Small Signal Amplifiers

today we are going to talk about small signal amplifiers so let's start with a microphone and our goal is to amplify the tiny signal that comes from this microphone and eventually put that signal into a power amplifier that can run a loudspeaker so what we have is basically a public address system or PA system and obviously it works at audio frequencies this could just as easily be a radio receiving antenna and then it would work at radio frequencies but a PA system is a good circuit to analyze to understand how basic amplifiers work so let's take a look at this microphone and see what it does and see what we have to do to make the signal useful if we imagine my pin is a microphone inside the head is a plastic diaphragm and attached to that as a spool with a coil of copper wire around it and that is suspended in a magnetic field so that it is free to move back and forth and when I talk pressure waves hit that diaphragm and cause it to vibrate when that diaphragm is moving away from me it pushes the coil into the magnetic field and as we know when you push a piece of wire into a magnetic field it induces a current in that wire so we get an electrical current that flows One Direction depending on how the microphone is oriented then when that diaphragm moves towards me it pulls that coil the opposite way through that magnetic field and induces the current to flow in the opposite direction so when the diaphragm is moving towards me the current goes one way when the diaphragm is moving away from me the current moves the opposite direction but there's no circuit here for that current to flow so actually what we get is voltage so when the current is trying to move clockwise we get a voltage that's positive to negative and then when the current is moving counterclockwise we get a voltage that is negative to positive this is going to be a pretty tiny voltage so let's assume that this microphone produces 20 millivolts Peak to Peak which means that when we have zero volts we would have this when there's no sound coming to the microphone when that diaphragm is moving away from me I'm going to get a voltage that goes up and then Peaks at when we have the diaphragm at the maximum acceleration it's going to Peak at positive 10 millivolts and when that diaphragm moves towards me it's going to Peak at minus 10 millivolts so we have a total of 20 millivolts Peak to Peak now my voice is pretty complex and it's going to be hard to analyze so let's just assume that I can produce a sine wave and this is what we get a nice clean sine wave that just repeats itself so as that diaphragm vibrates back and forth we get a wave that Peaks at positive 10 and minus 10 millivolts and it just as long as that diaphragm is vibrating we get the sine wave off of it okay so now that we've established that we need to amplify that so that we can drive a power amplifier so how are we going to to do that first of all we need to increase the voltage we only have 20 millivolts Peak to Peak so how are we going to increase that voltage well the first thing you might think of is a Transformer okay so let's put that on a Transformer a nice Step up Transformer and step up that voltage as a matter of fact some systems do use Transformers and microphones but not the way you think it does something a little different let's see if we just try to use this as a Step up Transformer so we have 20 millivolts Peak to Peak here over here let's say we step it up 10 times 40 millivolts great we've got some increase we could add some more coils over here and get it to increase even more but what happens well eventually we need to produce more power are we going to increase the power going across a Transformer and the answer to that is no we cannot increase our power so if our voltage goes up something else has to give how do we how do we calculate the power well in this case voltage times current so if our voltage goes up but the power stays the same our current has to go down and so if we have uh one microamp here we're down to 0.5 microamps here so our power stayed the same we need to increase the power to drive that speaker remember the ultimate goal of most electronic systems is to produce power power is produced when you have electromagnetic radiation or mechanical motion or heat our goal is to produce a lot of mechanical motion to move air and thus have a louder sound than we started with so we need to increase our power so stepping up the voltage with a Transformer we're not going to increase our power power is going to stay the same so we have to find something else to do so what can we do to increase our voltage and at the same time increase our power and that's going to be a transistor so what we're going to do is run this microphone into an npn transistor so there it is going from the base to the emitter of an npn transistor and so we have a circuit through here and as we know what does a transistor do well if we look at it not looking at anything else assuming I have a power source out here somewhere if I have a small current flowing into the base I get a considerably larger current flowing into the collector and of course those both go into the emitter and we'll talk about what happens with that later on so I get a tiny current into the base and a bigger current into the collector and to get that current what I'm going to do is I'm going to need some kind of a power source so let's put a battery over here and so small current into the base big current in The Collector but in preparing to run our power amplifier we need to amplify our voltage so we need to make this a voltage amplifier what does that mean well what's the difference between a voltage amplifier and a current amplifier for a current amplifier we basically want changes in our base current to cause proportional changes in our collector current what we need to do is design this so that changes in our base voltage cause proportional changes somewhere else and so what we're going to do is put a resistor right here so what does that do for us well let's examine what's going to happen at that resistor just by itself to see what it's going to do let's eliminate the rest of the circuit so we don't have any confusion we're just