What is the name of that OP-AMP topology - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: December 16, 2022, 04:52:49 am
content: I have encountered such a topology. As you can see, there is a constant 3.5V on the non-inverting Pin of the opamp. Does this topology have a specific name?Here a Falstad link.https://tinyurl.com/2mvbatrk

author: moffy
author_rank: Super Contributor
timestamp: December 16, 2022, 05:06:13 am
content: There is no point to R7, it contributes nothing to the circuit except a current path between 3.5V and "adjustable voltage", perhaps something is missing or incorrect?

author: jmw
author_rank: Frequent Contributor
timestamp: December 16, 2022, 05:23:01 am
content: This looks like a standard inverting amplifier topology, with an additional load of R7 placed on the input, right? Replace the 3.5 V at the non-inverting input with ground, and the 7 V single supply with a split +3.5 V / -3.5 V.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 16, 2022, 05:38:19 pm
content: Just for clarification:The opamp does not generate a 3.5V on that location, but you are supposed to put 2.5V on that location from some external voltage source.With that in mind, it's just a bog standard inverting amplifier with an 3.5V offset and some extra resistors.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 16, 2022, 06:01:24 pm
content: It looks like the schematic comes from an absolute beginner in electronic. Using the GND symbol for various voltage levels is a gross mistake, and the way the rest was drawn shows it was made by somebody who's not familiar with electronics (resistors drawn at the end of a wire, all asymmetric, leftover node, text instead of a proper symbol).Don't waste your time with that schematic. Find another source to learn, or tell what you want to build, ask for directions and you'll find proper help on this forum, or at least you'll be pointed to known good info.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 16, 2022, 07:26:36 pm
content: inverting opamp centered/offset at 3.5V with gain -3.32... R7 only source or sink current between 3.5V and adjustable input voltage, for what reason? only the maker knows...

author: Terry Bites
author_rank: Super Contributor
timestamp: December 16, 2022, 08:20:59 pm
content: A noted, its an example of an inverting amplifier with added offset. No more can be attributed to it. I has no famous discoverer.You may find this and similar circuits explain'd in this handy publication.www.ti.com/jp/lit/pdf/sloa030?keyMatch=SLOA030A

author: ledtester
author_rank: Super Contributor
timestamp: December 16, 2022, 10:50:08 pm
content: Many of the op-amp topologies are specializations of the generalized differential op-amp:whose output is given by:$$V_{out} = \left( \frac{(R_1+R_f)R_g}{(R_2+R_g)R_1}\right)V_2 - \left(\frac{R_f}{R_1}\right)V_1$$For instance, set V_2 = 0 and you get the inverting amplifier. Set V_1 = 0 and R_g = infinity and you get the non-inverting amplifier.(Image courtesy ofhttps://www.electronics-lab.com/article/the-differential-opamp-amplifier/)

author: RoGeorge
author_rank: Super Contributor
timestamp: December 17, 2022, 09:11:43 pm
content: No, that's how it looks to me.What do you call a net port?

author: RoGeorge
author_rank: Super Contributor
timestamp: December 17, 2022, 09:54:26 pm
content: I've wrote "absolute beginner", not "super noob". Absolute beginner is not a derogatory term. It is nothing wrong to be a beginner. You wrote your opening post as if that schematic was found somewhere else, so I've assumed you are following some shoddy blog instead of learning from better sources, so the advice to find another learning source.And I've never say it's wrong to ask. Beginners are welcome, and anybody is encouraged to ask. I did not intend to make you feel bad. Also, I've never heard of a "net port", didn't know what you mean by that, thus the question.This is a friendly place, please do not assume to malevolence what can be attributed to a misunderstanding.

author: mikerj
author_rank: Super Contributor
timestamp: December 17, 2022, 10:10:56 pm
content: Quote from: electronx on December 17, 2022, 09:28:28 pmAnd yet you couldn't recognise it as a simple inverting amplifier configuration?

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 18, 2022, 12:23:07 am
content: Quote from: electronx on December 17, 2022, 10:14:15 pmyou reminds me of my wife when in defense mode... then why ask what topology of this opamp? thats the question for beginner.. why dont just ask what R7 is doing? i already hinted (with my simple algebra that i did homework for you, that you surely can come up with easily in the first place if you passed laplace) that it possibly has something to do with what on the side of "adjustable input" which you havent provided. do you purposely want to give us open question?