Push-Pull Output Stage - Your Signal Needs More Power - Simply Put

let's say we have a signal any signal at all digital analog ac/dc whatever relative to a reference voltage of course the circuit ground and let's say that this is a well-behaved signal so we can plug the signal into whatever we want and it's not going to just short-circuit and run away current into ground and then we have whatever we're doing with the signal let's say we're driving a load with this signal of course also referenced to negative circuit ground so it looks great so far but we have a problem this signal may very well be generated by something that can't supply either sourcing or syncing meaning going out or going in it can't handle a lot of current flow such as a microcontroller a little C Mo's logic gate whatever let's say we need to amplify this signal we we need the signal to stay the same but we need more power so what's the first option that comes to mind a single transistor making an open collector output let's have an NPN transistor the signal will control the base the load will be connected to positive and instead of negative the load will go through the collector and out to negative there well that functions on a basic level but we've introduced a few problems the first is that there is a dead zone base to emitter is sort of like a diode it requires a minimal voltage to turn on so if your signal is low if it's just a bit above zero it's not going to output this is not a problem if you just have a digital signal that's either low or high it's not going to matter but let's say we're trying to handle any signal so that's a problem we have a dead zone second we can only drive the load in one direction current is going to go through positive through the load to the collector to the emitter to negative we cannot put current the other way let's say we're trying to drive a motor we might want the current to go backwards we could add other stuff to the circuit but let's say we're trying to keep it as simple as possible we want to just be able to use this output this way so that's a problem it only goes one direction another problem is we cannot feed this with a negative input voltage again we're trying to handle any reasonable signal so no good we can't use a negative input voltage or a negative signal really now if all you're trying to do is drive something with a purely binary input say know something very simple this is all you need but that's not good enough for us and there's even one more problem this is a floating output half the time when the signal is enough to turn on the transistor then current is flowing through and we have a nice clean signal if the transistor is not transistor if it's not conducting through you don't have a connection to negative-20 to the reference you have an open circuit which means signal noise such as electromagnetic radiation in the air static electricity whatever noise from the rest of the circuit can feed the load that's bad so let's try something a little more interesting let's hook the load back up the way it was and let's add a piece let's use a logic gate so we have our positive a resistor an NPN to negative and you take your output as normal you've seen an inverter before and there you go so we've solved the floating output problem you're getting an affirmative connection to the positive here or to the negative here no matter what the signal is but we've inverted the signal and we've introduced variable output impedance if your signal is low I did it again this goes to the base doggone it if your input is low the transistor is off and you've got this as your output impedance if your input is high the transistor is on and you've got effectively no output impedance that can be an issue well we can solve the inversion fairly easily we just add a second inverter so now the input signal is not inverted but we still have the problem that we can only drive the output one way we still have the deadzone and we still have variable output impedance and now we've quadrupled our component count what if we take an idea from seamos let's get rid of these resistors and what if I swap an NPN for a PNP so I have one of each we hook up our power through an NPN we have a junction our PNP now it's a negative our input signal is going to feed both of these bases and the output will be between them and then I can do something even more tricky and add a negative voltage so we've got our circuit positive our circuit ground and our circuit negative I suppose I should print out a ground symbol but I'm stubborn so what does this do if the signal is higher than zero and we've got the zero signal coming through the low - here then we have forward-biased the base emitter junction on the NPN and the positive can flow out through the load to zero whereas this PNP since the base is higher than the emitter at zero it's off so we are sourcing current out if the signal is below zero then the NPN based emitter Junction is not forward biased but the emitter to base of the PNP is and we're going from zero to a negative and we are sinking current through the PNP welcome to a push-pull output stage and the first thing you may look at is I've looked these up wrong remember we're always supposed to have the load going through the collector not the emitter well it's not that big of a deal it is an issue but right now we're just not gonna worry about it because this is not the final circuit and in the final circuit that's not going to be a problem here it is a little Wiggly your base emitter current is going to be contaminating so to speak the load let's not worry about it we have solved several problems but now we still have the deadzone problem if the signal is a little bit above or a little bit below zero it's still not going to turn one of these on so we're still losing signal near zero but we can drive the load both ways we can use a negative voltage which is how we get the bi-directional drive through the load everything's hunky-dory except that deadzone just for fun this is a Class B amplifier remember the audio you can make a similar amplifier like this for audio and it'll work but you'll get that clipping issue it's really crappy but it's incredibly power efficient but we talked about a Class A amplifier and how did we make a Class A amplifier work and have the best audio fidelity we biased the signal so that the transistor was always in the operating range well this splits the signal in half positive signals will turn on this negative signals will turn on this and in the middle we have that dead zone when the signal is near zero we still have a floating output so that's still an issue but we're gonna fix that anyway we need to change this input to be properly biased and we're gonna do it in a smart way instead of all that fuss and mess we used for the Class A amplifier to set a specific value we