going to have that resistor all by itself and let's assume that was a 10 volt battery so there's plus 10 volts and somewhere else there's other circuitry down here so this is to the positive side of the battery there's other circuitry down here causing things to happen but we just want to see what's going to happen at this resistor so somewhere we have a ground but we don't know what's between we just know that our current is going to change through this resistor let's assume that right now we have an open circuit here so there's no current flowing and let's just say that this is a 1 K resistor okay 10 volts 1K no current what's the voltage going to be across this resistor well the rule is to get any voltage difference between here and here we must have current so what do you what do you need to get a voltage difference you have to have resistance and current do we have any current flowing through here no we've established that this is an open circuit right now so there's no current flowing which means that the voltage here would be the same as the voltage here remember my demonstration with the soda straw right now we just have an open straw I'm going to pinch this down which represents the resistor so this is our wire the pinch represents the resistance right now we have 15 pounds per square inch on both sides of this and even if I block this off well since there's no current flowing through and there's nothing to prevent molecules from freely flowing from one side to the other I this is not completely pinched off there's a little room for air to flow between I will have the same pressure on both sides so I can pinch this off till the cows come home and it's not going to change the pressure now I'm going to suck on it well what happens remember that the negative side of a battery is like the suction side of a vacuum cleaner we usually think of the positive side as pushing electrons don't forget that the negative side is like a vacuum sucking them back the other way so it's acting just like me sucking on the soda straw so if I start sucking on this it's going to cause a drop in pressure on the side that I'm sucking from so this voltage is going to go to something lower and the harder I suck on it the lower that voltage is going to go so as I increase my current this voltage goes lower once again think of it like a vacuum cleaner where I have a suction side over here that's sucking the voltage down on this side so when we get current flowing through here we get a lower voltage and the more current we get the lower this voltage goes how low can it go well if I have no resistance between here and ground remember this is the positive side of the battery this is the negative side of the battery so if I have no resistance I'm going to have 10 volts on this side zero volts on that side so at that point I'm going to have my maximum current which is 10 milliamps at which point I will have 10 volts across this 1K resistor so once again we don't know what's going on here but something is causing the current to change when there's zero current 10 volts as I increase the current this voltage will go down and at my maximum current in this case of 10 milliamps I get zero volts okay so that's what's going on at that resistor so let's go back to our complete circuit and so now we know that as our current increases from the base to the emitter we get a bigger current through this so more current here means even more current here and if we take this as our output now what's going to happen to the voltage of this collector when there's no current let's make this a 10 volt battery so remember there's going to be 10 volts here zero volts here there's our ground so now when there's no current 10 volts when we get our maximum current which is going to be 10 milliamps we get zero volts so the greater current the lower the voltage so as our current goes up this voltage goes down and if we set this upright we can get it to the the voltage here is proportional to the voltage there okay so there's our voltage amplifier now let's look at it in operation so what do we have going into the base of this transistor we have a nice little sine wave because we said so that's going to positive 10 millivolts and to negative 10 millivolts and that's going to go into the transistor it's going to cause some current to flow from the base of the emitter and have you seen the problem yet how much voltage is it going to take between this base and the emitter before that transistor starts to really function minimum absolute minimum is going to be 0.5 volts if this is a silicon transistor so that's not even going to get this to start working and really we want to get this above about 0.7 volts remember the way the remember the way the diode curve works is that if we have our current on our vertical axis in our voltage on our horizontal axis we put about 0.5 volts there we get nothing nothing nothing nothing nothing no current until about 0.5 volts let me get a curve and it starts to straighten out at about 0.7 volts so we want to get the transistor operating in this area here we don't want it operated on this curve because that means that our collector voltage will not be proportional to our base voltage a tiny change in base voltage will not produce a proportional change in The Collector voltage we want basically to where if we double our base voltage we double our collector voltage so this means that it if we operate in this area well changes in voltage here don't make much change in current but a change in volts shared makes a big change in current and if we're operating in there it's going to be like well it's going to cause severe Distortion that we've got to get out of this region here so we really want to get this up to around 0.7 volts and operate in that area there so that's our problem at this point let's remove some of our clutter here and so what are we going to do here to increase our voltage on the base to about seven tenths of a volt well let's run a resistor down there from our power source and what we want to do is carefully select this resistor so we get just the right current that gets this up to about 0.7 volts and now changes from the microphone are going to cause changes over here