 Hello, and welcome to video session on Amplitude Modulation Generation, that is AM generation techniques. Myself, S. N. Chamath Gowder working as assistant professor in electronics and communication engineering, Walsh and the Institute of Technology, solar. This is the learning outcome. So, at the end of this session students will be able to describe the AM generation techniques, that is different types of AM generation techniques. Before we proceed with the this video session, let us recall what are the different types of modulation techniques and let us recall the definition of the amplitude modulation. Pause this video for some time and recall and write down your answers. I hope all of you have written your answers here. Now, there are two types in the generation of AM here, that is first one low level modulation technique and second one high level modulation technique. In the low level modulation techniques, generation of amplitude modulation takes place in the initial stage of amplification. Similarly, there are there is a second type that is high level modulation technique. In this technique, the generation of AM takes place in the final stage of amplification. As I said, there are two types of modulation techniques, that is low level modulation technique and high level modulation technique. First one that is low level modulator. Here in this circuit, you can see there is a field effect transistor and there is a feedback resistor, there is an operational amplifier, carrier signal generator, modulating signal input. Now, here you can see the connection that is carrier oscillator is connected to the non-inverting terminal of the operational amplifier, whereas the modulating signal is connected to the base of this field effect transistor. Now, by this connection we can understand that here in this circuit, the field effect transistor is used as a variable resistor, RF is used as a feedback resistor for this operational amplifier and this operational amplifier is used as a non-inverting amplifier for the carrier signal and the gain of the amplifier is 1 plus RF by R 1. Now, in this connection we can see that as I said, FET is being used as a variable resistor. That means, the modulating signal is controlling the resistance value of this field effect transistor. That means, when the modulating signal is increasing, that is from 0 to positive peak, the resistance of this field effect transistor decreases. When it is going from 0 to negative peak, the resistance of this field effect transistor decreases. Now, as I said, carrier oscillator is connected to the non-inverting terminal of the operational amplifier here. So, here to understand the working principle of this circuit, let us take two cases that is in the absence of the modulating signal and in the presence of the modulating signal. In the absence in the sense that is 0 modulating signal. During 0 modulating input signal, resistance of the field effect transistor remains constant, which in turn the gain of the amplifier remains constant, which in turn the air output remains constant. Amplitude of the air output remains constant. Now, in the second scenario that is after applying the modulating signal, there are two cycles. First positive cycle, second negative cycle. When we are applying the positive cycle, the field effect resistance value of the field effect transistor decreases. As the signal amplitude of the modulating signal increases, the resistance of the FET decrease and increases the which in turn increases the gain of the amplifier, which in turn increases the amplitude of the AM output. In the second cycle that is negative cycle, that is when the modulating signal is going from 0 to negative peak, the resistance value of the field effect transistor increases, which in turn decreases the gain of the amplifier, which in turn decreases the amplitude of the AM output. Now, here we can see there are two blocks in a high level modulation techniques. As I said there are two types in the AM generation techniques that is first one low level modulation technique and second one is high level modulation technique. In this technique there are two blocks as is as you can see first one that is a current pulse generator, which we also call it as a classy amplifier and second block is the tuned circuit. Now, input to the current pulse generator are the RF signal and the modulating signals. These are the two inputs given to the current pulse generator, which generates a series of current pulses which are fed to the tuned circuit. Now, tuned circuit converts each current pulses into a complete sine wave proportional in amplitude of current pulses. Here you can see the waveform here. Now, the first waveform is of modulating signal. Second waveform is a series of current pulses. Here you can clearly see that the amplitude of the current pulses is equal to the amplitude of the modulating signal. As the modulating signal is increasing the amplitude of the current pulses is also increasing. As the modulating signal is decreasing towards the negative peak the current amplitude of the current pulses is also decreasing towards the negative peak. Again from this point the modulating signal is increasing the current pulses amplitude of the current pulses is also increasing. Now, as I said first block generates the current pulses second blocks converts that those current pulses into a complete sine wave. Now, here we will see the circuit of high level modulation technique here. Here we call this circuit as a collector modulator class C amplifier ok. Now, here you can see the connections of this circuit here. Modulating signal is fed to the high power audio amplifier and the output of the high power audio amplifier is connected to the primary winding of the modulating transformer. Now, secondary of the modulating transformer is connected in series with the plus VCC and is connected to the collector of this transistor here. Now, carrier input is given to the base of the transistor here ok. Now, VCC is connected to the collector of this transistor and ground is connected to the emitter of the transistor. Now, you can see that VCC is connected in series with the secondary winding of the modulating transformer which in turn passes through the tuned circuit. Now, you can see this box is a tuned circuit. In the earlier block diagram we have seen two blocks is current pulse generator and the tuned circuit. Now, that tuned circuit consisting of a inductor and the capacitor this we call it as a tuned circuit and the secondary of this winding is connected to the antenna for the transmission purpose. Now, this setup we can understand that the transistor is used as a switch here. Now, the operation of this switch is controlled by the carrier inputs. We will see it in the waveform. Here you can see the resulting current currents whenever the switch is turned on the current passes through the transistor whenever it is off the current does not flow through the transistor here. Now, here in this circuit carrier input is controlling the switching operation of transistor collector of this transistor is connected to the plus VCC which is connected through the modulating transformer through the tuned circuit connected to the collector terminal. Now, here also to understand the working of this circuit we will consider two cases. First case in the absence of the modulating signal second case in the presence of the modulating signal. During the absence of the modulating signal what happens? There is no current passing through the primary of this modulating transformer winding. Now, due to which the voltage generation in the secondary winding of the modulating transformer is 0. That means, plus VCC is directly connected to the collector of this through the through the tuned circuit. Now, when the modulating signal is 0 plus VCC amplitude of value of the plus VCC remains constant that makes whenever the switch is on the current generated at this end is constant. As we can see in the waveform here the first signal is the modulating signal, second signal is the current pulses, third signal is the AM output. During the initial period of this waveform we can see the absence of the modulating signal which results in the constant amplitude of the current pulses which results AM output. Now, in the second case we will consider by applying the modulating signal there are two cycles positive cycle as well as the negative cycle. Now, as we can see the plus VCC is connected in series with the modulating transformer. So, whatever voltage generated across this secondary winding of the modulating transformer is being added to the plus VCC and subtracted from the plus VCC transistor here. Now, in this waveform we can say after some time period modulating signal is being applied to the circuit initially there is a negative pulse negative cycle. So, 0 to negative peak negative peak to return to 0 and 0 to positive peak and return to 0. During this period what happens the during the negative cycle of the modulating signal the voltage is being subtracted from the plus VCC that is why the amplitude of the currents reducing. Once it reaches a peak when the modulating signal is returning to the 0 the amplitude of the current pulses is also increasing. Similarly, at the output end that is output of the tuned circuit you can see there is a decrease in the amplitude of the sinusoidal wave ok. Now, after in the during the positive cycle that is when the modulating signal is going from 0 to positive peak the amplitude of the current pulses is also increasing similarly the amplitude of the A M output is also ok. Now, again from positive peak it is returning to 0 similarly the amplitude of the current pulses is also returning from peak to 0 similarly the sinusoidal wave is also decreasing.