June 13, 2012 AT 9:55 pm

ASK AN EDUCATOR! “Making a non-inverting op amp circuit on a breadboard”

Luis asks:

I am now comfortable with my electronics prototyping to want to try some things with operational amplifiers. I am still having issues reading schematics I guess, because whatever I do, I cannot seem to set up what seems to be a simple non-inverting amplifier! I am using an LM358N on a breadboard. I see that using two resistors connected in a specific way should allow me to amplify the voltage I have going in to 2x. So if I have 5v going in, should I not get 10v in the output? I actually get something like 3.58v!

Maybe a more general question would be, what are some tips for going from a schematic to a breadboard!? I know it is meant to be simple, but I cannot seem to get intuitive about this!

Great question, and I have spent a few days finding a good way to answer it. Before we get to the circuit problem, you might want to take a look at Fritzing.

Fritzing is a

Electronic Design Automation software suite with a low entry barrier, suited for the needs of designers and artists. It uses the metaphor of the breadboard, so that it is easy to transfer your hardware sketch to the software. From there it is possible to create PCB layouts for turning it into a robust PCB yourself or by help of a manufacturer.

One of the nice features of Fritzing is its ability to switch between your schematic view and a breadboard. You can either populate the breadboard, and get a rudimentary schematic, or vice versa. This would allow you to take your op-amp circuit and create something like the following:

Regarding your 3.58V output, make sure that you are sufficiently powering your op-amp. Your non-inverting circuit is configured to output 10V from a 5V input. This input should be a signal input, as the op-amp can only drive a couple of mA. If you are looking to boost your voltage from 5V to 10V in a power application, you might want to check out a boost converter. If your application is to boost a signal voltage, check out Todd Toporski’s article about “…Violating your op amp’s input common-mode range” which illustrates the devices range of common-mode input voltages that result in proper operation of the device and how close the input can get to either supply rail. So if you look at the data sheet for the LM358, the input common-mode voltage range is V+ – 1.5V. Which means that if you power your device at 5V you will only be able to output ~3.5V…which might describe your measured output. You would want to power the op amp at a minimum of 11.5V in order to get your 10V output.

I hope this has answered your question and good luck with your breadboarding!

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