New design with LM201A? - Page 1
Forum section: electronics / projects

author: maelli
author_rank: Contributor
timestamp: August 26, 2014, 08:00:07 pm
content: Hi there,In a product I reviewed, I saw a power op amp circuit +/-24V, 0.4A with a an LM201A as the main amplifier.And this should be a new design, 2012ish.An inductrial product which should be high precision, wide temp range (hence 201 and not 301 I guess)and high reliability... (but of course low cost)Would anybody know a reason why one would design-in such an old op-amp?

author: dannyf
author_rank: Super Contributor
timestamp: August 26, 2014, 08:04:00 pm
content: QuoteShould it?

author: sacherjj
author_rank: Frequent Contributor
timestamp: August 26, 2014, 08:49:47 pm
content: Quote from: maelli on August 26, 2014, 08:00:07 pmI know several.Cost.Meets the level of accuracy needed for that portion of the product.Cost.Meets the power needed for that portion of the product.Cost.Looks good soldered onto the board.Cost.Oh, and possibly Cost.

author: hlokk
author_rank: Contributor
timestamp: August 27, 2014, 12:15:29 pm
content: Also dont forget older parts are tried and tested, the designers are familiar with them and being older but still in production, are likely easier to source.As long as it meets the design requirements then why not go for it no matter the age.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2014, 03:19:07 pm
content: The LM201A also has an input common mode range which extends to the positive supply which for a long time made it uniquely useful in high side applications like high side current measurement. Now of course their are lots of operational amplifiers with an input common mode range which includes the positive supply rail.Another reason the LM201A might be used is because of the flexibility in tailoring its frequency compensation.The old LM318 is similarly useful because its differential input stage may be disabled and the comp/offset pins used to make it into the output stage of a high performance amplifier. I am not immediately aware of any modern amplifiers which can be used this way but examination of their simplified schematics should reveal some. TI amplifiers need not apply since they now like to hide details and do not publish full datasheets anymore.

author: coppice
author_rank: Super Contributor
timestamp: August 28, 2014, 05:03:32 am
content: Quote from: maelli on August 26, 2014, 08:00:07 pmWhy would being old make it less interesting? Analogue ICs are not like digital ones. They don't keep getting finer and better. In fact, its not practical to make most purely analogue ICs in fine geometry processes.The first generation amps from the 60's (e.g. the 709) had numerous issues, but the second generation (e.g. the 741) solved most of those, and derivatives of the 741 are still big today. I guess the highest volume op-amp today would be the LMV324, which is ancient. If you don't need something tricky, like operation outside the ground to Vcc range or wide bandwidth, the old simple parts are the cost effective choice.

author: maelli
author_rank: Contributor
timestamp: August 29, 2014, 07:35:40 pm
content: Thanks for your answers!The reason why I asked was the following:Company A designed a product, back in the seventies. Back then the LM201A was an obvious choice.Through all improvements, move from TO99 to SMD... they kept their basic design until now.Good idea, why fix it if it is not broken.Company B moves as a newcomer into this market, two years ago. They took a LM201A too.Now what I asked myself was: how much reverse engineering was happening here?Or simply coincidence?

author: jlmoon
author_rank: Unknown
timestamp: August 29, 2014, 07:49:38 pm
content: Just because you have an old dog (LM201A) doesn't mean you need to put it out to pasture."Also dont forget older parts are tried and tested, the designers are familiar with them and being older but still in production, are likely easier to source.As long as it meets the design requirements then why not go for it no matter the age. "I have to agree..

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 08:40:12 pm
content: Quote from: jlmoon on August 29, 2014, 07:49:38 pmAnd there will be applications where a newer part with greater performance provides no advantage at all.A LM201A has a typical CMMR of 96dB but the list of comparable amplifiers (supply voltage, speed, common mode input range) which beat that is small and they are more expensive. The LT1637 comes to mind and it is probably what I would use but it costs at least twice the price.