracy, it most frequently is not worth the effort and expense.

5. ### rwp289 Thread Starter New Member

Sep 30, 2010
11
0

Again, thanks!!

A few mV's is of no consequence to this application. Even 0.1V to 0.2V is acceptable.

I am stopping by and picking up a couple of op amps to test this weekend. Your direction will be a big help.

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You will most likely have to order the L2720/L2722. Digikey.com, AvnetExpress.com, and other suppliers carry them. ST Microelectronics makes these opamps. The L272 is an industry standard power opamp, but it's specifications are not as good as the L2720/L2722.