but there's there's a slight problem here of course we're going to get current going this way but we're also going to get current going through the microphone especially if this is a dynamic microphone because what is it it's a coil of wire in a magnetic field so what we're going to get is an electric current going through the coil what happens when you put a current through a coil of wire that's in a magnetic field well it's going to cause a magnetic field around that coil that's going to interact with the permanent magnet that's going to cause that diaphragm to move so it's going to actually physically move the diaphragm in the microphone even a crystal microphone which would act more like a capacitor the voltage across it would cause it to distort so we don't want any voltage or current going through the microphone so what are we going to do how are we going to block that current and this is called the biosine current we are biasing this transistor how are we going to prevent that DC bias current from going through the microphone do we have a device that can block the direct current from going to the microphone but yet allow the alternating current from the microphone to go to the rest of the circuit and what device is that it's a capacitor what's the mantra for a capacitor it blocks DC but passes AC so let's put a capacitor here how big of a capacitor we want the capacitive reactants to be low enough that it acts like a short circuit to our alternating current I don't want to get too deep into the calculations of this that's beyond the scope of this lecture but we want this to have a large enough capacitance so that at the frequencies we're working at it looks like a short circuit remember the bigger the capacitor the smaller the capacitive reactants so a bigger capacitor looks more and more like a short circuit to our alternating current so now our DC is blocked but our alternating correct from our microphone can go right through okay so now we're getting to a workable circuit here so we choose this resistor and it's going to be highly dependent on the characteristics of the transistor so I can't really give you a formula of how to choose this tell you the truth I'll probably do a trial and error get a variable resistor and play with it till I get it just right I want to get a current through here to approximately 0.7 volts and what's going to happen is we have over here our sine wave going to plus 10 millivolts and minus 10 millivolts remember this acts like a short circuit to the alternating current so if this goes up by 10 millivolts this voltage here will also go up by 10 millivolts so if this was 0.7 volts when this reaches the peak this will be point seven one volts and likewise when this goes down to the minus Peak it's going to pull this voltage down by 10 millivolts so when we get to our minus 10 millivolts here it's going to pull this one down to 0.69 volts so as this goes up and down by 10 millivolts this also goes up and down by 10 millivolts so we have 20 millivolts Peak to Peak here and 20 millivolts Peak to Peak here the only difference is this is centered on zero volts and this is centered on 0.7 volts but the wave is going up and down by 20 millivolts either side so we get our changes in voltage at the base that we amplify to become changes in voltage at The Collector now let's clean this up so we don't have too much clutter up here so there's our basic circuit and so what's going to happen here as this voltage goes up the current through here increases as this voltage goes down the current from the base of the emitter decreases so as this current increases this current gets larger remember what happens as this current goes up this voltage goes down as this current goes down this current goes down and therefore this voltage goes up when we have more current it means that this battery is sucking harder and it's going to pull this voltage down so as my current increases here my voltage goes down here as my current decreases my voltage goes up so basically what's going to happen is I have my voltage at my base there's my zero volts and as that goes up current increases this current increases this voltage goes down as this goes down this voltage goes up look what's happening as this voltage goes up this voltage goes down as this voltage goes down this voltage goes up so we get a 180 degree phase reversal from the input to the output that's no big deal if it's a problem there are ways to deal with that but usually it's no problem but this is going to be a bigger voltage than this voltage because of the changes in current okay let's look at some of the problems we might have now remember that over here when we had zero current flowing through the transistor and we had zero current through the resistor what was our voltage at this point remember that when there's no current there's no voltage difference so if it's 10 volts here it's 10 volts here let's move this battery make some room for a little chart over here in fact let's get rid of the battery entirely foreign that gives us a little room to draw over here remember these just connect to the positive side of the battery the grounds connect to the negative side of the battery now remember what happens over here when there's no current flowing through this transistor we have no current flowing through the collector resistor and therefore what's the voltage on the output remember no current no voltage difference 10 volts here 10 volts here so that's our lower limit on our current we cannot allow our current to fall to zero otherwise when our wave comes out it's going to top out at 10 volts and we'll get what's called clipping it'll flatten off the top of the wave so we have to keep our wave below 10 volts likewise what happens when we get our maximum current the maximum current remember was 10 milliamps because at 10 milliamps we get 10 volts across our 1K resistor we can't get any more we have start with 10 we can't lose more than 10. so when this reaches 10 milliamps this is going to be zero volts if we put that down here that gives us a lower limit we cannot allow this current to attempt to exceed 10 milliamps because that will cause us to go down below zero volts and we'll flatten out