op amp DC amplifier - Page 1
Forum section: electronics / beginners

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 24, 2015, 03:07:19 pm
content: Hi,I want to create a 100 times mutiplier of the input voltage.JPD

author: Shizaru
author_rank: Newbie
timestamp: December 24, 2015, 04:16:21 pm
content: Here you go, a circuit that multiplies your input voltage x100. I simulated it for you in multisim. To understand why this happens I really recommend you to read this:http://www.electronics-tutorials.ws/opamp/opamp_3.htmlIt does a good job at explaining the non-inverting opamp.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 24, 2015, 05:28:01 pm
content: Great.Thanks.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 24, 2015, 05:36:07 pm
content: Google is your friend.http://www.csgnetwork.com/opampcalc.html

author: Audioguru
author_rank: Super Contributor
timestamp: December 24, 2015, 07:24:54 pm
content: Multisim is very stupid to show an opamp with an output of 500V!The datasheet of a lousy old 741 opamp shows that its inputs do not work if their voltage is within 3V from the positive or negative supply voltage. Your input is only 0.2V above the negative supply voltage that is 0V. Therefore the 741 opamp needs an additional negative supply voltage. Also the 741 opamp is noisy so with a gain of 100 the output will have lots of noise.

author: c4757p
author_rank: Super Contributor
timestamp: December 24, 2015, 07:28:47 pm
content: Really? It's stupid to show a generic "opamp" model, with neither supply rails nor design limitations, performing as one?

author: daqq
author_rank: Super Contributor
timestamp: December 24, 2015, 09:05:36 pm
content: Note: Don't expect stellar DC (or any) performance from just any op amp - add the offset voltage (of the op amp) multiplied by the gain to the output. Depending on your chosen op amp, multiplying an offset voltage by 100 can give you even 1V of voltage error.For basic op amp circuits see:http://www.ti.com/lit/an/snoa621c/snoa621c.pdfhttp://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

author: Shizaru
author_rank: Newbie
timestamp: December 24, 2015, 09:30:07 pm
content: Quote from: c4757p on December 24, 2015, 07:28:47 pmI was just showing him the basic circuit on how he should build one. Of course I know that there are design limitations but without knowing the exact opamp he's going to use I can't help him any further. And I've even directed him towards a website where he can check the theory behind the schematic.Quote from: Audioguru on December 24, 2015, 07:24:54 pmJust showing him the basic circuit.

author: c4757p
author_rank: Super Contributor
timestamp: December 24, 2015, 09:36:36 pm
content: Glitch in communication? I don't have a problem with what you did. My issue is with Audioguru declaring the software stupid for doing what was asked of it. I wonder if hammers are also stupid for not automatically applying a twisting action when used with screws

author: Shizaru
author_rank: Newbie
timestamp: December 24, 2015, 09:47:13 pm
content: Quote from: c4757p on December 24, 2015, 09:36:36 pmWoops my bad!

author: LukeW
author_rank: Frequent Contributor
timestamp: December 25, 2015, 09:45:50 am
content: Perhaps it might be worthwhile for the OP to tell us more about the context - what is the actual problem that you want to solve?

author: LvW
author_rank: Frequent Contributor
timestamp: December 25, 2015, 10:22:42 am
content: Quote from: Audioguru on December 24, 2015, 07:24:54 pmMultisim is not stupid.Either the used models are (over-)simplified or (in most of all cases) it is the user who makes errors - not the program.

author: dom0
author_rank: Super Contributor
timestamp: December 25, 2015, 10:42:41 am
content: Quote from: daqq on December 24, 2015, 09:05:36 pmNot quite: In the non-inverting configuration noise gain equals signal gain, while in the inverting configuration the noise gain is the signal gainplus one. I.e. an inverting amplifier with a gain of 3 has a noise gain of 4. Btw. as noise gain is important for stability (and not signal gain) this small difference explains, why inverting amplifiers are more stable than non-inverting amplifiers, especially for uncompensated op amps near their minimum gain limit.

author: Audioguru
author_rank: Super Contributor
timestamp: December 25, 2015, 05:22:12 pm
content: He is using an old 741 opamp. I pointed out that it will not work with its input voltages so close to its negative supply voltage that is 0V. It needs an additional negative supply.Or he can use an opamp that has inputs that work all the way down to its negative supply (0V in his circuit) like an LM324, LM358, a few other older opamps or a modern rail-to-rail opamp.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: December 25, 2015, 07:21:07 pm
content: Hi,I am following all this with great interest. Thank youi for the comforting post).The input voltage should comes from a LM35D temp device working at 3.7 volts .JPDaviau