I am a second year EE student and I have been assigned a small filter project for the end of the semester. It is nothing crazy; just build a small filter on a breadboard and analyze the frequency cutoff with the Analog Discovery kit. The schematic is attached and shows the required components:



(x1) 1k resistor
(x1) 2.2k resistor
(x1) 1 uF capacitor
(x1) 741 Op Amp

My question, however, is the circuit kit that we were requested to buy does not contain a 1 uF capacitor. Instead it has a bunch of 100 uF, 10 uF, 22 pF, and 104 pF capacitors. While this isn't due until December, (so no rush to slap together a working circuit), but my curiosity is peaked as to whether I could use a combination of those capacitors to create an equivalent 1 uF.

 

Hi,

Ten 10uf caps in series will provide 1uf, but that's a crazy way to do it. Maybe just buy a 1uf cap from online. Also, ten 104pf caps in parallel creates a 1uf cap non polarized.
The cap should really be non polarized for this experiment because the output can go plus and minus relative to the other side of the cap. Maybe keep signal levels very very low less that 0.5v that might help.

Bigger problem though, i dont think the 741 will work too well at plus and minus just 5v, you might need 10v instead.
Alternately, get an LM358 ic.

 

Thank you, and yes that does sound crazy but I was curious if it were possible with what I had.

It's interesting you don't think the 741 would work too well because that is what our assignment specified. Could you elaborate more on that? Perhaps I can make a recommendation to my professor.

 

Thank you, and yes that does sound crazy but I was curious if it were possible with what I had.

It's interesting you don't think the 741 would work too well because that is what our assignment specified. Could you elaborate more on that? Perhaps I can make a recommendation to my professor.

Hello again,

I just checked the data sheet for the Texas Instruments ua741 and i can see that the MINIMUM voltage for the two power supplies is plus and minus 5v. That seems to make it OK. However, what i fear is that may limit the output to plus and minus 1v because with power supplies of plus and minus 15v the maximum output swing with 2k load is given as plus and minus 10v, which is of course means the positive peak is 5v less than the power supply source voltage, and ditto with the negative supply and negative peak output.
So there is a chance it might work to a certain point with a certain load, but the LM358 for example can make it all the way to the negative rail (-5v) and within 1.5v of the positive rail (3.5v with 5v power source) so the LM358 looks like a better choice.
It also depends on load too however, so some testing would be in order, but i see the LM358 being better and after all it is a better design.
There's one catch however, there is no input offset adjustment terminals so if you need that you cant use the LM358 as you would a 741, but in this practice exercise i dont see that being a problem.
Note also you could always increase the power supply voltages to plus and minus 10v. This op amp was designed back when plus and minus 15v supplies were like a standard in analog design.
You could actually use this information as a test bed too and find out the differences in output with the two op amps if you would like to delve into this phenomenon a little deeper. It would extend your understanding of op amp specs i can tell you that

 

Find a local electronics supplier or order online some (about 10) 1μF 50V non-polar capacitors. They will come in handy in future projects.
Or buy 100 and share them with your fellow classmates.

 

Thank you so much for the insight on how those Op Amps would act. I will definitely look into that more and include it in my report. Now that my curiosity is peaked, I will go ahead and get 358s and make both circuits and see how they compare.

 

Thank you so much for the insight on how those Op Amps would act. I will definitely look into that more and include it in my report. Now that my curiosity is peaked, I will go ahead and get 358s and make both circuits and see how they compare.

Hi,

Oh good, and i hope you will come back to this thread and report the results. Would be nice to see.