the bottom so we have a practical limit where we need to keep our output voltage from going above 10 volts or attempting to go above 10 volts or attempting to go below zero volts so likewise we cannot allow this current to go to zero and we cannot allow it to hit 10 milliamps now to make sure this is symmetrical what do we want to do at this voltage when we have zero volts on the microphone let's say there's no input so I have a steady zero volts which means this is going to be assuming everything's biased right 0.7 volts what do we want our voltage to be here so that we can still amplify our wave so we're going up 10 millivolts down 10 millivolts we want to be able to go up and down an even amount plus 10 millivolts minus 10 millivolts we want this to be able to go up almost to 10 volts and down almost to zero volts so our middle voltage is going to be right there so when there's no signal coming in there's our voltage just sitting there somewhere in the middle and when the signal comes in what's going to happen it's going to go up and down so what voltage do we want that to be we want to set this up so that when there's no input this is going to be half of our power supply voltage so in this case with 10 volts that's going to be positive 5 volts so now what we need to do is Select these components the transistor the base Bias Resistor and The Collector resistor we want to choose those such that we get about 0.7 volts on the base and with our quiescent current meaning why is it meaning no input and it's just sitting there we want to set this at about plus 5 volts so it's a very careful dance between the hfe of the transistor how much amplification does it have the base bias and The Collector resistor to get this just right and I'm not doing any calculations on this because well if you really want to do that there are books that can show you the formulas and tell you the truth it's you're probably going to end up doing it by trial and error anyway but let's say we've got it working and we've picked just the right resistor and I picked one k here might end up being a slightly different resistor because another thing to look at is how much amplification we get and as this resistor increases what's going to happen to this voltage the point here is the larger I make this resistor the more voltage gain I get so once again it's a very careful balancing of what's the size of this resistor what's the size of this resistor what's the hfe of the transistor and to tell you the truth you're going to have to do this by trial and error because there's so much interdependency there but we can finally get an amplifier that's going to amplify our signal just the right amount and that's basics of a Class A small signal amplifier class a meaning that it amplifies the entire signal both the positive and negative sides so a quick review on what we did here we increased the base current to bring this voltage up to where the voltage coming from the microphone would push this up and down in a region where the transistor will operate fairly linearly this capacitor keeps DC currents from going to the microphone but allows AC currents to come through as this voltage goes up and down this voltage will go up and down the same amount so it's we have an offset here this is of this is centered on zero volts this is centered on 0.7 volts either side goes up and down 20 millivolts as this goes up we get an increase in current increasing current what happens our voltage goes down how much does it go down it depends on how much current and how big this resistor is the bigger this resistor is the more it goes down so we want to increase the voltage gain we increase the size of this resistor but once we get it all balanced out to do just what we want we can get a nice amplifier that takes our small signal here and gives us a much bigger signal here only difference is I really should draw this reversed because as this voltage goes up this voltage goes down once again it's centered on 5 volts so as this voltage goes up this voltage goes down 180 degree phase shift but if we carefully match all of these resistors to the hfe of this transistor we can get just the right voltage here so that when there's no signal this sits at 5 volts when the signal comes in it doesn't go above 10 volts doesn't go down below zero volts and We Have A Nice Class A small signal amplifier except it can't work the reason is that everything is so dependent on the hfe of this transistor which changes with temperature changes with our current changes with our base voltage changes with our collector voltage it's so interdependent on so many things that if it is so fiddly that you can't really get this to work and on top of that it has way too much gain this is going to be a huge resistor like a 1.5 Mega ohms or bigger to get this to be just the right amount of current to get just the right voltage here get just the right voltage there this one's not so critical but the real problem is going to be this transistor right here because even if I buy um transistors from the same batch from the same manufacturer they're all going to have slight differences in hfe which is going to throw my voltages all off and even the temperature is going to change that and because the hfe of the transistors dependent on other factors I'm going to get severe Distortion over here it just doesn't work so even though you might see this in textbooks well here's a way to make a Class A small signal amplifier what's the next thing they do is they say well here's a way to make it a little better one thing we can do to improve this amplifier is to move this resistor to hear and that gives us a little bit of negative feedback negative feedback meaning that as my base signal increases and my collector signal also increases some of that inter some of that energy from The Collector is fed back into the base remember when this goes up this goes down so as this voltage goes up some of that voltage comes through here and tends to drive the base voltage down so it gives us some negative feedback and that lowers the gain of the system and improves both the biozene and the lat and the Distortion remember the higher gain your amplifier the more Distortion you're