only need a rough one we only need to get most the signal back so let's say we have positive and a negative let's have a couple of resistors and let's have a couple diodes our input signal is going to feed in between the diodes and on either side of the diode will be our output signals so if you remember your Kirchhoff loops you've got positive through a resistor a diode a diode a resistor and a negative that's a complete loop so we can decide our voltages from that the diodes are each going to have fixed forward voltage drops and if the resistors are equal in value then this is still going to be a midpoint these are equal these are equal roughly so it's going to be roughly in the middle so I've got positive here negative here this is going to be roughly zero but up here is going to be roughly plus a voltage drop and down here is going to be roughly minus a voltage drop the voltage drop of a diode is about the voltage drop of a base emitter junction so when the signal is zero then this is at it's nice midpoint and this is about a diode up and this is about a diode down so both of these are just on a razor's edge of being on they may be on a trickle but they're on enough and then if the signal goes up or down it's going to change the voltage here one of these resistors is going to have a bigger drop and one of them is going to have a littler drop a smaller drop which is going to stop forward biasing one of these and more strongly forward bias the other so now we've gone into something called a class a/b amplifier it takes more power but it doesn't take as much power as a Class A amplifier you still have some power drain but you have less power drain it's still not perfect the signal does have a small amount of distortion around the center point but we've mostly solved the problem but we have four more components and it's messy and it's still not great but we have our negative voltage we have actually gotten rid of the floating output this time we can drive the load both ways everything's hunky-dory we're done but we can do better remember the flavor of the month the op-amp the op-amp uses something called feedback it's basically a magical little device that has a little gremlin inside that manually turns the knobs up and down to get the result you want and we are going to employ this gremlin to great effect let's have an op-amp and slap it down as a reminder I've made videos on op-amps so I don't have to explain the whole thing essentially the intuitive way to look at an op-amp is you have a non-inverting input a positive input and an inverting input a negative input the differential input is the positive minus the negative so if five volts here and five volts here is a differential of zero five volts here and four volts here is a differential of one four and five is minus one the op-amp is a feedback device you connect the output to the input in various different ways and the op-amp will do whatever it takes to make sure the differential input is zero the feedback and the internal circuitry of the somme PAMP we all wiggle its output there's that little gremlin he's going to turn those knobs until his inputs are zero whatever the output ends up being the inputs are going to be zero between them one minus the other will be zero so the inputs will be the same that's the magic so what if you take the input and we shove it right into that positive non-inverting input and of course we need to connect it to power which is not usually drawn but I'll draw it in this case just to remind us that the op-amp is receiving the positive and negative voltage well what's the negative input what's the inverting input well we need our transistors back don't we and we're gonna do it just like before we're gonna hook them up the positive and negative and the output is gonna be between them so we're kind of backsliding a little bit here just like so to the output the op amps output is going to drive both transistors and the only thing left is the feedback the feedback is the output what the load gets also goes into the inverting input there's no resistors remember in those videos about op amps I had resistors in different configurations that was to control the gain if you don't use resistors it's what's called unity gain in this case in this configuration you're going to get unity gain it's going to be input and output is the same let me show you the input signal is going into the positive the non-inverting input the output signal is going to go into the inverting input positive and negative so our signal minus our output we want the signal and the put to be the same we're trying to amplify the power we don't want to change the voltage we're trying to amplify the power so signal minus output if they're the same voltage should be zero volts that's what the op-amp wants that's what that gremlin wants so whatever this output ends up being whatever the voltage current whatever going into these transistors it's going to be adjusted so that this spot is equal to the input signal minus output equals 0 therefore signal equals output now I would show you this on a breadboard I made the circuit tested it it works just fine but there's nothing to show unfortunately the the whole point of this is to change nothing but allow you to drive a bigger load the input voltage is the output voltage it just works and I tested it was positive and negative voltages works just fine so let me walk through this a couple times to make it clear to you if the signal is greater than zero we have a positive on a non-inverting input so that means we need the same value here so this in order to get a positive voltage here we're going to have to turn on the NPN and turn off the PNP so the output voltage is going to go up we're going to have something that is above zero on the base of the PNP and zero through the load cz Earth of the load on the emitter so not forward biased remember it has to be forward biased in the direction of the arrow we've got higher on base than emitter so it's closed but this one we've got higher on base than emitter because base is a positive value emitter is zero so the NPN will turn on and you'll get a positive voltage out here and then it'll feedback and the positive voltage will go up and down until the beta amplification whatever of this transistor is giving you the right current but we're talking about voltage remember load load is an effective resistance whether it's a resistor or not it's an effective resistance Ohm's law voltage equals current times resistance whatever the effective resistance from moment to moment is so a certain amount of current going to the load is going to give you a certain voltage drop or you could look at it as there's a voltage drop whatever is causing it omma car not the load is going to have a certain voltage drop and so you've got a Kirchhoff Loup positive through the collector