Don't want much do you, Malcolm ?:-)
I'd just use a conventional OpAmp, and add discrete devices on the supply rails... a boost technique discussed here in the past few months.
Then a few additional discretes to tri-state the add-ons.
...Jim Thompson
TI has lots of +-15 rail opamps that are *very* fast. THS3061 and such. Fairly cheap.
*That* I don't understand.John
Hi,
I need an op-amp to drive 30mA, on 24V supply, signal is quite high frequency, 120kHz so I reckon I want at least 40dB to give low distortion, i.e. at least GBW 20MHz but 200MHz would be better. I also need to be able to turn the output high impedance.
Is there such an op-amp? And is it cheap?
TIA
--
Malcolm
Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK ( snipped-for-privacy@fullcircuit.com, snipped-for-privacy@fullcircuit.co.uk or snipped-for-privacy@iee.org). Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:
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He needs to "tri-state" his OpAmp.
...Jim Thompson
Think that's what he meant? Downstream analog switch, then.
John
Malcolm said "30mA". That's a bit rough with an analog switch.
...Jim Thompson
Hey! Hey! Hey! Hey! A technical discussion. Will miracles never cease!
As I said before, I'd use booster discretes at the output, making it a piece-a-cake to tri-state.
...Jim Thompson
Yep! Malcolm needs to provide more details.
...Jim Thompson
More like a fet based relay.
Sounds good. The devil would be in the details of opening the feedback loop and the recovery on closing it, again. Especially if this had to happen quickly or often.
Not any more. Some of the parts are down to 1 ohm. And I suppose you could also use an SSR.
John
It's surprising how many people post a question and then vanish from the thread.
John
They are taking their answer off the air.
Me? I fret over charge injection.
...Jim Thompson
But I wish they'd check in now and then to clarify just what the hell they really need.
John
30mA into what impedance? This matters a lot because at smaller swings, the problem gets easier.
How big is the signal when it is tri-stated? Simply putting a power MOSFET in the signal path may be enough.
In article , Jim Thompson wrote: [...]
The requirement wasn't clear but I wonder if the LT1206 wouldn't do the job. It isn't exactly low cost, but it may run cheaper than a multicomponent circuit.
I believe that the LT1206's output is high impedance when you disable it.
In article , John Popelish wrote: [...]
You could use two pairs of MOSFETs as switches. One MOSFET would isolate the output from the load and the other would connect a dummy load. This would keep the loop in bounds except for the switching charges. The switching would partially cancel.
Hmm, various folks are wagging their tongues, failing to address your question. Several opamps meet your needs, such as these:
part f_T Vsupply Iout slew tristate number manuf MHz max max V/us output? comments ------ ----- --- ----- ---- ---- ------- ------------ CLC431 NSC 120 33V 60mA 2000 yes G = 2 min CLC411 NSC 200 36V 70mA 2300 yes +/-6V Vout max
Check the rest of the Comlinear (now NSC) product line. There are lots of muscular opamps, like the Analog Devices AD815. If you can run the opamp at lower voltages (e.g. add three-terminal regulators) there are lots of good choices.
The world is round remember? This fellow is in the UK, for god's sake! While you are bitching about his lack of response, he's probably fast asleep.
Jeroen Belleman
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