High voltage op amps? Looking for ±30V or similar dual supply op amps. - Page 1
Forum section: electronics / projects

author: denialriversun
author_rank: Newbie
timestamp: October 08, 2016, 10:22:41 pm
content: I am indeed looking for ±30V or similar dual supply op amps. Preferably in an 8-pin package. The opa549 has 11-pins, but I need to replace a couple 8-pin LM358s on a PCB I made for a power supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2016, 10:29:54 pm
content: What's wrong with using more LM358s?JFET amps may work too (with lower noise!), if the CM range is okay.Tim

author: Someone
author_rank: Super Contributor
timestamp: October 08, 2016, 10:36:58 pm
content: Quote from: T3sl4co1l on October 08, 2016, 10:29:54 pmBecause they're asking for +/- 30V bipolar supply range, sounds like an upgrade of an existing design.Quote from: denialriversun on October 08, 2016, 10:22:41 pmIts off in the unusual region for opamps, if you dont like the specficiations or prices of the drop in replacements (easy to find through the manufacturers parametric searches) then you'll need to spin your own output stages:http://cds.linear.com/docs/en/application-note/an18f.pdfCompare that price to the integrated opamp!

author: denialriversun
author_rank: Newbie
timestamp: October 08, 2016, 10:38:30 pm
content: Well, you see, I have a 20V 8A transformer, which is sitting at 30V under no load, and minus one Volt per Amp, roughly. It's a CCCV powersupply, so I need to pull the control voltage to ground somewhere to get the current to go (at least close) to zero, you see. I'm using a diode, but then the op amp needs to output -0.7 on the one side of the diode to have 0V at the other side of the diode, which effectively result in a 0V control voltage, and thus, both constant voltage and current control.It all works, except over 16V.I also have a negative zener reference which gives -2.8V.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2016, 11:45:37 pm
content: Your requirements are not really clear but there are some inexpensive dual operational amplifiers which will operate at up to 44 volts.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 09, 2016, 12:17:22 am
content: LM12. The opamp *IS* the power supply.

author: KerryW
author_rank: Regular Contributor
timestamp: October 09, 2016, 02:30:28 am
content: Can't you use something like +30V & -5V? They don't have to be equal.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2016, 10:28:21 pm
content: You've not stated your requirements very clearly.If all you need is to increase the output voltage/current then a buffer will do that, but it won't improve the common mode range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2016, 11:26:15 pm
content: Yeah, the only thing you should ever need to do is have a local supply rail for the op-amp. Which can be well filtered to improve PSRR. The op-amp output is followed by a simple voltage gain stage.The stage gain does not need to be exact, nor does the offset. It doesn't even need to be very linear. It just needs to be faster than the op-amp, so that it effectively scales the op-amp output (over its entire range, from negative saturation to positive saturation) up to the desired output voltage range (zero to +V?).This is easily achieved, using no more than a few discrete transistors. They can be selected for any range, up to several thousand volts (and more with a little trickery).On the feedback side, the voltage sense divider reduces the output by at least the same ratio the gain stage multiplied it by, so that the op-amp thinks its input and output voltages are in the same range, and therefore it won't be subject to excessive loop gain (which causes instability, even with compensation, which you'll need anyway).The best bonus is that the gain stage can be designed for rail-to-rail operation, allowing you to achieve rail-to-rail output with cheap precision op-amps, without needing the op-amp to strain itself pushing up to +V.Tim