Another opamp question

Lazy. here are the simulations...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson
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Or, to be a bit more precise, always the same voltage, whether ground or something else.

So I put the scope on it, and guess what - it's not ground. In fact, it's a small version of the input.

Well it would have to be, wouldn't it. The slew rate on my opamp is whopping 0.6 V/uS, but even with a reasonably fast amp, there's always going to be some delay. And as someone said earlier, if there weren't some signal there, there wouldn't be any output.

Still, it seems the rule of thumb works well enough for most purposes.

Reply to
Peabody

The "gain" of an OpAmp is OmegaGBW/s

(OmegaGBW = 2*pi*GBW, "s" is the Laplace/Heaviside variable)

Do the math, or "the rule of thumb" will snag you. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

We all do that, but we need to keep aware that opamps aren't ideal.

I got embarassed earlier this year. I was running some opamps with asymmetric power supplies, to optimize for expected signal swings, and forgot to consider PSRR. Got big DC offsets. Well, those amps had terrible PSRR.

I originally had a trimpot in each amp to factory-trim offsets, but the customer has a corporate-wide NO TRIMPOT policy, and made me take it out.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

Somebody got an award for pushing that rule through, no doubt. About 15 years ago, some bright lad in Watson Safety got the idea that since some labs were noisy--with hundreds of servers in racks or a big cryopumped vacuum system--ALL labs were going to have large horn PA speakers installed. To save my hearing, I went down to the stockroom and got a handful of skinny rubber stoppers. One of them fit perfectly down the inside of the horn, reducing the volume to a perfectly audible but not painful level.

Most of those sorts of idiocies can be traced to perverse incentives.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Yup, that's OK I done the calc's a few times. Those curves look familiar, you can get a stronger roll off if you don't mind some gain peaking. You can roll off the opamp some too, at that point.

George H.

Reply to
George Herold

I've got Sergio Franco's opamp book, it does a nice job of doing the math.. (simple one pole opamps) If you need help. It's always nice to have someone working along with you. (That's to the OP an obviously not to you JT. :^)

George H.

Reply to
George Herold

I put a pair of anti-parallel diodes across the 70V feeder to the speaker near my benches. It made many of the speakers in that area sound like they were blown. ;-)

Reply to
Michael A Terrell

Wow, So you did some, measure offset-> table-two resistor (through hole?) thing?

George H.

Reply to
George Herold

The banning of incandescent bulbs should forever be held up as a shining example. (NPI)

George H.

Reply to
George Herold

Sergio and I have been conversing a lot recently... one, about current-feedback OpAmps... two, he's compiling a history of the designers of my era (including himself ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Bob Widlar had the optimum solution... a plumber's helper... load with cherry bomb and hold against ceiling speaker >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I shipped about 10 first articles, with eight "failing" their offset spec. It actually doesn't matter, since other system offsets are a lot bigger. This is a photodiode amp, and they already do "dark" shots to acquire the baseline offset.

I don't want to solder in selected resistors. Fortunately their "copy exact" rules would require every unit with a separate resistor value to have its own part number, so I don't think they'd want do that.

I could use a nonvolatile DPOT, but then I'd have to connect to it and program it.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

K/s implies infinite DC gain, which few opamps have.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

For most purposes the DC gain isn't the critical parameter in stability analysis. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I would expect significant capacitance at the inverting input would result in oscillations. In any event, a resistor isn't likely to have much impact on the gain of the circuit. The voltage on this pin is related to the output voltage swing by the opamp gain. So with very little voltage, the resistor will have *very* little current. It will have an effect on rates as that is when the inverting input will have the most voltage on it.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
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Reply to
rickman

I was at the iAPX-432 announcement presentations during the grand opening of the Grand Hyatt in NYC. The hotel had a lot of teething issues, one of which was the fire alarms constantly going off. One of the Intel VPs solved the problem in the room we were in with a full swing of a chair, right to the horn. I think the horn cleared the fence at Yankee Stadium. ;-)

Reply to
krw

Name dropper, Sergio wrote a one nice book, so his reputation may out live him. (nudge nudge :^)

George H.

Reply to
George Herold

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