#79: Op Amp Power Supply Considerations: split, single, virtual ground, etc. - a tutorial

here's a quick video to talk about some op amp power supply considerations and the difference between split supply operation and single supply operation most times when you look at an op amp the power supply connections are often labeled B plus and B minus sometimes they're labeled VCC plus and VCC minus or VCC de VDD VSS but but the typically typically labeled in this way because most the times are op amps are many op amps are specified for a split supply operation so neither of them are called ground ok but the reality is is that even up op amps that are SPECT or split supply operation can be used in a single supply application the op amp doesn't really care okay now an example of kind of that equality is looking at this situation here here's a split supply for an op amp plus some minus 5 volts okay and often if you see something like this you know the non-inverting input to the op amp is reference to ground so that means your input signals are reference to ground and everything is kind of reference to ground but the we can bias the op amp in this way say with a 10 volt supply okay and ground I still have 10 volts across the supply pins and then bias the non-inverting input to +5 volts okay and so it's still midway between the two supply rails okay just like it was here so that's an equivalent you know biasing situation as far as the op amp is concerned and often times you'd see this implemented in a circuit like this okay where I've got a single supply biasing up the op amp and oftentimes just a simple resistor divider to create a voltage here that is halfway between VCC and ground that would get decoupled to make it kind of stable and that becomes the reference for the non-inverting input for the op amp when you do this this point is often called a virtual ground because it's not really truly ground ok but it does become a point a reference point for any of the signals that are applied for the op amp and a reference point for the negative feedback so it's often called a virtual ground for that that reason is that it becomes a reference point a new reference point workaround was often a reference point in the past okay well so why do we care about this and and why can't we just simply operate all a pipes this way okay so one of the reasons is that traditionally op amps when they were designed you know did have the ability for the inputs to get all the way or to be driven nearer the positive supply or near the negative supply typically had to have a couple of volts between where these inputs are and the positive or negative supply rail so the one first question you have to ask is how close can the inputs get to V + or V - still operate properly that's called the input common mode range or input voltage range of the op amp and many of the original op amps couldn't get to within a few volts of either supply so hence we use split supplies and just kept everything kind of in the middle okay similarly the outputs typically would not be able to drive all the way to either supply rail in many cases okay so again we kept the supplies you know you know split supplies had everything referenced around ground adjusted all of our work in that area okay so so and for most of these op amps that if the inputs and outputs can't be driven close to the supply rails the op amp was usually SPECT with split supplies okay but it can still be used with a single supply we just have to take care to keep the input and output voltages within the allowed range let's take a look at a spec for example to kind of show what we mean by by these voltage ranges that we'll take a look at the implication of that so what I've got here is a datasheet for an old 14:58 op amp which is really just a dual 741 741 op amp is probably the most popular op amp that people have heard of okay and is often SPECT with a plus and minus 15 volt supply this 1458 is no different if we take a look at some of the DC characteristics here inspected plus or minus 15 volts okay that's on the important specs we want to worry about or right here here's our input voltage range sometimes called the input common mode range an input voltage range is SPECT minimum plus or minus 12 volts and typically plus a minus 13 okay so that says that we're only guaranteed that we'll get proper operation as those input signals are kept away from the positive and negative supply rails by 3 volts sometimes you get away with getting up too close closer to two volts but it's only guaranteed to operate properly when you get to within 3 volts of the supply rail okay so similarly the output voltage swing we take a look at that that has some similar numbers a couple of different specs for different load resistance but again same numbers here plus a minus 12 volts plus a minus 13 14 volts so we're not going to guaranteed to get any closer than about 3 volts from the supply rail with a high impedance load worse than that with a low impedance load but you know typically we'll get closer but not always okay so you might conclude from looking at this saying well I don't want to use this on any kind of a single supply operation because I really have to keep far away so maybe I can't use it for anything other than you know plus or minus 15 volts the reality is is that you look deeper into some of the specs you'll find that you certainly can use the op amp for lower supplies I mean here's an example in some of the performance characteristics graphs if we look at them here's one that plots out output voltage swing as a function of supply voltage so here's our plus or minus 15 volt line here and this says that we'll have in about 25 volts guaranteed peak to peak output swing okay when we had this 30 volt supply plus a minus 15 volts we can see the curve extends all the way down to plus or minus 5 volt operation or even which would be a single supply of 10 volts really guaranteeing about 5 volts of swing okay but it certainly still be used so so don't be afraid of all you've got on hand for example as a 741 op amp you need an op after your circuit and you've got say 10 volts to play with or 12 volt supply to play with you probably could still use it you just have to take care of where those voltages land so the way to investigate that I've put this little circuit together on my breadboard and we'll go take a look at its op raishin so it's a simple inverting amplifier okay and so I've got a 2.2 k feedback a 1 1 & 1/2 K input that's going to give me a voltage gain of about one and a half or so okay and just a little bit greater than negative one okay it was an inverting op-amp okay inverting the figuration the output is connected to channel 1 on the scope which will be the yellow trace and then the I've also set up rather than setting up the resistor divider to set up the virtual ground here I've set the virtual ground up with a power supply okay so I can vary that power supply you know plus or minus I'll be able to vary that and change our operating point within the voltage range here okay and then my input signal I've actually got a signal generator that can do