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Current time:0:00Total duration:5:56

Video transcript

so now I want to talk a little bit about the concept of feedback this is a this is a really important concept it was developed in the 1920s the idea of using feedback and it was done at Bell Labs Bell Telephone laboratories remember we talked about this on the op amp this being the the non-inverting input so if this voltage went up this voltage went up and the negative sign is the inverting input so these these voltage here and here move in opposite directions and the key to understanding what feedback is is to pay attention to this inversion path than these non-inverting paths we're going to start over here with just isolating the voltage divider part of our circuits so that's copied over here and we think about let's look at what happens if V out goes up and what happens to V minus in this case well V minus goes in what direction in a voltage divider it goes up so this is a this is a non-inverting structure if V out goes up then the thing we care about V minus goes up as well and likewise if V out goes down then V minus goes down so that's isolating just on this part of it we have no inversion happening around here so we go around here now and let's look at what happens on this path here if we go up in V in then we know that V out goes up if we then change colors to let's try this if V minus goes up then V out goes down that's the inverting path so there's so there's one inversion in this circuit and it happens right here where V minus goes to V out so that's where the inversion is so so now let me set up just an example circuit we'll set R 1 equal to R 2 and from the last video we developed a gain expression and we said that V out equals R 1 plus R 2 over R 2 times V in and with these with these resistor values V out equals 2 two times VN all right so this is equal to two times V in and what does that make this point here V - this is V - and from our voltage divider we know a voltage divider says that V - equals V out times R - which is just R over R plus R or V - equals one-half V out so we have let's put let's put a voltage on here let's put a real voltage on here let's say this is it one volt all right and going through our amplifier we know that V out equals two volts and that means that V - equals one half of V out so V - is one volt so this is one volt here so let's say for the moment that something happens to this circuit like we heated up or something like that and let's say the gain goes up a little bit now what that means is that this amplifier which is amplifying this voltage difference right here is going to be a little higher so the voltage here is going to go up a little bit let's use this color a goes up a little bit and that means that this output voltage is going to go up a little bit and we already decided from looking at this voltage divider that if this point goes up that this point will go up it goes up half as much but it goes in the up direction when this voltage goes up that means this voltage goes up and now we find ourselves we're at the inverting input we're at the inverting input to the amplifier and that means what when change that the inverting input goes up that means the output goes down and that's in the opposite direction of the original change so this is the mechanism of feedback a went up a little bit we thought that V out we go up a bit which meant this point goes up which meant it gets fed back to the input to the inverting input and then it goes back down and this balancing act that's going on right here that is the mechanism that is what we call feedback you get this feedback effect when this connection is made right here back to the inverting input to the op-amp and in particular because it's the inverting input this is called negative feedback so this is the mechanism of feedback in particular negative feedback and what it does for us is it provides us a way to exploit and to use this enormous gain that these amplifiers have to create really stable really nicely controlled circuits that are controlled by the values of the components we attached to the to the amplifier so that's the idea of feedback a really powerful idea and really at the heart of analog electronics