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Perhaps it was asked a million of times, but I could not find the answer.

Say for example I have a basic voltage divider used as a reference for TL431 or a simple resistor in pairs with a zener diode. I can stepdown the voltage, but I can't exceed the current above some point, because of the upper-leg resistor. Am I right? I just can't workout an idea into my brains about whether or not I can amplify this current and still remain the same voltage even after doing some research. Could I do it using FETs? I want to supply 555 with 5V from mains input, but this part of the circuit will also be used to drive a pair of transistors, so I want to have an advantage of higher current.

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2 Answers2

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Your terminology is incorrect here. You ideally want the divider/reference to output zero current while replicating its output voltage at a lower output impedance somewhere else so that output currents don't drag down the voltage as much. But zero current from output of the divider/reference multiplied by any gain is still zero current. So what you want is to actually buffer voltage, not amplify the current.

In which case an opamp wired as a voltage follower is the simplest approach that will do the job (albeit with limited output currents only but you have not given us any current requirements). Higher currents can be achieved by using the opamp voltage follower only as a closed-loop feedback control device (rather than a power device) to drive external transistors in a push-pull configuration or open collector (depending on what you need). It gets increasingly complicated from there the more current you need.

Also, add a circuit to your original post. This website has a schematic editor and you can edit it into your post.

DKNguyen
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  • Though many opamps are quite feeble in output current specs. Might need some transistors in the feedback loop to boost it. – Scott Seidman Apr 02 '20 at 17:58
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    @ScottSeidman Indeed. This is just the simplest approach in the absence of any current requirements from the OP. – DKNguyen Apr 02 '20 at 17:59
  • Generally, a voltage buffer **is** a current amplifier. – The Photon Apr 02 '20 at 18:08
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    @ThePhoton Well, in my mind a current amplifier doesn't just provide bigger current willy nilly. It predictably provides bigger current in proportion to some input current, and in this case the load current could change while the input current remains more or less the same which means your current gain now varies all over the place which isn't much good as an current amplifier. – DKNguyen Apr 02 '20 at 18:23
  • @DKNguyen I think the OP is probably asking for what The Photon is pointing towards. – jonk Apr 03 '20 at 09:19
  • @The Photon, is the source follower (FET) a current amplifier? If so, what is its input current? The voltage buffer is not a current amplifier; it is a voltage amplifier (with gain of 1). So its output behaves as any constant voltage source that can deliver big enough current through the load. I share DKNguyen's view that in a current amplifier the output current is proportional to the input current. Obviously, OP needs a voltage buffer. – Circuit fantasist Apr 03 '20 at 20:17
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If I get the question right (not always a great assumption, but anyway), what's being asked for is to make a 'stiff' reference for some other devices in the circuit.

There's a type of device called a 'buffered reference', often used to make a virtual ground for single-ended op-amp circuits.

Here's an article about this very topic: https://www.electronicsweekly.com/market-sectors/power/ten-best-ways-split-voltage-rail-2015-02/

Now, about your 555... that's a whole 'nother topic, which I've spoken to here: Is the NE555 the IC I need, and if not, what do I replace it with?

hacktastical
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