emitter and out the load there so the voltage drop of the load varies by the current through this varying and so forth but the point is it's turning on the NPN and not turning on the PNP the amplifier here is sourcing current it's going out to the load into zero if the signal is negative it's below zero so it wants the output to be below zero minus two minus minus 2 equals zero so in order to get a negative voltage we have to open up this PNP so this voltage is going to go down we're gonna have zero and something below zero forward biasing emitter to base PNP will turn on and current will be going from the circuit ground through the load through emitter collector and out the circuit negative and then we're gonna have zero on emitter or something below zero on the base so this NPN is going to be off and we don't have a dead zone because this feedback is going to tweak them at zero or at very close to zero it's going to be super fiddly and there's going to be the tiniest currents running through this thing but you get yourself a nice quality op-amp it's gonna have good accuracy and you'll still have that nice exact or near exact signal at zero because whichever you know small differences in manufacture maybe this one will turn on maybe this one will turn on but you'll get that very close to zero voltage and then of course the stronger the signal is that long as it doesn't clip but the stronger the signal is within the normal operating range of your positive and negative supply voltages the easier it's going to be the less you're going to be affected by just random noise so that's the magic the magic of an op-amp solves the problem of floating outputs it allows us to have bi-directional supply bi-directional current flow through the load we can have positive and negative signals we can have everything we want and no resistors we assume the signal is well-behaved that the signal is not going to produce like we could hook the signal directly to circuit ground and it would not be a short circuit so we're assuming that we're also assuming that the load is well behaved that we could hook the load directly to positive or negative power and it wouldn't be a short-circuit that there's resistors in there or whatever so if those two things are true then there is no current pass through this thing that is going to be a short circuit and we don't need resistors the only possible path that'll short is through positive collector emitter emitter collector and negative only one of these is going to be on at the same time just like a c MOS configuration one of the C Mo's one of the MOSFETs and the C Mo's is on at the time and the other one is not near zero or when it's switching there can be momentary flow through that is technically a short circuit but it's gonna be brief it's really probably not going to be a problem if you need to you can add resistors add yourself some small resistors put an equal value resistor on the top and bottom so on the collector here in the collector here you could add resistors and you can do this with C Mo's too if you really need to if it becomes a problem or you're just super worried just add a resistor here and a resistor here of equal value to preserve your mid point at zero and it's going to slightly reduce the response rate of your circuit because resistors always make everything slower if it's a small resistor it's gonna be negligible we're not working at gigahertz terahertz frequencies here it's obviously gonna make your circuit bigger and increase part cost and it's going to cut your room so you've got positive and negative supply voltage to work with it's going to reduce your window just like for the Class A amplifier you have a window to work with so it's going to reduce your window slightly because those resistors will have a voltage drop and the op-amp is wiggling the output voltage up and down within that window but you're probably going to have a supply voltage much greater than your signal anyway for example I hooked my op-amp up to positive and negative five volts but my op-amp is only able to output +3 some vaults and like minus 4.5 volts when I hook it up to plus and minus five now you can get a nice high-quality op amp with CMOS that's gonna go all the way to the top and bottom but mine doesn't so my op amp can't even output signal anyway so it's not a problem but you can do that but you shouldn't need to so with one op amp one PNP and one NPN transistor and nothing else well and a negative power supply so you need a dual power apply or you need to take one power supply and split it somehow you can do that but however you do it that's all you need those three pieces and you have an output stage that you can stick on anything and some integrated circuits actually include this output stage in them for example I'm about to get into the 555 timer and its output is a push poll which means it can drive more current more power than the internal circuitry of the timer can anyway but keep this in mind you still want to use this you still want to keep this in mind and make it yourself out of discrete components if you're driving something like a motor because when you have an integrated circuit even if it's something like this it's still going to be somewhat limited we're still going to be talking about maybe hundreds of millions which is quite a bit for digital circuitry for signal circuitry but if you're trying to drive a motor if you're trying to drive some big load or if you have a big fan out let's say this output you're trying to drive 10 things and each thing doesn't take much power but you're driving 10 of them at once so then you need a total output power to handle the fan out so you're going to need more power in that case then the chip can probably supply so you're going to still want to be able to make this circuit out of discrete components for the highest power things you ever do but once again we are impressed by just how much magic is done by the op-amp now you won't be that surprised if you ever look at the actual circuit diagram of an op-amp I'm going to make one someday but that'll be for in the future because there's a lot of parts in there making your own homemade discrete op-amp is not a simple one night task so it is a complex beasty but it's a common one and you can get a whole bunch of them for cheap so now you know how to supply a signal with more power and while you do that I'll be seeing you

Channel: Simply Put

Views: 6,096

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Keywords: simply put, simply, put, circuit, circuits, electric, electrical, electronic, electronics, electricity, math, maths, mathematics, ai, neural, network, networks, push, pull, push-pull, pushpull, output, stage, buffer, amp, amplifier, amplification, power, signal, current

Id: 9UYXzstco60

Length: 19min 35sec (1175 seconds)

Published: Wed Jul 03 2019