going to get and also sort of because as our signal comes through the hfe is changing I have a different hfe here than I do here it distorts terribly this will reduce some of that Distortion but then again you look at books on small signal amplifiers and they say here's an improvement and then they move on because it doesn't really improve it that well so this style of small signal amplifier if you see it it's going to be if you see it at all it's going to be very rare because they're just too fiddly too dependent on the transistor tiny changes in temperature everything else can throw this all off so what do we do to make this a stable small signal amplifier first of all let's get this off of here put it back where we have it and what we're going to do is add a resistor here what does that do for us let's get some of this clutter out of here well what this does is it adds a significant amount of negative feedback to the system what does that do well it reduces the gain of the amplifier we don't want to amplify too much in one stage because we're going to get Distortion you if we're going to get less frequency range less bandwidth more Distortion so we want to do it in stages and have only so much gain per stage this is going to decrease the gain because it adds negative feedback how does that happen well remember that what happens here as this voltage goes up this voltage goes down so they're always going in opposite directions well what's happening we have current flowing through the transistor now we have the opposite thing happening here what happens when current encounters a resistance well if I pull out my soda straw again now we're going the opposite way no current flow same pressure on each side but when I blow into it I get a backup of pressure where the current goes in so where conventional current enters the resistor I get an increase in voltage so what we have are two counteracting voltages here my voltage is trying to go down here but this voltage is trying to go up and push that voltage up so that's negative feedback and that greatly reduces the gain of the amplifier so adding that emitter resistor reduces the gain of the system to where actually the gain is no longer dependent on the hfe of the transistor the gain is reduced to such a point that the gain of the whole system is now there's our collector resistor here's our emitter resistor our gain is now so our gain is now our collector resistor divided by our emitter resistor transistor has nothing to do with it so the biggest problem which is the changes in hfe here but temperature with current with voltage and different transistors is eliminated and now we have a nice stable circuit that's going to be easy to buy us and once we get it biased changes here are not going to change the voltages we can pull that transistor out put almost any other transistor in and everything's going to be the same because we're no longer dependent on the hfe of that transistor and another thing that we do to improve this is add a little more negative feedback by making this a voltage divider so what that does is as current flows into the base there's a sum current that would have gone into the base goes into this resistor here which means that we don't get as much current here so we get less ample there's less current to amplify so this adds a little bit more negative feedback even more important it stabilizes our voltage at this point because now it's more dependent on these resistors than it is on the characteristics of the base to emitter Junction of the transistor so this resistor makes our overall gain independent of the transistor and dependent on the two resistors and adding this resistor makes our base voltage more dependent on these two resistors than it is on the characteristics of the transistor so there is our basic finished small signal amplifier let's erase it and redraw it just a declutter it a little bit here and I'm going to eliminate the microphone just because I can so there is our finished small single amplifier once again we're not talking about specific numbers here that's beyond the scope of this lecture but there are plenty of design manuals that can help you choose these resistors to design your amplifier if you should need to design one now let's look at another Factor here is we're going to go into another stage here and we have DC voltages here we don't want to go that way so what can we use to block the DC voltages that buys this up and but allow the AC to go through of course we're going to put another capacitor over here on the output and that isolates the next stage from this stage there's one other Improvement we can make if we want to now the gain of the system is going to be this resistor divided by that resistor however what if we want a little more signal gain than the gain for our DC biasing so our gain for our biozene that sets up what's this resistor what's that resistor what's the hfe of the transistor getting everything balanced just right to get just the right voltage here and and get everything to work that's determined highly by these two resistors here but also the gain of the whole amplifier so if I have a gain of two let's say this is 1K and this is 500 ohms which is a good starting point this will have a gain of about two that means my amplifier is going to have a gain of about two what if I want considerably higher gain main reason we have less gain is because of adding this resistor is there any way to make our alternating current ignore this resistor where our direct current that sets up our biosine doesn't ignore it why don't we take another capacitor and bypass that resistor so to alternating current that's a short circuit so as far as our signal is concerned that's our circuit so our gain is dependent on this resistor and the hfe of the transistor so for our AC gain we have a higher gain but then for DC that resistor is visible to the direct current let's put the bypass capacitor back in So to alternating current this resistor does not exist so the gain is the hfe of the transistor combined with this resistor but for DC which cannot go through this capacitor this resistor does exist and the gain is this resistor divided by that resistor so we increase our