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current-mode opamps

I'm still trying to figure out current-mode opamps.

In a conventional opamp, the emitters of an input diff pair are fed by a current source, and the diff input voltage steers that current. In a current-mode opamp, the complementary input transistors have a small quiescent bias, but the non-inverting (bases) input signal invokes large collector currents, which are amplified all the way to the output. So in a sense we are getting information from the signal applied to a regular opamp, but we're extracting power from the input to a current-mode opamp. Which is why slew rates can be so high: more error loads the input signal harder, and drives the outputs harder, without running out of current.

If you look at this,

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you'll see that the ni input impedance is spec'd as 780K + 1 pF. They also provide an S11 plot, which, after it's untangled, models as a high resistance shunted by about 2 pF or so, presumably because of extra C on the horribly-laid-out eval board.

OK, I have an open-drain (a phemt) with a 50 ohm pullup to, say, +2 volts. Unloaded, risetime is 100 ps or so. If it's then coupled to the THS3201 as a gain=+2 amp, the output falling edge is screaming fast, but rise is slow. Looking at the fet drain, it looks as if the pullup resistor is loaded by about 10 pF. Pig city.

So at large-signal swings, the input transistors are running out of beta as frequency goes up. That makes the input appear to be capacitive. That's not suggested by the input specs nor by the small-signal S parameters.

Tricky.

John

Reply to
John Larkin
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Sez 1pF in the data but they only give a typical value, no max. But it depends on what you have hanging on the output.

Maybe it didn't look good on the datasheet? But seriously, what's on the output?

I've never really warmed up to the CFB concept. Kept things discrete, mostly. One of our profs told us that using a FET in common gate configuration would be outright stupid. I laughed so hard I had to leave the auditorium.

--
Regards, Joerg

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

You've got Vin = 2V and G = +2. What are your two CFB "-input" resistor values?

Reply to
Winfield Hill

At G=2, 100 ohm load, they suggest that both resistors be 715 ohms for good large-signal performance. Playing with the resistors changes transient response but doesn't much affect the apparent 10 pF input capacitance.

The eval board, and the S-params setup, effectively drive the ni input from a 25 ohm source. The frequency response curves are unclear on what the generator source impedance may be. They give inverting and ni curves small-signal, but the large-signal stuff is all inverting. Are they trying to hide the actual performance?

TI is beginning to disappoint me.

John

Reply to
John Larkin

Ask them. But don't hold your breath, I had a case a while ago where they could not figure out why a TPS regulator went "exotherm" despite everything around it being well within specs. They didn't want to give me the SPICE model either so that kind of tied my hands. It was a LDO, by the way ...

--
Regards, Joerg

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

I commented earlier on our experience THS3062's frying. It took them about 8 weeks to respond to our many requests for help. Finally prodded by TI's VP of Ethics, they admitted that the amp is fundamentally broken, and that the datasheet effectively lies.

It's sad that they have about the fastest linear process on the planet, and they use it so badly.

My finger is much better. I may not have a permanent SO-8 shaped scar.

John

Reply to
John Larkin

They have a VP of Ethics? Dang, I could have forwarded the TPS regulator issue to him because IMHO the handling was ethically, ahem, let's say borderline. Do they give an "ombudsman" link somewhere? But it's water under the bridge now, I designed it out for good. No more LDOs.

Their non-CFB THS amps are fantastic. CFB I don't know, I tried those out years ago from another manufacturer. Back then CFB was hailed as the ultimate cat's meouw. To me it sure wasn't ...

A friend does have a permanent scar. 16 dots. He leaned on a table and failed to notice that 16 pin DIP chip laying there, upside down ...

--
Regards, Joerg

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

John, have you had a chance to compare the TI CFBs with the AD versions?

(Great thread, by the way...)

Thanks,

Tom

Reply to
Tom2000

I stepped on one once... in my socks :-(

Completely cured me of walking around with no shoes in the lab.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | America: Land of the Free, Because of the Brave

Reply to
Jim Thompson

Me too, dip24 barefoot. And once I knocked a soldering iron off the bench and, superb athelete that I am, caught it in mid-air.

John

Reply to
John Larkin

The ADI parts are great if you don't need a lot of voltage swing. We use AD8009's, 8014's, 8001's, all great parts without quirks.

The TI's have insane slew rates and bandwidth with +-7.5 or even +-15 rails, like nothing else I know of.

TI also makes some insanely fast fixed-gain amps, THS4302 and 4303.

John

Reply to
John Larkin

Thanks, John. Great info. I'm keeping this whole thread for reference.

Tom

Reply to
Tom2000

The ultimate nightmare happened to a telco engineer. He wore those climber's sandals where it takes a while to get them off, with thick wool socks. A blob of molten solder rolled off a board, onto the toe area, then phsssss... through the sock.

--
Regards, Joerg

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

I reached for the iron without looking up, ONCE ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

An engineer in The Netherlands (where they drink a lot of coffee) wanted to place his back without looking up. Instead of the Weller spring he landed it in a full styrofoam coffee cup.

--
Regards, Joerg

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

Surely a superb athlete would have caught the correct end though? I've caught a dropped iron a couple of times - with the presence of mind to catch it by the *cord*.

I left the soldering iron on the ground while working on the car's tail-lights, *once*. My three-year-old stepped on it barefoot, and didn't realize for several seconds. No idea why it didn't do more harm, but he was fine after a couple of days, though it was a painful burn. Still makes me feel queasy 18 years later...

Reply to
Clifford Heath

Today's experiments suggest that the "fake capacitance" at the THS3201 input is different depending on the polarity you happen to be slewing. Even more fun!

We're looking into output stages that use discrete GaAs fets.

John

Reply to
John Larkin

On Feb 25, 9:22 am, Joerg wrote: [...]

If someone has a plumber on staff, I tend to think they have or at least had a problem with pipes leaking.

[... TI ...]

CFB op-amps were the "new and improved" product back then. Today the dust has settled and they have found use in a few places. I like the LT1206 because you can "overcompensate the heck out of it" with just resistor values.

For a while I had a nifty pattern on the bottom of one foot. I worked bare footed at home back then.

Reply to
MooseFET

I think it is the copper in the leadframe that makes it hurt out of all proportion to the real damage.

Reply to
MooseFET

Could be, but the time I stepped on one and some of the the pins hit bone and folded over like staples, hurt for another reason.

--
Regards,

John Popelish
Reply to
John Popelish

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