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what does 741 operational amplifier latch up mean? what are the reasons IN GENERAL for that? i need a general answer only

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    \$\begingroup\$ Although the comments could have definitely been kinder, the problem with your question is that you just copied here the question your professor asked, without showing any research effort. You could have found a lot of information in the Wikipedia definition, without having someone putting effort in writing your answer. \$\endgroup\$ – clabacchio Mar 24 '13 at 15:59
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The attached is a sketch of a dual well CMOS process cross-section contrived to show a parasitic structure known as a SCR.

enter image description here

The diodes shown are junctions that are present and the red line shows a current path that starts on (for example) Signal 2, contacts to P++ S/D implant, moves through NWell into the PWell and then finally into the N++ S/D implants into the contacts for signal 1. That describes a PNPN structure which is also know as a A SCR (Silicon controlled rectifier).

This is a parasitic structure that exists in ALL modern processes and must be accounted for during design by following what is known as design rules for spacing. You're not allowed to place different structures too close together for example.

An SCR, once triggered will "latch-up" and conduct current until the current path is broken. In the case of a chip, this either when the power supply fails or the chip self-destructs. The wells are very large structures and can handle large amounts of currents so once activated they win out over any deliberately designed structures.

I don't know the details of the process used to fabricate the '741 but I show the CMOS process example as a cautionary tale that a parasitic SCR does exist even in todays modern CMOS processes even after all these years -- it is inherent.

In general a device will latch up when the parasitic device is triggered by voltages being out of range. In my sketch signal 1 would need to exceed Vdd and Signal 2 would need to be lower than ground and the SCR depends upon doping levels and electric fields and spacing.

Reading the 741 specification and observing its limits in normal operation is advised. Although it would be educational to sacrifice a few of these devices in an experiment.

Regardless of what people think of the 741, it is a nice starting point to understanding simple op-amps and contains the key element of what is the core of an op-amp the "differential pair".

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    \$\begingroup\$ +1 for also demolishing the "newer is always better" myth. Not that the 741 doesn't have limitations : but so do ALL its replacements - the limitations just have different numbers on them. \$\endgroup\$ – Brian Drummond Mar 24 '13 at 13:13
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It could be a number of things, input getting close to or exceeding the supply rails, common mode input voltage exceeding maximum limits, and a capacitive load has been known to make some op amps do strange things. I'm sure there are other reasons I'm not thinking of at the moment. The moral of the story is the average modern op amp doesn't have these problems. The 741 is a dinosaur, and needs to be run out of EE programs around the world on a rail.

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Not sure if you are referring to the fact that many IC's have a very similar physical build as an SCR. Not sure if it is true for a 741 either. The problem occurs when voltages at various pins occur in wrong order or get too high, it may latch as an SCR does, effectively shorting out the circuit, and probably dissipating itself to death.

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