Protection Schemes for BI-FET Am

　　To use integrated circuits in real applications， designers must know the limitations of the devices. The majority of the limitations are published in the datasheets， and these fall into two categories： Absolute MaximumsÐwhich， if violated， can cause damage or destruction of the device; and Electrical CharacteristicsÐwhich indicate the performance limitations. Unfortunately， these specifications don’t explain the consequences of a violation， nor what may happen between the violation of an Electrical Characteristic limit and an Abs. Max. limit. This information is needed so the designer can design an appropriate protection scheme. This article will focus on National Semiconductor Corp. BI-FET op amps： how to improve their reliability and performance. In most cases， the results are similar for bipolar op amps also

　　THE BI-FET FAMILIES Our BI-FET op amps are divided into families： LF411， LF441 and LF356. The LF411 family consists of LF411， LF351， and TL081 （all singles）; LF412， LF353， and TL082 （duals）; and LF347 （quad）。 This is a good general purpose set of op amps that all have the same internal design， differing only by grade. The LF441 family consists of the LF441 （single）， LF442 （dual）， and LF444 （quad）。 This family gives nearly the same DC performance as the LF411’s for 1/12th the supply current. However， because they are low-power devices， they are also proportionally slower than the 411’s. The LF356 family includes the LF355， LF356， and LF357 （in the commercial temperature range）。 The 355 is the low-Icc part， and the 357 is the wide-bandwidth part. All of the family have good DC specs and can drive a lot of capacitance （to .01 mF， typically）。 They are also among our faster op amps， having slew rates which vary from 5V/ms （for the 355） to 50V/ms （for the 357）。 The operating restrictions for these families are nearly all the same， as are the methods of protection. In some cases （as will be pointed out） certain families are better at surviving the abuses than others.