Purpose of opamp follower / buffer in power supply

[FiveseveN]

That doesnt say much then for the Angry Charles Vref voltage follower if that case holds true.

No, it doesn't. Josh is more of a historian than an EE. Definitely don't load a TL072 with 100u.

[bluelagoon]

Yeah well I figured as much 100uF was a bit of overkill, judging by general consensus, but more to the point, where is it right to be placed, before the opamp or after the opamp?, Like I said earlier one person commented concerning Vref Voltage followers elsewhere that -
"Op-amps generally can't handle excessive capacitance on their outputs"
Is that statement relevant to where this capacitor should rightly be placed?

[FiveseveN]

"Op-amps generally can't handle excessive capacitance on their outputs"
Is that statement relevant to where this capacitor should rightly be placed?

Well, it's a big ass cap and it's right on the output, so it's very much relevant. Plus it acts like a short to ground while it charges (TL072s love that ).
Look at the original post in this thread: C3 shunts R1 and R2's Johnson noise. But seeing as the op amp's input is roughly infinite ohms, the resistors can be bigger and the cap proportionally smaller (thus cheaper).

LE: Some 4 years late I just noticed the inverting and non-inverting inputs are swapped in that schematic. You get the idea, it's supposed to be a buffer.

[antonis]

If you DO insist to place a cap on buffered Vref, you'll have to place an output series resistor..

https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

[bluelagoon]

Yeah But Nah But, what if I place that filter cap before the op amp, as in the first image posted first post this thread, will I still need this output series resistor? Thanks.

[antonis]

 

No..!!
(unless, of course, buffer output drives some other significant capacitance.. - stray one included..)

[amptramp]

An op amp used as a buffer for Vref has some advantages.  If you are providing the bias for two or more stages, the low output impedance of the op amp buffer keeps the signal at one Vref point from contaminating the signal at any other point because the output impedance of the buffer is very low, on the order of a few ohms.  In some cases, without the Vref buffer, you might want separate Vref dividers for each place where Vref is used to avoid feedback from one circuit that uses Vref to another circuit that uses Vref.

As far as adding a large capacitor to the output of an op amp, DON'T!  The op amp used as a buffer is set for unity gain and the feedback from output to inverting input maintains this gain.  But each op amp has an output impedance that varies with frequency, so imagine a resistor connected to the output of the op amp.  If this is connected to a capacitor to ground, this acts as a lowpass filter that reduces the amount of feedback at higher frequencies.  Then the feedback connection that provides unity gain is turned into a lowpass filter giving a rising gain characteristic that intersect the op amp's falling gain characteristic and you may get oscillation or at least a poor transient response.

[antonis]

In some cases, without the Vref buffer, you might want separate Vref dividers for each place where Vref is used to avoid feedback from one circuit that uses Vref to another circuit that uses Vref.

What Ron said..

https://aionfx.com/app/files/docs/theseus_kit_documentation.pdf
Look at page 28 VB & VD configuration..

[ElectricDruid]

+1 what the others have said.

The cap goes on the voltage divider, so that the op-amp sees a stable filtered voltage to buffer.

You *can* add a proper RC filter after the op-amp, but then your nicely buffered reference voltage will move about with changing current draw again, just like you were trying to avoid - so why would you?

Adding hefty caps to ground directly on the output of the op-amp is just wrong and likely to cause problems. The fact that some circuits have done it and apparently got away with it in most cases doesn't make it a good idea. At all.

[bluelagoon]

Yeah, What Ron said!

[bluelagoon]

Thankyou all for your patience and kind explanations. Cheers.

[duck_arse]

This is contrary to the circuit from the opening post in this thread.

And then I read from a contributor on the electronics stackexchange that
"Op-amps generally can't handle excessive capacitance on their outputs"

Well, it's a big ass cap and it's right on the output, so it's very much relevant. Plus it acts like a short to ground while it charges (TL072s love that ).

As far as adding a large capacitor to the output of an op amp, DON'T!

+1 what the others have said.

there. now you have a collection of people on the interwires saying it is the wrong thing to do. it is now up to you to root it out at every occurence and flag the error whenever you see it.

[Ben N]

I just want to chime in and say what a great example of a perfect DIYSB thread this is. Clear, thought-out questions followed by clear, intelligent answers by a number of contributors, with documentation (love that capacitive-loads-on-opamps-and-what-to-do-about-them article, Antonis!)--i.e., real learning going on--and even an entirely appropriate necrobump, plus the courtesy and friendliness this place is rightfully known for.
Kudos, friends.