Op-amps - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: December 02, 2014, 06:08:42 pm
content: I am looking for a relatively high-speed op-amp (eg 40MHz+ GBW) and relatively high voltage (+/-25V). Any ideas? I have tried the OPA-445 but it is too slow.

author: tggzzz
author_rank: Super Contributor
timestamp: December 02, 2014, 06:24:00 pm
content: Quote from: akis on December 02, 2014, 06:08:42 pmAnalog Devices ADA4870?

author: nctnico
author_rank: Super Contributor
timestamp: December 02, 2014, 06:56:16 pm
content: That ADA4870 is an interesting amplifier! I have used the TCA0372 in the past because it has short circuit and thermal protection but the ADA4870 looks like a perfect replacement and more!

author: akis
author_rank: Frequent Contributor
timestamp: December 02, 2014, 07:05:34 pm
content: Thanks but I need +/- 25V which is 50V, the ADA4870 can do 40V only.

author: c4757p
author_rank: Super Contributor
timestamp: December 02, 2014, 07:09:38 pm
content: Yeah, I've been getting emails about the ADA4870, and it looksreallynice. I can think of so many things I could use it for...Was going to suggest it, but then noticed the supply range is 10V off. Any chance you could relax that requirement a bit?Otherwise - look at the output stages of old analog function generators. They often had high bandwidth and high voltage (had to drive quite a bit into 50R for general-purpose use); typically made from discretes. Alternatively there are tons of ways of bootstrapping op amps to higher voltage use.

author: free_electron
author_rank: Super Contributor
timestamp: December 02, 2014, 07:16:44 pm
content: 40MHz . 50 vpp ? that is a serious opamp...apex.

author: akis
author_rank: Frequent Contributor
timestamp: December 02, 2014, 07:21:56 pm
content: I typically use the LM4562 for hi-fi or the LM6172 for higher BW, but now I want to use two 6 cell batteries giving +/- 24 V and no op-amp I know of can accept these kind of voltages and furthermore the speed I am looking for.

author: c4757p
author_rank: Super Contributor
timestamp: December 02, 2014, 07:23:31 pm
content: Ooh, I forgot about Apex. They have someseriouslycool parts. Seriously expensive too though!

author: Marco
author_rank: Super Contributor
timestamp: December 02, 2014, 07:29:55 pm
content: OPA552 is a bit faster, still only 12 MHz GBW though.

author: dannyf
author_rank: Super Contributor
timestamp: December 02, 2014, 07:33:59 pm
content: High voltage amps are typically audio amps; High speed amps are typically video amps and CFB opamps - many video amps are CFB opamps.

author: Leadfootin
author_rank: Contributor
timestamp: December 02, 2014, 07:42:02 pm
content: Look for video amps. I can't recall the numbers but quite a few were made but at lower voltages. An output circuit could then be designed to increase the output voltage and current capabilities.

author: akis
author_rank: Frequent Contributor
timestamp: December 02, 2014, 08:01:01 pm
content: Had an idea. I am thinking of using a bridge configuration like the attached. Each part has an Av=19 and both together slightly less than 38 !!!!The next hurdle now is, How to drive the post-amp which expects this voltage referenced to ground.

author: dannyf
author_rank: Super Contributor
timestamp: December 02, 2014, 08:09:35 pm
content: 1. None of that is going to solve your problems.2. Take a look at the input section of a typical instrumentation opamp.

author: nctnico
author_rank: Super Contributor
timestamp: December 02, 2014, 08:34:33 pm
content: 2 ADA4870 chips in a bridge should do it. It probably takes a single to differential amplifier to create the 180 degrees phase shifted drive signals and pre-amplification.

author: MK
author_rank: Regular Contributor
timestamp: December 04, 2014, 08:09:57 am
content: AD815 might do it for you, it has a bridged output as standard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2014, 10:43:51 am
content: What are you driving?

author: metacollin
author_rank: Contributor
timestamp: December 11, 2014, 06:49:42 am
content: Do you want an op amp that can accept rails of ±25V? Or can have an output voltage swing of ±25V? It sounds like you just want the rails to be that based on your 6 cell battery setup. What kind of output swing do you need though? And your GBWP is 40MHz, but what is the maximum frequency you hope to amplify at what gain? With such large output voltage swings, you could find yourself in the slew rate limited triangle distortion land of sadness all too easily (but that depends on the actual gain and signal frequency you need).