1 00:00:00,560 --> 00:00:02,990 In this lesson, we will discuss amplifiers. 2 00:00:02,990 --> 00:00:07,919 As I mentioned in the introduction to the course, this course is about amplifiers. 3 00:00:07,919 --> 00:00:13,090 So, what we will do in this lesson is to see the characteristics that an amplifying device 4 00:00:13,090 --> 00:00:29,140 must have. 5 00:00:29,140 --> 00:00:33,820 Now in this lesson, we will not be looking at any amplifier device in particular, but 6 00:00:33,820 --> 00:00:38,490 only about general characteristic that amplifier devices must have. 7 00:00:38,490 --> 00:00:45,730 And amplifier has an input and an output; so we will assume that it is a two port network. 8 00:00:45,730 --> 00:01:11,990 So to one of the ports, we will feed the input and from another port, we take the output. 9 00:01:11,990 --> 00:01:20,070 And let me consider just for simplicity, a three terminal two port network like this. 10 00:01:20,070 --> 00:01:25,679 You would have seen representations of such a two ports in your basic electrical circuit 11 00:01:25,679 --> 00:01:35,340 class and this is port 1, port 2 and this is the common terminal. 12 00:01:35,340 --> 00:01:52,479 Now let say that I apply a signal, let say it is call v 1 to port 1, and I will also 13 00:01:52,479 --> 00:02:01,189 write down the definition of port voltages and currents v 1 and i 1; and v 2 and i 2 14 00:02:01,189 --> 00:02:02,939 in the second port. 15 00:02:02,939 --> 00:02:11,870 And let us say that we take the output from the second port; again I shown a voltage source 16 00:02:11,870 --> 00:02:13,900 here, but it does not have to be like this. 17 00:02:13,900 --> 00:02:17,879 It could have a series resistance, it could be a current source and so on, because all 18 00:02:17,879 --> 00:02:23,709 the arguments we will make now will be general and in terms of only the port variable. 19 00:02:23,709 --> 00:02:31,069 It does not matter what is actually connected to this side or this side. 20 00:02:31,069 --> 00:02:38,500 Now every network that we take is passive that is it does not have a source of energy 21 00:02:38,500 --> 00:02:43,099 inside, it can only dissipate power. 22 00:02:43,099 --> 00:02:52,540 So what does it mean this two port network N, this is passive. 23 00:02:52,540 --> 00:03:06,120 So it means that v 1 i 1 plus v 2 i 2 is going to be more than zero, or maybe it is equal 24 00:03:06,120 --> 00:03:09,400 to zero in which case it is not dissipating any power. 25 00:03:09,400 --> 00:03:12,580 So it cannot generate any power; it does not have any source of power inside. 26 00:03:12,580 --> 00:03:21,780 Now what does it mean for an amplifier; first of all, we wanted amplification, what it means 27 00:03:21,780 --> 00:03:35,950 is that the power that we put into the amplifier P in must become bigger when it comes out 28 00:03:35,950 --> 00:03:49,530 that is for an amplifier, we would want P out – the power coming out to be more than 29 00:03:49,530 --> 00:03:51,939 power going in. 30 00:03:51,939 --> 00:03:57,129 Otherwise there is no amplification at all, so this is what we would want to have. 31 00:03:57,129 --> 00:04:05,890 Now let see if our circuit can amplify power, because just now I said that all our networks 32 00:04:05,890 --> 00:04:14,290 are passive so that means that v 1 i 1 plus v 2 i 2 will be more than zero. 33 00:04:14,290 --> 00:04:25,880 So now what is the input power - P in, P in is v 1 times i 1 so you see that the current 34 00:04:25,880 --> 00:04:30,770 flowing out of the positive terminal of v 1 is i 1, so the power supplied by this voltage 35 00:04:30,770 --> 00:04:36,500 source which is going into the port one of the network that is v 1 i 1. 36 00:04:36,500 --> 00:04:41,130 And what is the power delivered into the load that is the voltage across the load which 37 00:04:41,130 --> 00:04:47,000 is v 2 times the current flowing into the load according to the passive sign convention 38 00:04:47,000 --> 00:04:48,950 which is minus i 2. 39 00:04:48,950 --> 00:04:55,720 So the output power is minus v 2 i 2. 40 00:04:55,720 --> 00:05:03,650 Remember we choose the signs of v 2 and i 2 according to the passive sign convention. 41 00:05:03,650 --> 00:05:08,479 So the power that is coming out of the two port is minus v 2 i 2. 42 00:05:08,479 --> 00:05:17,250 Now, if you just rearrange this by taking v 2 i 2 to the other side, you will easily 43 00:05:17,250 --> 00:05:22,890 see that minus v 2 i 2 is less than or equal v 1 i 1. 44 00:05:22,890 --> 00:05:29,290 In other words, P out is less than or equal to P in. 45 00:05:29,290 --> 00:05:35,580 Now this is on very surprising, this looks in fact like roundabout way of saying that 46 00:05:35,580 --> 00:05:42,699 the device is dissipating power, because we already said that the device is passive so 47 00:05:42,699 --> 00:05:47,780 that means it is dissipating power and remember there is no other source of power here. 48 00:05:47,780 --> 00:05:54,569 The input v 1 is the only source of power, so whatever is coming out of the two port 49 00:05:54,569 --> 00:06:02,080 is less than or equal to utmost equal to the power delivered by the source v 1. 50 00:06:02,080 --> 00:06:06,930 Now the reason I even showed this calculations is we will elaborate on this later, we have 51 00:06:06,930 --> 00:06:13,180 to make a some of the arrangements to amplify a power, so to get use to that I have shown 52 00:06:13,180 --> 00:06:18,490 you these calculations, but the calculations basically show what common sense would have 53 00:06:18,490 --> 00:06:23,030 told you there is only one source of power here which is v 1 and it is a passive network. 54 00:06:23,030 --> 00:06:27,669 So it can only dissipate power, so what can come out can only be less than what is going 55 00:06:27,669 --> 00:06:30,789 in through a single source of power.