Opamps as voltage sources (C-loading)

[George Herold]

On Tuesday, April 30, 2019 at 7:47:21 PM UTC-4, John Larkin wrote:

On Tue, 30 Apr 2019 11:57:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

snip.
Not all RRIO amps have the topology I referenced. You've got to do
what you did, try some load cap and a pulsed load to see if it
oscillates or rings.
I was pulsing the input. I'm not sure how to pulse the load.
Well, simply with a function generator.
How about a 10 k ohm load R and I toggle the bottom from gnd to +1 V.
(it's a 5V supply)

I'll see if that looks the same.


Input is just as good. I'd go from, say, +2 to +3 and not start at
ground. The amp will slew into a big cap, and current limit, which can
be more interesting. So play with the square wave amplitude on top of
some DC offset.

Well, silly me, I was coupling ground through the func. gen.
oscillations everywhere ...wow.! (~1-200mV p-p)
(with the FG at ~0V the oscillations were damped, but
TC >1 ms. ~ 800kHz freq..weird DSO display :^)
I added the series 10 ohms and everything got clean... huh?
I'm not sure what I was testing with my circuit F-up...
But I'm keeping the 10 ohms. :^)

So this 5V supply is going out into the world, and
someone (most likely a physics student) is going to connect it
wrong. For an opamp grounding is fine. But what about an
over voltage* on the output? Maybe I want more series R?

(or the lm317 in to92 clip and replace when someone fries it)

*Not AC voltages, but a 60 V DC lab amp...
maybe 2 or 3 (4-6 total) lnd150's in parallel?
5V at 3-5 mA.
or some beefier depletion fet and a resistor?
(say 10 to 100 ohms)

You can AC couple a square wave, from a 50 ohm fungen, into the output
too.
Yeah I was trying to do a DC thing...

George H.

GH


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tim Williams]

<tabbypurr@gmail.com> wrote in message
news:dd13a58a-f266-49d8-93dc-8ca66ea64173@googlegroups.com...

The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue
rightwards
into the right half plane (i.e. pushing it into oscillation).

yes - and R_out dominates massively over C_ESR in that situation.

But L_out dominates even more, that's the problem. (The goal of the RCRC
network, of course, is to dominate both, so the amp is stable by itself,
/and/ the output is kept accurate.)

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

On Wednesday, 1 May 2019 02:14:13 UTC+1, George Herold wrote:

On Tuesday, April 30, 2019 at 8:29:56 PM UTC-4, tabby wrote:
On Wednesday, 1 May 2019 00:46:30 UTC+1, Tim Williams wrote:
tabbypurr> wrote in message
news:0aa3f735-4e20-4187-922f-0f76ce041ab9@googlegroups.com...

Why does opamp R_out not dominate?


That was in context of C only (and whatever ESR it has), no explicit R_out.

not sure I'm following you there.

Do you mean the amp's output (source) impedance?

yes

The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue rightwards
into the right half plane (i.e. pushing it into oscillation).

Tim

yes - and R_out dominates massively over C_ESR in that situation.

Yabut, R_out is inside the opamp feedback loop, and C_esr isn't.
(If I'm following correctly)

Yes, and it's the whole feedback loop that determines stability. R_out is part of that.


NT

 

On Wednesday, 1 May 2019 05:05:26 UTC+1, Tim Williams wrote:

tabbypurr> wrote in message
news:dd13a58a-f266-49d8-93dc-8ca66ea64173@googlegroups.com...

The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue
rightwards
into the right half plane (i.e. pushing it into oscillation).

yes - and R_out dominates massively over C_ESR in that situation.


But L_out dominates even more, that's the problem. (The goal of the RCRC
network, of course, is to dominate both, so the amp is stable by itself,
/and/ the output is kept accurate.)

Tim

I guess just treating it as (increasing) R_out was my error.

NT