two things number one I can vary the amplitude of the sinusoidal signal that I'm applying and I can also vary the DC offset okay that we can apply with respect to this so if we've dial the DC offset to zero then this voltage is going to just going to swing around my virtual ground but this allows me to inject an offset above or below there so we can investigate what happens with the circuit without all that happens so my virtual ground is going to be plotted on channel 2 with a scope and then my input signal the plotted on channel 3 of the scope okay so if we turn on my power supplies here okay there's my 1458 op-amp sitting in the breadboard and we'll take a look over here at the scope okay and on the scope I've actually got some cursors on here this cursor and this cursor here I've got setting sitting at ground and +10 volts where my supply is okay so my power supply is right there let's see that's set to 10 volts and then my virtual ground voltage right there set to about 5 volts okay and so we take a look again at the scope okay so the focus is in there there we go so this point here is my virtual ground ok the DC line that I see there this lower smaller amplitude signal that's in the pink that's my input signal okay and then the larger output signal is the yellow trace number with trace 1 and we can see I've got a gain of just a little bit greater than minus 1 in here so again this op-amp we determine from the datasheet that we can we're not gonna be able to get you know anything more than within a volt or two of the supply rails so let's kind of see how that happens if I change my virtual ground point to shift these signals up or down okay let's move it down let's say I had a signal it was reference near ground you can see what's happening here as I move that down the output is getting clamped about two volts above ground okay this op-amp can't swing anything closer than that okay if we go up the other way we can see we start to clamp at about a bolt or so little less than a volt from the positive supply we start to plant okay so that's with these voltages moving back and forth okay so we can kind of see how much range I've got I can't get too close to ground I can't get too close to that positive rail okay we could also we wanted to play with the offset voltage which we can move say the input signal above okay the virtual ground point and now I can see I'm burying my signal level my signal down below I kind of get them everything kind of offset right and get that back in some way I bring my offset the other way okay we have to bring that back down again to kind of fit it all in there okay so again as long as you kind of adhere to the requirements you know for where the input signals need to be and where the output signal can swing to you're certain can use these op amps that are SPECT at split supply in a single supply application now since most modern circuits and modern systems operating are lower and lower voltages these days most of the manufacturers have started to develop op amps that are designed for lower voltage operation and single supply operation and the biggest distinction with the single supply operation devices is that normally now the input and output voltage ranges okay the input the allowable input range sometimes will include one supply rail or the other okay like the inputs can be brought down very close to or even surrounding ground okay or the negative supply or the positive supply and that the output can swing nearly to the supply rails okay and nearly two meaning within tens of billions or something like that and you'll start to hear terms like rail-to-rail op amps and usually when you hear a term called rail-to-rail op amp that means that either the inputs or the outputs or both or just the output can swing rail-to-rail or can be brought from one supply rail to the other sometimes the op amps will have maybe the inputs can go to ground and the app I can swing rail-to-rail will be a different combinations but those are the things that you could look for depending on where you need your signals to be used so I've got a little rail-to-rail op-amp here this little analog device is a t32 and this happen to have a couple of samples of these happens to be a really fast op-amp but it's also a really nice op-amp in terms of a lot breakdown at two point seven volts minimum supply and it's inputs and outputs will operate rail-to-rail okay so let's take a look at that what I'll do is I me to turn off my power supplies here and we'll swap this guy out for the 80 32 okay so let's pop him in then let's get him over here and pop in in here boom turn the supplies back on power supply the bias point and my signal so everything looks kind of normal here okay just like we were looking at before but now if I increase the signal fight kind of increase the offset and move it we can see that that output signal doesn't clamp until we hit the supply rail okay now see if I bring my inputs all the way up still operating normally even though I'm clipping even though my inputs are going even up slightly above that positive supply rail if I come down the other way okay the output clamps at zero so we can certainly see the output hang swing rail-to-rail okay and the inputs can go even slightly below that supply rail everything is still kind of working okay so and that's kind of the difference between you know a single supply op amp or one that's specified for rail or rail operation I increase the gain of a signal level here I can see I'm starting to clamp okay on the positive because where I am if I Center myself up here I can get kind of a rail rail swing out of this op amp so so certainly op amps that are designed for kind of single supply applications will generally have you know rail to rail swinging outputs and inputs that at least include one of the supply rails and those are the kind of things that you'd look for for those types of applications so you know I hope this helped to answer some questions about the difference between split supplies and Simo supplies what's meant by a virtual ground and some of the considerations you might want to think of you know in terms of where these voltages need to be when using an op-amp in a single supply application and also probably the most important thing is to think about is that if you need an op-amp for a circuit that you're developing you don't have to seek out one that is specified for single supply operation as long as you take care of where these voltages land and where they are you could certainly use an op-amp that is typically expect for split supply operation so again any questions you have I'd be happy to answer them for you and thanks again for watching

Channel: w2aew

Views: 87,242

Rating: 4.9804182 out of 5

Keywords: W2AEW, Tek, Tektronix, op amp, opamp, op-amp, power supplies, power supply, power, supply, supplies, operation, performance, split, single, single supply, split supply, rail, rail to rail, input, output, electronics, basics, tutorial, virtual, virtual ground, 741, lm741, 1458, Ground, design, voltage

Id: MtccB9K09ck

Length: 15min 8sec (908 seconds)

Published: Sun Feb 10 2013