Limiting Op-amp Output

Tim Wescott · 2011-08-15T18:23:06+00:00

OK, this is a dumb question (because I really should know the answer), and it's probably one that I can answer myself with a bit of digging:What's a good way to limit the output of an op-amp to within the 'normal' capability of the device? I.e., I have an integrator that I want to allow to go up to some voltage (say 5V) and stop cleanly, and be able to immediately start descending cleanly from that voltage when the input reverses polarity.Being able to tweak the limit voltage would be ever so good, too.I tried doing this with a common-base transistor, connected so that when my bound was exceeded the transistor would short out the integrator cap. It worked dandy, except that if the circuit could get "stuck" if it started up in the wrong state.I'm sure this problem has been solved in a dozen different ways, I just don't know what cookbook to look in...--

Aug 15, 2011 82 Replies

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Joerg 14 years ago

when

cap.

Thanks. Slurped it off the page and put it in my sim directory.

Behavioral is what you have with all those Linear Technology parts and I've found quite a few boo-boos in those. But they are blazingly fast and sometimes that's more important. Simulating an upchuck episode on a buck converter is kind of impossible with a real device-level model for the PWM chip. At least when you want the sim to finish some time today.

Regards, Joerg http://www.analogconsultants.com/

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Phil Hobbs 14 years ago

the

when

cap.

just

Could get boring, doing your sound effects that slowly. ;)

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net

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Spehro Pefhany 14 years ago

Hmm.. I tried this one, and it shows about 1.5usec to get up to 5mA with a fast ramp of 2 volts per microsecond (overshoot from 5V to 8V).

With the slow 10mV/usec ramp, it only overshoots about 100mV (~10usec to get up to 5mA).

Quite different, and probably acceptable for a crowbar provided there's a biggish cap on the power supply rails (and no 5.5V abs max parts).

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

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