Understanding Op-amps From Simple To Hard

[Tim] wanted to help out a ECE student struggling with some Op-Amp problems. He put together a video which does a good job of explaining what an Op-Amp does, then tackles each of the questions one at a time.

His analogy is illustrated in this image. There’s an operator using a crane to lift a crate. He is watching a ‘radio man’ in a window of the building to know how high it should be lifted. These roles are translated to the function of an Op-Amp in a way that makes understanding the common parts quite easy. The crane is the Op-Amp and the floor to which it is trying to lift the crate is the input pin. The current height of the crate is the output signal. The radio man is the feedback resistor which is trying to get the desired height and current height to equal each other. Watch the video after the break and all becomes clear.

After this analogy is explained [Tim] tackles the actual homework problems. He’s going through everything pretty quickly, and doesn’t actually give the answers. What he does is show how this — like most circuit solving problems — depends on how you group the components in order to simplify the questions. Grab a pen and paper and put on your electron theory hats to see if you can solve the questions for yourselves.

17 thoughts on “Understanding Op-amps From Simple To Hard

  1. I’d like to see hack a day make a youtube guide to electronics from free examples like this it would be cool for people like me who are stary eyed for hackers doing cool stuff but have no idea what they are doing to accomplish it ^^, I’ve thought about getting into electronics but for all the scary terms he bypassed into this simple explanation its a little insurmountable and kinda scary =.=

    oh well at least there is hack-a-day

  2. I just did a mechanical engineering exam (Mechatronics) involving Op-Amps. As in just a couple of hours ago. I’m pretty sure I got the Op-Amp problems wrong or mostly wrong, partially due to a difficulty understanding exactly what goes on inside them.

    This would have been so incredibly helpful yesterday night that I find it difficult to put into words.

    It’s fine, though, aced the second order transfer functions.

    Cool post!

  3. Looks pretty good to me. I’ve used the trick at the end of the video on exams many times where you turn everything into a lumped impedance model and it pretty much boils down to basic circuit theory at that point.

  4. the basic opamp function is simple. the output will replicate the input but with a gain (multiplier) of K based on the ratio of the 2 resistors. things get more complex when you toss in an ac signal, but not too bad. until you get into designing them, the 3 resistor technique will work well enough. if you add capacitors with or in place of resistors, you make the system frequency dependant.

  5. after watching the video, it is a decent primer. you needed to cover slew rate. this is how quickly the crane can lift the box from a to b. using a common value given by manufacturers called the gain bandwidth product, you can take the opamp you want to look at, divide the GWB value by your max gain, then see if that number is in the frequency range you need. this is a back of the napkin calculation that gives ee’s a dirty calculation to estimate what kind of opamp to use. second. Ideal opamps are great, but they don’t exist. it is easily possible to get an unbalance on the front end of the opamp. that is why you place a 3rd resistor between the non inverting input and ground. those are the basics you need to work with real world opamps and choose the right one for the job!

  6. I just found the constant doodling and erasing to be distracting.

    He’s talking about electronics, and yet before my eyes I see little guys building stuff and having little exclamation marks appear over their heads.

    This is far more confusing than a good textbook on electronics.

    1. I have to agree, video is not good for this kind of stuff (well, for a teacher who is giving a class it might work, but as a standalone teaching aid not so much imho)

      There are issues like the ones you mention, then there is the issue with not only understanding a possibly foreign language (also an issue with text) but spoken by someone who may not sound very clear to you

      A good text, with pictures (and then possibly a round-up on video) is much preferable imho.

      All that “negativity” aside, good for him trying to simplify something so people will understand it – and it might work well for his class. but for anyone not in his class it will til have the above issues (to me, you may or may not be bothered by it :-)

  7. After watching the video, from a basic overview of ideal opamps, it was pretty good. However, there are a couple of things that need to be added to convert this into a real opamp model. First is a term called “slew rate”. This is analogous to how fast the crane is able to move the package from point A to point B. Slew rate dictates what frequencies the opamp will be able to function with.

    A simple calculation (back of the napkin) that many ee’s use when working with opamps is based off of a number that manufacturers give called the “gain bandwidth product”. Simply put, this takes into account various different things in the opamp and gives you a quick and dirty value to tell you if the opamp will work for your purpose. It means that for any value of K (gain), you can pass a signal of X bandwidth. So you take the gain bandwith product provided, and divide by your wanted gain. This will yield a maximum (barely) usable frequency of that opamp. Keep it significantly below this number and you will be great.

    The other important thing is that because we are not working in an ideal world, we have flaws in the system. Simply put, if you use the 2 resistor gain model, you will have an improper bias voltage. This will feed through to the output to give an incorrect value by some factor. That is where the 3rd resistor comes in. In a standard inverting amplifier, it hangs off of the non inverting pin to ground. To calculate it… it is simply R1||R2.

    These are some of the common pitfalls that new engineers run into when learning opamps and have a problem that they can’t seem to fix (that or they are trying to use a bipolar supplied opamp off of a single rail without a floating ground…)

  8. This is for anyone wanting to contribute an oddball project:
    Don’t forget that op amps have undocumented features like an amplification that increases with decreasing temperature and an ability to detect microwaves (which includes smartphone transmissions and microwave oven leakage).​
    For beginners: Connected as comparators, op amps oscillate readily, which can be remedied by connecting a capacitor from the output to the noninverting input, but trial and error is needed to find the right value. The inputs ideally do not source or sink currents, but they do a little bit, and if that little bit finds no path to ground, the circuit does not work. They also oscillate if the load has too much capacitance. If you have a leftover op amp in a dual op amp package, you can connect it as a buffer to drive the output with better tolerance for capacitance. Op amps like to draw as much power as they want, whenever they want, so connect a fast ceramic decoupling cap as close as possible to the +Vcc pin, with the other lead going to ground (this assumes a single-sided power supply), to provide your op amp with a local power cache. They also hate batteries that aren’t fresh; a battery with a lot of internal resistance will cause the signal being amplified to get into the power rails, from where it can go upstream in the signal processing train and cause feedback.

  9. Correction to my comment above: the temperature effect I observed was probably due to the temperature dependence of an offset voltage that looked like a change in gain because it was in a comparator stage.

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