Electronics > Beginners

Strange behavior - simulated op-amp rectifier

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rstofer:

--- Quote from: FriedMule on January 02, 2020, 08:31:29 am ---
I believe you, my thought on using this method is that I have seen many examples where other have used it as a power rectifier and read articles where people have praised this method. But on nearly all of the schematics have there not been shown any supply pins, so maybe it's why it do work in some simulators?
Is there any way to avoid having to use supply? I have thought about using compactors instead and I have thought of an, properly stupid idea, letting some of the finished DC power, back to the Op-Amps supply?

--- End quote ---

It is common practice on op amp circuits to omit drawing the power supply wiring but the device almost always shows the pins and labels them something like VC1, VE1 or +9, -9  as shown in your two schematics.

How do you expect all those little transistors inside the op amp to work if they don't have power?

See the schematic about half way down:

https://en.wikipedia.org/wiki/Operational_amplifier

Zero999:

--- Quote from: FriedMule on January 02, 2020, 10:15:01 am ---
--- Quote from: capt bullshot on January 02, 2020, 09:17:46 am ---This circuit works as a precision signal rectifier, intended to e.g. measure AC voltages by converting them to DC voltages. It doesn't deliver any power, and requires an external supply to operate at all, indeed it uses more power from the external supply than it uses from the AC input. It's quite common to not show the supply pins in such schematics, they're just implied to be there and appropriately connected to an appropriate supply.

So it's not applicable for your purpose, for minimum voltage drop at low power, use schottky diodes as the rectifier.

--- End quote ---
Okay thanks, just a small question, I have seen some use Mosfets with diodes or even schottky's, why do that?


--- End quote ---
The circuit you're looking for is an ideal diode. The easiest way to do it is to use an IC.
https://www.google.com/search?client=firefox-b-d&q=ideal+diode+controller+IC

The schematic you've posted with the MOSFETs, will not work with a capacitor on the output, so is only good for loads which don't require a smoothed power supply. It is also limited to peak AC voltages of no more than the gate-source voltage rating of the MOSFETs, which is typically around 20V.

There are ICs available to make a proper active bridge rectifier which will work with a capacitor load or some ideal diode controller ICs can be configured as a bridge rectifier.
https://www.nxp.com/docs/en/data-sheet/TEA2208T.pdf
http://www.ti.com/lit/ds/symlink/lm74670-q1.pdf

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