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I know this is not an easy question but at least I thought someone may relieve my pain by explaining the big picture.

How is a voltage source made from current mirror or a constant current source by using BJT or MOS transistors? Can you give an example with a big picture explanation. Is that how linear voltage regulators built?

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  • \$\begingroup\$ you forgot about bandgap voltage reference with ratio feedback to comparator or fixed gain error amp \$\endgroup\$
    – Tony Stewart EE75
    Sep 4, 2017 at 17:47
  • \$\begingroup\$ Do you understand negative feedback? Have you done any research on the internet about this subject because there are plenty of resources. \$\endgroup\$
    – Andy aka
    Sep 4, 2017 at 18:04
  • \$\begingroup\$ @Andyaka I understand opamp's negative feedback concept but not in transistor level. Analog IC design texts are too complicated. I thought some one could give me at least a big picture view or some sort of foggy view whats that about. imho \$\endgroup\$
    – GNZ
    Sep 4, 2017 at 19:02
  • \$\begingroup\$ Most liner regulators like a 7805 for example are series regulators. Where as what you describe is common to voltage references which are designed as shunt regulators (like a TL431 for ex). \$\endgroup\$
    – sstobbe
    Sep 4, 2017 at 22:23
  • \$\begingroup\$ @sstobbe I saw the datasheet and the schematic of 7805 and it has like 18 transistors, many resistors and capacitors. Looks too complicated to analyze by looking. How can one understand the logic of it? Or at least is there a simpler one that is easier to understand whats going on inside? \$\endgroup\$
    – GNZ
    Sep 4, 2017 at 22:33

2 Answers 2

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Good question, and the fact is it's not just voltage regulators but many integrated circuits (especially analog) go to great lengths to replace certain components (resistors) with current sources and current mirrors. This is because resistors are expensive for ICs, and take up a lot of space. In a cost-benefit analysis, more complex designs without resistors are better than simpler designs with resistors.

Take the case of biasing a transistor, for example. A common emitter amplifier or an emitter follower buffer is really familiar with resistors all over the place. But then your boss says "resistors cost x, transistors cost 100x less, use more transistors and less resistors." The result of a lot of work is the typical amplifier stage design you see in many op-amp internal schematics (found on many datasheets). More transistors in current source and current mirror configurations and fewer resistors.

So how can a current source or current mirror replace a resistor? We can talk about that in another question, but essentially you remove the idea of a resistor and look at the absolute purpose of the resistor with respect to voltage, current, biasing, setting gain of an amplifier, etc and why it is that way. Then you can change the topology to get the same effect with a current source or mirror.

In the case of a voltage regulator, well thats going to require a lot of detail for virtually the same answer. If I get the time I can try to write something up, but if not, try doing some research of your own and see if you can understand anything.

Good luck,

Sam

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  • \$\begingroup\$ Thanks for the introduction to topic. I didnt know that. If possible could you recommend me a text you used to learn or enjoyed the subject? Most texts jump from one to another topology without explaining the logic and principles behind these. Regards \$\endgroup\$
    – GNZ
    Sep 4, 2017 at 22:25
  • \$\begingroup\$ One book I recommend is "Analysis and Design of Analog Integrated Circuits" by Gray and Meyer. It gets into this around chapter 3. And of course, a general text like the Art of Electronics will provide a fair amount of information about current sources and mirrors and general circuit topics, thats always good. Theres a good chapter on voltage regulation in AoE. Other books on analog CMOS circuits or similar could be helpful. Thats about all I can say, though! \$\endgroup\$
    – Sam Gallagher
    Sep 4, 2017 at 23:05
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Here is schematic

schematic

simulate this circuit – Schematic created using CircuitLab

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