What did I miscalculated ?? - Page 1
Forum section: electronics / beginners

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 03:51:38 am
content: Fraction of schematic .... I didn't work with OP07, but OK with MC34071I'm guessing R6+R7 too high? What exactly in opamp datasheet i need to look at , and calculate

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 04:35:49 am
content: "Input voltage range." -- How so ?I'm basically amplifying 200mV. The symptoms was: opamp wasn't able to balance , so positive has 200mV, and negative input around +450mV. so the output was around 3.5V I did try couples old opamp, only MC, and TL081 works.I assumed the current wasn't enough due to high R6 + R7 ...

author: MarkF
author_rank: Super Contributor
timestamp: October 24, 2018, 04:42:41 am
content: The OP07 op-amp is not single supply. You will not be able to get that close to the negative rail (ground in your case). The input voltage range would need to be a couple volts away from the power rails. While the MC34071 is single supply.Your circuit needs a single supply or rail-to-rail op-amp to work correctly. Give the OP07 a negative voltage and test it again.

author: boB
author_rank: Frequent Contributor
timestamp: October 24, 2018, 04:52:26 am
content: Well, not sure what you're expecting exactly but looks like the DC voltage output of the op amp is approximately correct.Gain should be approximately 15 with 70K/5K + 1 and 0.193V DC in and 2.9V DC output.AC component is almost nothing so the frequencies around 12 kHz are probably meaningless here.boB

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 05:13:13 am
content: Wait wait, where you found RR operational mode??200mV * to coeff == up to 2.9. And it single rail , +12VAhh .. get it: AC - are simulator's nuances, so we looking only to DC , no any AC floating around at all ; don't look at , only DC ; that all ....

author: spec
author_rank: Frequent Contributor
timestamp: October 24, 2018, 09:55:39 am
content: @ GigaJoeOpamps have an input voltage range and an output voltage range where they will operate as opamps. Outside that range their behavior is unpredictable.(1) RRI (Rail To Rail Input) means that the input voltage range goes from the negative supply rail to the positive supply rail.(2) RRO (Rail To Rail Output) means that the output voltage swings from the negative supply rail to the positive supply rail (but normally under no current load).(3) RRIO (Rail To Rail Input And Output) means that both (1) and (2) characteristics apply.But for your design (if I interpret your figures correctly) you only need an opamp where the input voltage range goes down to the negative supply rail. The MC34071 does this but the OP07 does not. That is why your circuit works with an MC34071 but not with an OP07.This is what the other members have been telling you!

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 01:54:37 pm
content: Folks,Where do I have negative rail on my schematics ?Why: "input voltage range goes down to the negative supply rail." - where is it ?I'm so really surprised ......Schematic has single +12V positive supply. It has 2 LM336-5 that produce 10V, Then transistor emitter follower with output 9.3, Then we dividing it to 200mV. and THEN 200mV amplifying by opamp at 3 Volts. - DONE.PROBLEM: OP07 doesn't work, it can not balance itself, so input on positive +200mV on negative +450mV, so the output was around 3.5+ Volts. MC34071 ; TL081 (not all what I have) works as expected.I ASSUME: R6 + R7 has so low current output that it not enough for the opamp? I'm speaking about Input bias current. I'm I right?

author: dmills
author_rank: Super Contributor
timestamp: October 24, 2018, 02:26:43 pm
content: Nope, the non inverting input is just 200mV above the U- pin of the opamp, which is way too close for an OP07.They only give data for a +-15V use case for the OP07, but reading between the lines the inputs need to be at least a volt or so above U- and volt or so below U+, 200mV will not do it.While there are no 'single supply opamps', it is just a case of the bias conditions, there are parts that have a common mode range that includes one or more of the supply rails, and you want a part where the common mode range includes (or at least gets very close to) the negative rail.Regards, Dan.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 02:44:42 pm
content: OK, great , Thank you very much,I understand now, as MC actually designed to run on a single rail. OP07 doesn't, I will not happen if i have some negative bias to feed opamp ... (I simply forgot about opamp input range , silly me)

author: Nerull
author_rank: Frequent Contributor
timestamp: October 24, 2018, 03:23:13 pm
content: Quote from: GigaJoe on October 24, 2018, 01:54:37 pmConnected to ground. Which means your negative rail is at 0V and you will be limited by how close you can get to 0V.The negative rail is pin 4 on your opamp. Whatever voltage you connect it to becomes the opamps negative rail.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 03:30:42 pm
content: yes i'm fully understand ,now , OP07 has some behavior when input close to 0, also output can not be lower then 0.7V ; when opamp running in a single rail mode.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2018, 04:25:58 pm
content: In the past, typically an OP90 or LT1006 would be used in place of the OP07 for single supply operation.

author: spec
author_rank: Frequent Contributor
timestamp: October 24, 2018, 06:55:56 pm
content: + GigaJoeJust read the above posts. I now see where the confusion comes from.Just to clarify, an opamp has two supply lines: a negative supply line and a positive positive supply line. The negative supply line is always more negative than the positive supply line. This says nothing about the absolute value of the supply line, just the difference between the two.So, for example an opamp's negative supply line could be -6V and the opamps positive supply line could be 6V, giving the opamp a supply voltage of 12V.Alternatively, an opamp's negative supply line could be 0V, and the opamp's positive supply line cold be 12V, as in your circuit. Once again the opamps supply voltage would be 12V.And, in a third arrangement, the opamp's negative supply line could be -12V and the opamp's positive supply line could be 0V. Once again, the opamps supply voltage would be 12V.To take this to the extreme, an opamp's negative supply line could be -100V and the opamps positive supply line could be -88V. Yet again, as far as the opamp is concerned, it still has a supply voltage of 12V.Any opamp can be used with a single supply, as opposed to dual supply, even the classic 741 opamp, but some opamps are more convenient to use with a single supply.To minimize the many pitfalls of using opamps, I would recommend that anyone experimenting in electronics, especially newbees, should choose a 'perfect' opamp. Of course, there is nothing perfect in electronics but an opamp with the following characteristics comes pretty close in practical terms: RRIO Practically zero input offset voltage Practically zero input bias current High output sink and source current Reasonably low power Reasonable gain/bandwidth, at least 1 MHz Stable with feedback for a gain of 1 Capable of operating with a supply voltage of 5V, or less, and 32V, or more. Available in single, dual, and quad versions. Reasonable cost.There are probably a number of opamps that meet these requirements, but the best that I have found so far is the Texas Instruments OPA192.http://www.ti.com/lit/ds/sbos620e/sbos620e.pdf

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 24, 2018, 07:17:51 pm
content: or OPA197 ; it probably whatever didn't fit in 192, but just US$1.13.

author: spec
author_rank: Frequent Contributor
timestamp: October 24, 2018, 07:26:40 pm
content: True. The OPA197 is an OPA192 with a wider input offset voltage. Nonetheless the OPA197 would still be a good choice for a 'perfect' opamp.By the way, this family of opamps in only available in surface mount, whereas experimenters like to have a through hole package for easy handling. The solution is simple: fit a through hole conversion header board to the surface mount package.