AC gain or signal gain by putting this bypass capacitor here so that's the purpose of each one of these components what does each component do this component turns us into a voltage amplifier so as our current changes through our transistor we get voltage changes at The Collector this resistor biases our base it brings up our base voltage so that it's high enough to operate the transistor and then our alternating signal comes in goes through this capacitor that blocks our DC from going that way but allows the AC to come through that causes our base voltage to go up and down which causes our collector current to go up and down which causes our collector voltage to go up and down this resistor decreases our gain so all of the selections we make for resistors here are not dependent on the hfe of the transistor because that's just too sensitive it's just not going to work so this resistor makes the gain of the system equal to this resistor divided by that resistor making it a lot easier to set this up and get it stable and then finally this resistor adds a little more negative feedback and also makes it makes the voltage at the base of the transistor less dependent on the characteristics of the transistor and finally this capacitor blocks our DC from going to the next stage but allows the AC to go through so that's the basics of a small signal amplifier now that we've seen the typical small signal amplifier let's take a closer look now in reality as an Electronics technician you're much more likely to be troubleshooting one of these that someone else built than you are to build one yourself as a matter of fact if I'm going to build a small amplifier I'm probably going to use an op-amp it's just so much easier let's go ahead and build one of these and see how that can be done and let's see how to troubleshoot it if something goes wrong now there are some formulas we can use to design this down to the T but for a quick and dirty amplifier if we're not too worried about the input impedance we can design it pretty quickly without going through a lot of mathematics and since engineering is beyond the scope of this class we will do this with this little mathematics as possible so what I'm going to do is design a small signal amplifier with a gain of two let's get this out of the way for the time being and remember that the gain of the amplifier is going to be this resistor divided by that resistor so we have 1K here let's put 500 ohms here well I have some 510 ohm resistors so let's make that 510 ohms and for the power for experimental purposes I just used a 9 volt battery that was actually a little weak down to eight volts so let's go ahead and just make this Plus 8 volts nice even number so we've got those two resistors we have this resistor I'm going to use a 2N 22 22 and how are we going to choose these two resistors well it's fairly easy if you want to make a quick and dirty amplifier and trying to avoid the calculations we want this resistor to be fairly big compared to this one maybe at least about 10 times as big so let's start by using a 20K potentiometer in place of these two resistors and let's get this out of the way just for uncluttering the board here and we already know what that is so we get that out of the way so here's a simple amplifier we've eliminated our capacitors and we've replaced those with a 20K resistor all we have to do now is power this up and put a DC volt meter right there and start turning this until what remember that we want the quiescent voltage in other words the voltage with no signal input to be one half of our power supply voltage that way our input can swing it all the way up to eight volts or all the way down to zero volts without hitting the limits we cannot go above eight volts we can't go down below zero volts and if we hit those limits we get clipping distortions so we don't want that to be uh we don't want our sine wave flattened out at the top of the bottom so to make it symmetrical to give us the room to go either way as much as possible we want to make this voltage half of our maximum voltage so all we do is crank this potentiometer until we see four volts there easy peasy now let's take a look at what we've got if we look at the rest of the circuit with the volt meter we would see that we probably have about two volts here and expect about 2.7 volts there so it's 7 10 of a volt above the base so the base is seven tenths of a volt above the emitter and pretty much like we expect now what we'll want to do just to find out what these resistors are is simply measure this and when I did this I came up with let's put the resistors back there 13k and 7 K now let's put an input on here let's start by putting in a coupling capacitor because of what I had in my junk box and just off the top of my head I made that a two microfarad capacitor and I put an input of 2 volts Peak to Peak so so I picked it plus one volt and negative peak of -1 volt now we put a oscilloscope probe at this end and voila I ended up with four volts Peak to Peak with the output inverted compared to the input as we see in the inset up here so quick easy peasy made a small signal amplifier with a gain of two and we're ready to move on now we might want to put a bypass capacitor around this resistor and that will increase the gain and we want that to be a fairly big capacitor um several microfarads maybe 10 to 20 to 30 microfarads want it to be big enough that has a total short circuit to all the frequencies that you might be amplifying but remember that if we do this we're going to increase the gain but as you increase the gain you increase the Distortion and you decrease the bandwidth so it we might not want to increase the gain we've already got a gain of two that might be enough if I reduce the size of this resistor we'll increase our gains we can get a gain of three or a gain of four fairly easily without having to deal with the extra game we get from this capacitor remember if we put the capacitor here our gain is going to be proportional to our hfe and it might go high enough that it's really too much gain so we control our game with these two resistors and if we want lots and lots of gain we can put this bypass capacitor in here but I'm going to