Rail to Rail output stage for 2A 5V

I need a more-or-less (say 125'C and IC's like the OPA567 would get too hot. I probably need op-amps around a discrete output stage, but what's the best topology? Preferably no charge pumps used and no bias adjustments.

Cheers

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Clive
Reply to
Clive Arthur
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Linear amplifier? Or switching to the rails?

Is 5V the only supply available?

Complementary follower mosfets would be easy but the gates would have to be driven past the power rails. Flip the fets if only 5v is available, but the bias gets a little trickier to avoid crossover distortion.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

A classic op-amp power booster that should be able to go more-or-less rail to rail is the op amp output feeding the emitters of a PNP and NPN, connected like the output stage of a complimentary emitter follower, but facing back towards the op amp and driven common-base, through a resistor for V->I conversion. the bases are both grounded to the mid point for a single supply.

then resistors in the collector lines of each, complimentary MOSFET gates to the resistors, sources to the supply and ground, drains drive the output. AC and DC feedback back to the op amp from the junction and then couple the output thru a cap

Reply to
bitrex

I'd consider a bank of OPA567, configured with small series output resistors, to insure current sharing. Are you serious about running the electronics at an 125C ambient? OK, time to add some active cooling.

And compensation would be painful.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I am considering just that, but these parts have such a small heatsink pad area. It seems they're rated for 85'C ambient operation knowing that the part itself will run much hotter.

Active cooling is out of the question, it's in a well. 125'C is minimum, 180'C is a better target. Luckily the OPA567 thermal shutdown is optional :-)

Yes. It's linear BTW.

Cheers

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Clive
Reply to
Clive Arthur

If you're down in a very deep hot well, how is it you only have 5.0 volts? That's weird.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I do have lots more volts and there's a step down SMPS.

I'm driving a low-impedance load. Normally, I'd use say a 30V supply for the amplifier - conventional class AB - and a step-down transformer to couple to the load, but I'd like to try direct coupling. This means a lower supply voltage, and rail to rail becomes important.

Cheers

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Clive
Reply to
Clive Arthur

Maybe it's time to develop a valve IC for downhole use. I always wondered if a VFD could be rejigged to somewhat do that.

NT

Reply to
tabbypurr

Larkin's complementary source follower is a great solution. Is the objection to two low current charge pumped auxilliary rails philosophical or is noise a worry?

piglet

Reply to
piglet

It's reliability - capacitors are best avoided at high temperatures and high vibration if possible.

Cheers

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Clive
Reply to
Clive Arthur

If you can power a small opamp from, say, +10 and -5, it could drive complimentary mosfet followers powered by the big +5/0 rails.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Probably not hard at 50 KHz.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Interesting. Thanks. What are the best capacitor types for that kind of environment?

piglet

Reply to
piglet

piglet wrote in news:qdbe30$797$ snipped-for-privacy@dont-email.me:

up to 260C

Reply to
DecadentLinuxUserNumeroUno

Korg makes a VFD-based dual triode:

Reply to
bitrex

How to make triodes much more expensive for no known reason.

NT

Reply to
tabbypurr

no. cylindical tubes handle pressure much better than flat-sided ones.

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

both form factors are well established & handle it without any difficulty. That has simply nothing to do with developing an IC

NT

Reply to
tabbypurr

neither were designed to handle downhole pressure, but the box shape is particularly vulnerable.

--
  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

more so per given quantity of glass. Hence it uses thicker glass. Anyway the enclosure shape of an IC-like array is not a challenge. Plenty of tiny VFD style triodes can fit in a short tube if you so want, and of course VFDs can be made with thick glass. It doesn't strike me as a problem.

NT

Reply to
tabbypurr

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