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What is the difference between using a nFET vs OP-Amp as a unity gain amplifier, i.e. voltage follower.

Application: Provide constant voltage with a variable load, see example below:

Min Current 0.1mA, Max Current 600mA

Voltage Follower Example

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    \$\begingroup\$ Your ground is upside down. All the electrons are going to fall out. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 25 '15 at 19:39
  • \$\begingroup\$ Assuming the electrons won't fall out of the upside-down ground, why wouldn't you just simulate (if not solve analytically) the two proposed circuits and see the difference? \$\endgroup\$ – Eugene Sh. Jun 25 '15 at 19:42
  • \$\begingroup\$ @IgnacioVazquez-Abrams, sorry, i don't get it. Are you specifically referring to the symbol? why does it have to "point" down? or is it just by convention? \$\endgroup\$ – reaver shadow Jun 25 '15 at 20:02
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    \$\begingroup\$ He's just making a joke ;-) We EEs make jokes all the time. It's common practice to have the ground pointing down. Doesnt mean 'we can't understand your schematic though. \$\endgroup\$ – Bimpelrekkie Jun 25 '15 at 20:30
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    \$\begingroup\$ @IgnacioVazquez-Abrams Nah! That's not ground, it's a Yagi Antenna! \$\endgroup\$ – Lorenzo Donati supports Monica Jun 25 '15 at 23:00
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You can do what you want with an NFET or an opamp.

If you use an NFET there will always be a voltage difference between Vin and Vout, this is caused by the Vgs of the NFET. This Vgs is also somewhat dependant on the load current. So if for example Vin = 4 V, your Vout could be 2 V if you draw 600 mA. For very low Vin (below 2 V) it can be that you will get 0 V at Vout.

Using an opamp will give more accurate results, however 600 mA is too much for most opamps. There are opamps that can deliver such a current but I am unsure if they can do so with a 5 V supply.

You could also combine an opamp and a NFET, something like discussed here But again the 5V supply might be limiting, for 600 mA.

Also note that you will need some cooling (a small heatsing) for the NFET or the power opamp in case you will be drawing the 600 mA.

As suggested above, it will be a good excersize to use a simulator (LTspice or QUCS) and try this out for yourself at 0 cost (OK just some time).

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  • \$\begingroup\$ My pleasure ! Enjoy :-) \$\endgroup\$ – Bimpelrekkie Jun 26 '15 at 21:11

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