 Hello everyone. Welcome to this session. Myself Deepali Vadkar, working as assistant professor at W. T. Solapur. In this session, we will study Perl's amplitude modulation PAM. At the end of this video lecture, student will be able to describe working of PAM modulator using emitter follower circuit. Student will be able to explain working of PAMD modulator circuit. The Perl's amplitude modulation, it is the type of analog modulation in which the amplitude of train of pulses changes according to the amplitude of modulating signal. Here two types of PAM signal. One is a double polarity signal in which positive as well as negative levels are present. And another is a single polarity or we can say it has a unipolar PAM in which one sufficient DC biasing is inverted with the signal so that the signal or train of pulses having the only positive levels. We will see the block diagram of PAM modulator. In the PAM modulator, the continuous time signal is given to the multiplier. The another input to the multiplier that is the train of pulses. The amplitude and frequency of train of pulses is adjusted according to the sampling theorem and at the output of multiplier we get a PAM signal. Now you will see in detail the PAM modulator circuit using emitter follower. So this is the circuit diagram for emitter follower PAM modulator circuit. In this circuit the transistor circuit is used. Here the input is given to the base of the transistor. The input signal is a continuous time signal. We can say this signal as a modulating signal. The another input to the base of transistor that is nothing but train of pulses which is nothing but clock signal. Here the clock frequency is nothing but sampling frequency and the amplitude of clock signal is adjusted such that the higher level is at 0 volt and its lower level is at some negative voltage which is sufficient to bring this transistor in a cutoff region. The output of this circuit is taken across the emitter. Next during high clock pulse to the base when the high clock pulse is given to the base of this transistor then at that time the positive voltage is get applied to the base of this transistor. So when we apply the positive voltage to the base of any transistor then at that time this transistor works in a saturation region. So due to this this transistor acts as on switch and the input current flows through the output through the emitter. So here the input current is present across the output for that time period. So we can say that here the output follows the input. So here the circuit behaves like a emitter follower circuit where the output follows the input. When the input signal is low clock pulse at that time the negative voltage is get applied to the base of the transistor and due to this this transistor works here in a cutoff region. And when the transistor works in a cutoff region at that time the transistor act as a off switch. So the output here which is present that output is a zero. So in this way when we give any continuous time signal input to the base and another input that is nothing but train of clock pulse then we get across the emitter of the transistor that is nothing but PAM signal. Now next pulse amplitude modulation demodulator. In the demodulation it is the process of extracting modulating signal from a modulated signal. So in the PAM demodulator the PAM signal is given input and across the output we get a continuous wave signal that is the original signal. So this is the circuit for PAM demodulator which in which the envelope detector followed by the low pass filter circuit. So here two basic circuit one is the envelope detector and another is a low pass filter. Let us see the working of envelope detector circuit. So this envelope detector circuit consist diode with RC circuit. Here the input PAM signal is given to the diode and we get the output across this resistor. So in this circuit when we give the input high pulse to the diode then at that time this diode works in a forward biasing mode means it act as a on switch that is close switch. So here the capacitor across switch whatever the voltage that voltage is less than the input voltage and due to this the capacitor start charging. This capacitor charge through the load resistance RL here. Now next when we give the input signal to the diode which is the low pulse then at that time this diode works in a reverse biasing mode. So it act as an open switch that is the off. So at this position the voltage across capacitor is greater than the input voltage and due to this the capacitor start discharging. So this capacitor will discharge through this load resistance R. So in this way we get the charging and discharging charging and discharging voltage across this load resistance R. So this is the output of envelope detector. Now next circuit is a low pass filter. In the low pass filter here we are using the op-amp circuit which is this circuit pass only lower frequency signal below the cutoff frequency for which it is designed and atonate the higher frequency signal. And the whatever the op-amp circuit is used here this op-amp circuit amplifies this signal and at the output of the op-amp circuit or low pass filter we get this type of signal means again original signal. So in the PMD modulator first input is given the pulse amplitude modulated signal then across the envelope detector we get the charging discharging means the continuous signal with some ripples and across the low pass filter we get the original signal that is the continuous time signal. Next the transmission bandwidth of any signal generally the transmission bandwidth is refers to the range of frequency which is used to transmit the signal or the transmission bandwidth we can say it is the difference between the higher frequency and lower frequency. Now we have studied the PAM signal we have studied what is the transmission bandwidth then the question is what will be the transmission bandwidth of PAM. Pause the video for a while and think for the answer first let us see the frequency spectrum of PAM. In the PAM first input is a continuous time signal its frequency spectrum is this in this frequency spectrum the maximum frequency is FM then the sampling function its frequency spectrum here this sampling function it is it is sampled at the frequency sampling frequency FS and third is a PAM its combination of these two spectrum that is the PAM spectrum. In the PAM spectrum this type of spectrum is for a rectangular pulse. So the rectangular pulse spectrum continuous time spectrum and sampling function its combination that is nothing but frequency spectrum of PAM and this is like this. Now in the PAM if tau is a time duration then in that case this tau or time duration of this PAM is less than very less as compared to the sampling time period. But the sampling frequency is always greater than equal to twice FM. So this is according to the sampling theorem. And this sampling frequency and the sampling period its relation is FS is equal to 1 by TS. So we can say that the 1 by TS is greater than equal to twice of FM. So we can write this equation as TS is less than equal to 1 by 2 FM. So by using both these equation we can say that the tau is less than TS and TS is less than equal to 1 by 2 FM. In the PAM if the time duration for on and off time pulses is same then in that case the maximum frequency of PAM fmax is less than equal to 1 by tau plus tau that is for time duration for on pulse plus time duration for off pulse. So it becomes equal to 1 by 2 tau. Then the bandwidth required is always greater than equal to maximum frequency. Then can say that the bandwidth required is always greater than equal to maximum frequency. But the maximum frequency is equal to 1 by 2 tau. Okay, hence the bandwidth always greater than equal to 1 by 2 tau. We know that the tau is less than 1 by 2 FM. So we can say that the bandwidth is greater than 1 by 2 tau and it is again greater than FM. So finally we can say that the bandwidth is greater than FM. So the transmission bandwidth for PAM is greater than FM. FM is nothing but maximum frequency of modulating signal. These are the references. Thank you.