leave it out for now just leave this a nice gain of two amplifier and there we have it that'll work we have a working amplifier all we have to do is have a coupling capacitor to take it to the next stage to remember this capacitor blocks the DC bias voltages on this side from going to the next stage just as this capacitor blocks the DC voltages from going to the input and we have a ready to go small signal common emitter amplifier now let's redraw this circuit and take a look at some scenarios just in case things go wrong and with some jump cut magic There It Is I've eliminated the coupling capacitors and the resistances just to keep this uncluttered while we look at it and here are the voltages we expect at The Collector at the emitter and the base of the transistor now let's make some things go wrong I'll put in some other voltages here and we'll try to guess what went wrong with the circuit so let's say our amplifier we look at our input remember one volt Peak to Peak but our output is something lower so I have a loss so we pulled out our DC volt meter and start looking around and we see plus 6.5 volts on the emitter 7.3 volts on the base and 6.5 volts on The Collector so same voltage on The Collector and the emitter DC voltages and let's look at these voltages here 6.5 to 7.3 volts that means we have a difference of 0.8 volts from the base to the emitter of this transistor so transistor is obviously bad isn't it remember don't start zooming remember don't start assuming your transistor is bad because your voltages don't add up is it possible to have 0.8 volts from the base of the emitter of this transistor remember that's a forward bias diode can we get more than 0.7 volts of course we can we've demonstrated that over and over how would we get 0.8 volts here sounds like our base to emitter current is too high what could cause our base to emitter current to be too high our base voltage is set by this voltage divider is pretty stable but the current is basically the current through this resistor here and notice the high voltage here it looks like there's something wrong with this voltage divider this should be just about 2.7 volts should be about this should be about 2 volts something is going wrong here as long as these two resistors are okay this should be pretty close to 2.7 volts I think there's something wrong with our bias resistors here which one is most likely bad well we have a lot of extra current flowing in here so I would suspect this resistor right here and in fact this is what we would get if this resistor were a short circuit so if this resistor is shorted we can expect a higher than expected base to emitter voltage and notice all these voltages are higher over here we have a signal coming through the transistor but it is a loss so High bass to emitter voltage base voltage too high start looking at your bias resistors here and in this case this resistor is shorted here the input looks normal but our output looks half wave rectified what's happening is our voltage is somehow hitting its lower limit whatever that is that's going to depend on how much current we have and it'd be pretty close to zero volts but uh we're obviously hitting the bottom and look at this this voltage is too low it should be four volts it's down to three volts this voltage is just a little bit High which means we have a higher than expected current flowing through here so what could cause that and also look over here our voltage is a little high here which looks like once again we have more current than expected flowing into the base it looks like and what's that going to do the more current we have going into the base it's going to increase our collector current what happens to our collector voltage as we increase this current that collector voltage will go down so this voltage has gone down it's gone down to such so now our classic voltage is too low and when this hits the peak we're hitting the bottom over here so so now our DC voltage here is too low so we are able to go up far enough but we can't go down because we hit that lowest voltage so once again I'm going to be suspecting something is going on over here because this voltage still should be pretty close to 2.7 volts no matter what happens anywhere else so the last time this resistor was shorted this resistor is open and so it's not quite as dramatic why is it not as dramatic so we still have a voltage divider here our current flows through this resistor then through the base to emitter path of the transistor and this resistor even though it's only 510 ohms that resistance is multiplied by the hfe of the transistor so it's a It's Kind of a Funny thing if you look here you see 500 ohms or 510 ohms but if you look in the base because the transistor acts like a magnifier this looks like it's going to be much larger so this resistor becomes a fairly big resistor compared to this one and it pushes our voltage at the point of the voltage divider up so what's happened here is we've lost this resistor it's an open circuit so once again if this voltage isn't right over here suspect there's a problem with these two resistors okay in this scenario our input looks normal but our output is simply 8 volts DC we look at our DC meter 8 volts put our oscilloscope on there and we just see straight DC at 8 volts so there is no output getting out of the transistor our input voltage looks like about what it should be so I suspect that these are okay because that should be pretty close to 2.7 volts depending on what's going on it might get just a little higher you'll see that a little later but that looks good so we're good over here uh what could cause this to be eight volts this voltage is the same as that voltage it could be just a coincidence but it looks like this resistor could be shorted or is it what if this resistor is open well notice we have eight volts here and eight volts here yeah I wouldn't expect eight volts here if this resistor was shorted because we would have a large current flowing through the transistor and there would be some voltage across it so we wouldn't have the same voltage here as here remember if we have current flowing through a resistance which a good resistor would be we will have some voltage