# Help with voltage follower

#### igorcarajo

Oct 30, 2014
2
Hello, this is my first post on this forum. I'm a mechanical engineer. I've built some simple circuits before, but I'm really an "outsider" in the world of electronics. This is my problem: I have a circuit with an output signal that is 0-10 VDC, with a maximum current of 1 mA. I need to feed this signal to another circuit that requires a much higher current, maybe as high as 500 mA. I've been researching, and what I think I need is called a unity gain voltage follower, or voltage buffer. It is not terribly important that the output voltage exactly match the input voltage, so long as they are with 2-3% of each other. I saw some articles showing a very simple schematic using an op amp. The part where I'm stumped is finding an op amp that can handle 500 mA. So, am I looking at the wrong type of circuit? Any suggestions? Also, is there any commercially available product that can do this and that comes in an enclosure with electrical connections, so I don't have to build a bunch of circuits and mount them in a box and all that? (I might need 20). Thanks in advance.

#### Gryd3

Jun 25, 2014
4,098
Hello, this is my first post on this forum. I'm a mechanical engineer. I've built some simple circuits before, but I'm really an "outsider" in the world of electronics. This is my problem: I have a circuit with an output signal that is 0-10 VDC, with a maximum current of 1 mA. I need to feed this signal to another circuit that requires a much higher current, maybe as high as 500 mA. I've been researching, and what I think I need is called a unity gain voltage follower, or voltage buffer. It is not terribly important that the output voltage exactly match the input voltage, so long as they are with 2-3% of each other. I saw some articles showing a very simple schematic using an op amp. The part where I'm stumped is finding an op amp that can handle 500 mA. So, am I looking at the wrong type of circuit? Any suggestions? Also, is there any commercially available product that can do this and that comes in an enclosure with electrical connections, so I don't have to build a bunch of circuits and mount them in a box and all that? (I might need 20). Thanks in advance.
Can you share some additional info for us?
What device are you wanting to feed the signal to?
500mA for a signal wire seems kind of high, and because current is a function of voltage and resistance, it would help to know a little more about the devices.

#### Supercap2F

Mar 22, 2014
550
Is it a analog signal or digital? If it's digital you can use a transistor. If it's analog your going to need something more complex.
Dan

#### Arouse1973

Dec 18, 2013
5,178
You could use an OP-amp with an NPN transistor on the output as a current amplifier. Choose a rail to rail opamp powered from the 12 Volt supply. The transistor should be rated for at least 1A if not 2A and you might need a small heat sink. You could look at using a Darlington transistor instead which will reduce the output current needed from the opamp . You will need a bit of supply decoupling also. A 100nF ceramic and a 22uF Tant or electrolytic capacitor across the supply close to the opamp, these need to be rated higher than 12 V, 16 V should be fine.

#### igorcarajo

Oct 30, 2014
2
Thanks for the responses. To answer a couple of the questions above, the device being controlled is a driver for a dimmable LED light. The analog 0-10 V signal determines the intensity of the light (doesn't provide the power for the light, it's just a signal). The thing is, I need to control a bunch of these LED drivers with a single signal, that's why the current gets high.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I recommend Adam's circuit in post #4. But you will need a higher voltage power supply, especially if you use a Darlington (which I recommend). I think your power source should be around 15V DC because most op-amps can't drive their outputs closer than about 1.5~2.0V below the positive rail, and the Darlington could need up to 2V base-emitter voltage.

The op-amp needs to be a "single supply" type whose input range includes the 0V rail. There are many suitable op-amps - LM358 (two op-amps per package) and LM324 (four per package) are widely available and cheap.

#### Colin Mitchell

Aug 31, 2014
1,416
In circuit #4 you are not gaining anything by adding the op-amp. Just use the emitter-follower transistor.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
In circuit #4 you are not gaining anything by adding the op-amp. Just use the emitter-follower transistor.
The reason for using the op-amp is that the output voltage will then follow the input voltage accurately.

If you use just an emitter follower, the output voltage will be about 0.6~0.8V lower than the emitter due to the base-emitter voltage drop (twice that, if you use a Darlington transistor). Also the base-emitter voltage will vary somewhat with load current, which will normally vary with output voltage.

With the op-amp, the input-to-output voltage difference is reduced to roughly the input offset voltage of th op-amp, which is typically a few millivolts for older designs, and less for modern devices.

Last edited:

Aug 31, 2014
1,416

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
First he doesn't need accuracy, then he wants an INSTRUMENT AMPLIFIER ACCURACY. He's only driving LEDs !!!!
In post #1 he mentioned an accuracy of 2~3%. That may just have been a guessimate. A 0.7V base-emitter drop is a 7% error (offset), or a 14% offset if it's a Darlington.
You're going to have to do a lot of fancy footwork to get a 0v - 10v into a 0v - 12v output with an op-amp.
Yes. I pointed that out in post #6 where I suggested a power supply voltage of 15V.
He just needs a Sziklai Pair
That will still have the 0.7V offset, but it would be better than a Darlington, and a 12V supply would work.

Depending on the characteristics of the drive circuit, removing most of the base-emitter offset could be as simple as adding a diode between the input and the base (anode to the base) and a pullup resistor from the base to the positive power supply.

But I think given the requirements (undefined, low-current source; 2~3% accuracy) an op-amp feeding a Darlington is probably his best option.

#### BobK

Jan 5, 2010
7,682
500mA sounds really high for a dimming signal, even if you had 100 of them. Can you post a link to the LED driver specs?

Bob

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