AoE x-Chapters, High-Speed op-amps section, DRAFT

Read and comment. AoE x-Chapters, High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that was meant to come after each relevant chapter, can skip on a quick first read, go back later for detailed info. But as main book was growing over 2000 pages, we opted to bring out main book first, including x-Chapter cross refs, follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6 Chapter 4x is to supplement H&H AoE III, chapters 4,5,8. (Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing? Examine tables, any of your favorite parts missing?** Want a separate low-power table? Can we skip that? Is the CFB op-amp scene explained well enough? Did you like VFB scatterplots? Need some for CFB? Have a favorite trick that should be included? Make comments here or send to snipped-for-privacy@yahoo.com

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** For op-amps that come in multiple variants, i.e., single, dual, quad, we list only one part, and show # per pkg variants in next column, so you may not see your part, e.g., we show OPA192, not OPA2192 or OPA4192. As you can see, we were forced to economize on space.
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 Thanks, 
    - Win
Reply to
Winfield Hill
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On a quick read, I didn't see any mention of c-load stable amps. That could be an interesting section.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

I could scribble up a half page or so on the subject.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

What p/n would you include for that?

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    - Win
Reply to
Winfield Hill

There are only a few declared c-load stable opamps. We use a lot of LM8261s. In reality, many rro amps are c-load stable, because adding a capacitive load reduces the rolloff corner frequency but doesn't add a pole.

LTC has a dozen or so c-load opamps, LT1200 and such, but LTC tends to be expensive.

We often distribute a heavily-bypassed reference rail, and it's convenient to drive that with a c-load-stable amp.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB. "AB. stable with unlim cap-load." Four types are marked with the AB comment.

There aren't very many C-load opamps, and my memory is most of them have fT under 10MHz, which would be too low for these tables. But a separate section labelled C-Load, would not have that restriction.

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    - Win
Reply to
Winfield Hill

Oops, missed that on a quick read.

Some opamps are unstable over a fairly narrow range of cap loads, but are happy with big caps. Use the Jim Williams technique: a big enough capacitor will stabilize anything.

With a big c load, one might not expect much bandwidth!

We developed a fancy I/Q modulator box to simulate a blade-tip sensor in a jet engine. It looked great in Spice but it oscillates big-time in real life. We used some really fast current-mode amps, AD8009s. I sent my guy your opamp tables and he'll use that to pick some slower (roughly 250-300 MHz) voltage mode amps, which we hope will tame things.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

I went looking for the LT1200 that JL mentioned. I found this,

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George H.

Reply to
George Herold

Pretty good.

Two suggested additions:

  1. EMI behavior of opamps. This is generally not understood at all by engineers and (still!) not taught at universities from what young EEs told me. A bipolar input stage will rectify RF at the first BE junction, even stuff at cell phone frequencies. This rectification or demodulation is very inefficient but since that is inside the loop any resultying baseband AM will hit at full tilt because it happens at "open loop".

This has brought me several consulting assigments so maybe don't add it. OTOH I am trying to retire, so maybe put it in, so I can cycle more, bike more and brew more beer.

No kidding, two weeks ago I explained that to two engineers at a client and they went "WHAT?!". Many don't believe it until demonstrated. GSM phones are good for doing that, when they connect to a new tower. Switching to a CMOS opamp usually makes that problem go away 100%. When I did that for the first time I almost achieved guru-status at that client because nobody had ever fixed an EMC issue without a serious re-layout there.

  1. Back-to-back input protection diodes between IN+ and IN-. Very often overlooked. In datasheets they are sometimes only mentioned in a footnote under the abs max table but often there is only a +/-0.3V diff limit. Aside from pouring gasoline on the above mentioned EMI issue these diodes can really throw people a curve when using opamps in an unorthodox way or as a comparator. Thou shalt not do that but ...
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Reply to
Joerg

Thanks, Joerg, very good suggestions. I've used the CMOS trick before, people should know about it. I'll go through all the datasheets, and flag the ones that have input diodes.

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Reply to
Winfield Hill

I've been using the National (is that now TI?) LM8272 for some years now. It's 15 MHz so I guess not much different. Works a treat for audio work.

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  Rick C. 

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Reply to
gnuarm.deletethisbit

AD8011 is available in DIP8

Reply to
Johann Klammer

Am 24.04.19 um 21:35 schrieb Joerg:

It's not that FETs are any worse at demodulating than BJTs, it's just that they need a higher source impedance for the same dBms. Say, a different cable transformation. At the same time their higher Cs make it harder for that match to exist. The loop gain is the same.

It's much like in HP's app note "Every Schottky diode is a zero bias detector".

These diodes are anti-parallel, not an efficient rectifier. It's a very good thing that they are there. A low noise op amp with zenered BE junctions at the input is no longer a low noise op amp. And an op amp with 100 dB open loop gain and a Volt between its inputs is a design error. Using an op amp as a comparator is not unorthodox, it's wrong. The diodes are there to protect other stuff from dying.

BTW an ADA4898 can draw a _lot_ of supply current just when one of its inputs is open circuit. It can get too hot to touch.

cheers, Gerhard

Reply to
Gerhard Hoffmann

Good to know, thanks, added to fixes.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Main thing is to explain the mechanism, why it happens. IME people really do not know this, often even after decades in the industry. Then they engage in expensive and time-consuming system level fixes to muffle a susceptibility problem where a simple module level remedy (a switch to CMOS) would have done the trick.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

The misbehaving circuit is an all-pass phase shifter

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(from the Williams book) which tops out about 30 MHz. The AD8009s simulate fine here but oscillate wildly in real life. We just tried some LT1818s, 400 MHz VFB amps, and they behave.

This all-pass doesn't like current feedback amps.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

MOSFETs don't demodulate at all because there is no conducting diode path. I have found cases with hardcore EM susceptibility to the point where a person outside the concrete walls of the building could upset a circuit operating at single-digit kHz range, just by turning on their cell phone. GSM ones were especially bad. After switching to a CMOS opamp ... nad, zilch, not even when holding a cell phone right above the open circuit and then turning it on.

A downside is that one can often not achieve the same low noise performance with CMOS opamps.

For RF in, say, the cell phone range there is no or hardly any loop gain unless you have a super-fast opamp.

Yep :-)

Or in American "Every opportunity has its tax".

Can be enough. There is always an offset.

Not if you know what you are doing and the mfg has blessed railing the output.

Nah, BTDT. You have to know the limits and not exceed reverse Vbe anywhere. Which can be a mere -2V in RF stuff.

Real hot stuff doesn't have diodes. For example the SD5400 quad FET array, a marvelous IC. One of my circuits almost drove a tech insane because every one of them he soldered in died. Until I showed him how to avoid every last bit of ESD. Once inside the circuit everything was safe.

They should explain that in the datasheet.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

There are a few specifically EMI hardened zero-drift opamps around now, like ADA4522. They seem ideal, but they are complex inside and have some quirks.

JFET opamps probably rectify a lot less than bipolars.

Maxim makes a *comparator* with back-back input diodes!

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Some opamps are pretty good comparators, just the right speed for some applications. AD8033, OPA171.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

You're not using 10K feedback resistors, are you? That could lead to odd behavior with CFB amps.

-- john, KE5FX

Reply to
John Miles, KE5FX

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