difference so if we have eight volts here with current flowing through the transistor I would expect something lower than eight volts here so I think I see a situation where there's no current flowing from here to here because we have no voltage drop we have resistance and this has resistance too a transistor does have resistance it tends to change but it does have resistance but we have the same voltage everywhere I suspect that this resistor is open also what if we just have a broken connection broken leap broken solder joint or whatever where we have no connection to that resistor so this scenario with the voltage the same all the way across the back it's a good indication that we either have a open resistor or a broken connection from the transistor to the resistor so now we have almost the same situation the only difference is that the emitter of the transistor is at zero volts so what could cause that well the first thing you might think of well if this resistor were a short circuit we would have zero volts on the emitter but that would not cause this voltage to go up to Plus 8 volts in fact if this was shorted it would increase the gain of the amplifier so these voltages might be messed up this voltage will probably drop to a lower voltage but I certainly wouldn't go up and I certainly wouldn't be a DC output I'd have a greater output so this resistor is probably not shorted what could be going on well if the transistor were an open circuit if the transistor were not here at all yeah no current the output would be up at our positive 8 volts no current through here this would be zero volts and of course this would be sitting at pretty close to 2.7 volts actually it might be a little different so if you see a slightly different voltage here maybe just a little bit higher not a lot higher I would strongly suspect that the transistor was a short circuit but let's say this isn't higher it's exactly where we expected especially if we've looked at the circuit and it's the exact voltage we saw when the circuit was working now I have a new suspicion and that is that I simply have no connection to the base of the transistor so there's no connection coming in here so there's no DC bias voltage no current going from the base to the emitter no current running through this resistor zero volts likewise there's no current flowing through here no current flowing through the collector resistor that would be eight volts so this could be a bad transistor an open transistor very likely the only difference is if the transistor is open this voltage might be slightly higher than you expect but it might not be so this is a case where if you see that the collector voltage is all the way up and the emitter voltage is all the way down and the base voltage looks normal could be an open transistor or a bad connection to the base of the transistor okay here's another interesting situation and I think this one might be the most obvious of them all I've got 2.7 volts here like I expect might be just a tad higher which might be a indication of what's going on but we have the same voltage here in the same voltage here we have an input of one volt Peak to Peak and an output of one volt Peak to Peak looks to me like this transistor is a short circuit or there's something shorted around it of course watch for other possibilities shorting out the transistor but it looks like we've got a short circuit in here either a bad transistor or something touching it and shorting it out just another thing to say because I've seen it and it looks weird you need to watch out for it let's say that these two resistors are whacked somehow and we don't have the right bias voltage on the base such that this voltage drops down and starts to clip on the bottom as we showed so we have a normal looking input but our output looks something more like well let's draw our sine wave here and clip off the bottom this is sort of what we expect when we get clipping but what you might get is a little bit of a bounce up when it hits bottom and it tries to go below zero volts or below its lowest possible voltage because of the characteristic of a transistor and the saturation voltage when you start going into saturation the voltage might start to rise and here's what we have here so it starts to go into saturation actually tries to go deeper into saturation and the voltage actually comes up and so instead of seeing it flat on the bottom you see these little humps so that's another symptom of out of whack bias resistors here and the voltage here has gone down and now we are clipping on the bottom going into saturation if this gets bad enough we can get something that looks sort of like kind of a weird wave shape we have a long a long pole short pulse long pulse short pulse so once again this is happening because it's trying to go into saturation here and as it goes deeper into saturation the saturation voltage goes higher and gives us these little bumps here and in fact if you go far enough this will eventually get to where the output is the same as the input and we no longer have the inversion it's no longer 180 degrees out of phase and we end up with the output being the same as the input if we get these out of whack far enough just pointing that out that if you see something really weird like this it's trying to go into saturation so it's trying to flatten the bottoms but as it goes deeper into saturation that voltage actually comes up and so we don't have the flat bottoms there so just something to watch out for so it doesn't throw you for a loop saying what the heck's going on here well transistors trying to bottom out clip on the bottom and go into saturation but that voltage tends to bounce back up because of the characteristic of a transistor you found this video useful and informative please give me a thumbs up down below it really helps the channel and subscribe because that not only informs you when I put new videos up but it really helps the channel also and a big thank 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Channel: Vocademy - Electronics Technology

Views: 295,044

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Id: WgOKPF8LkA8

Length: 57min 52sec (3472 seconds)

Published: Mon May 29 2023