uA741 Operational Amplifier - Need Help with Discrepancy on Output. - Page 1
Forum section: electronics / beginners

author: JJ_023
author_rank: Frequent Contributor
timestamp: January 09, 2024, 11:23:50 pm
content: I am asking for the communities help as well as any suggestions or criticism in my pursuit on the subject of operational amplifiers.A positive and negative supply was provided for the operational amplifier ( +10 V, -10 V).The input into the operational amplifier was a CR2032 lithium battery with a voltage of 2.865V. I decided on this choice to minimize any variables when inputting into the operational amplifier.The circuit was constructed for a basic inverting amplifier.Both the breadboard and schematic is attached and labeled.There are several Print screens with descriptive title names attached.The resistor values were 10K and 5.1K. I verified the resistor values and they were slightly off from specified but not by much so let's assume them at face value.Vout = -Vin(10k/5.1k) is the gain ( as I understand it). Which would be a gain of 1.9607So the voltage should be either +/- 5.617V depending on the polarity of the battery and how it was connected.However the actual output that I am getting is +/- 4.8V.The inverting of the voltage is happening so that is not a problem.What am I missing here?I've done a little bit of research and reading on uA741 and it is problematic in certain situations but unfortunately this was the only operational amplifier I have to experiment with.This is the operational amplifier that I used.https://www.ti.com/lit/ds/symlink/ua741.pdfThank you to everybody who participates in advance.

author: dmills
author_rank: Super Contributor
timestamp: January 09, 2024, 11:39:21 pm
content: Check your battery voltage under load, 5.1k may be enough to pull the Cr2032 down a fair bit, they are as I recall only rated to something like 2mA and only really specified to 0.5mA or so.4.8V at the output would correspond to 2.45V from the cell assuming the resistors are right and the opamp is rather better then a real 741, so the cell loading down by 400mV seems believable.

author: Benta
author_rank: Super Contributor
timestamp: January 10, 2024, 12:03:07 am
content: Where are your power supply decoupling caps?

author: JJ_023
author_rank: Frequent Contributor
timestamp: January 10, 2024, 12:21:23 am
content: Quote from: dmills on January 09, 2024, 11:39:21 pmSo you are correct about the battery being rated to roughly 2 mA.https://data.energizer.com/pdfs/cr2032.pdfI tested it under load the result was 2.45V which would give you 4.803715 with the gain that I calculated.I believe that is the problem.Thank you for your help.

author: JJ_023
author_rank: Frequent Contributor
timestamp: January 10, 2024, 12:26:57 am
content: Quote from: Benta on January 10, 2024, 12:03:07 amFrom what I read if your power leads are longer than 6 inches (the shorter the better) then use roughly a .1uF from supply to ground. I knew about this and decided to try without the decoupling capacitor to see what would happen especially since my leads were longer than 6 inches.I did not notice any ill effects and got carried away with the voltage discrepancy that I got and just completely overlooked it in the end.Thank you for pointing that out.My question is do you always needed a decoupling capacitor, even if your supply leads are short?

author: magic
author_rank: Super Contributor
timestamp: January 10, 2024, 09:11:28 am
content: Supply capacitors do two things: reduce supply rail ripple if chips draw AC current from it and reduce supply impedance at high frequencies, regardless of how long the cable is and what's on the other end. The former is irrelevant in a low frequency (here: DC only) circuit, but funky supply impedance in absence of decoupling may cause analog chip outputs to oscillate (often hundreds kHz, maybe low MHz) and this in turn may have other weird effects such as significant offset voltage appearing out of nowhere.

author: JJ_023
author_rank: Frequent Contributor
timestamp: January 10, 2024, 03:47:51 pm
content: Quote from: magic on January 10, 2024, 09:11:28 amThank you for the explanation. I actually played with this a little bit more. I added a .1 uF capacitor on a zoomed in wave. It cut the peak to peak by almost half. This is definitely Significant. However the peak to peak swings on this particular set up were fairly minor to begin with.So my question is if the supply is somewhat clean as was in this particular case (off a lab supply) is it still necessary?What are the practical limitations on the size of capacitor you can utilize before it has detrimental effects on a circuit like this?If I wanted to make this particular set up Oscillate so that I could get a better understanding of how this happens and the effect on the circuit. What would be a way of testing that safely. Injecting a signal on top of the DC?Please keep in mind that when I ask these types of questions it's not to challenge the advice you are giving or to save on components used. I know what you are saying is accurate and was actually aware of this before this experiment as I explained in a previous post. I ask these questions as often times they lead to more knowledge overall.