 Hello, welcome to the video session on AM receivers, myself SN Chamath Gauder working as assistant professor in electronics and communication engineering, Walsh and the Institute of Technology, so on. In this video session we are going to study about the envelope detector and TRF receivers. This is the learning outcome at the end of this session students will be able to describe the envelope detector and TRF receiver. Pause the video for some time and recall which are the types of AM generation techniques. I hope you have recalled the types of AM generation techniques here. Now, let us proceed with the video session, this circuit is an diode detector, it is also called as envelope detector, it is also called as AM dot demodulator. This circuit consists of an input transformer, diode and RC filter circuit here, as you can see in the diagram. Now to understand the working principle of this circuit, let us divide this into two parts. So, first part consisting of a input AM signal, input transformer and a diode, in the second part let us consider this RC filter circuit here ok. So, AM signal is given to the primary winding of the input transformer and due to the variation in the current, a voltage is been generated in the secondary transformer, secondary winding of the input transformer and this voltage is given to the diode here ok. So, as we have studied the half wave rectifier circuit, in the half wave rectifier circuit, the same circuit is used for the half wave rectifier circuit. Here the diode is connected in such a way that the diode conducts current during the positive half cycle and do not conduct current in the reverse bias condition ok. So, in this circuit also the diode conducts during the forward bias and do not conduct during the reverse bias ok. So, here as I said we will divide the circuit into two parts ok. First part is a rectifier part and second part is the envelope detector part ok. Envelope detector in the sense it is a RC filter circuit ok. So, AM wave is given as an input to the rectifier circuit and half cycle we will get at the output end and this signal is given as an input to the envelope detector and we will get the envelope of the input signal here ok. So, in the signal we can see that there are two envelopes, one is the upper and second one is the lower ok. Here it consisting of a high frequency carrier signal here ok. Now, as I said as I said in the in this circuit the diode conducts during the forward bias that is during positive half cycle of the AM signal and diode is reverse bias during the negative half cycle during which the diode will not conduct ok. So, here in this diagram we can see the carrier signal which is having both positive cycle as well as the negative cycle ok. So, when the carrier signal is positive going during that time diode conducts and it is passed through the diode and this waveform comes at the output terminal of the rectifier circuit ok. So, during the negative half cycle the diode is reverse biased hence that negative cycle does not appear on the waveform at the output terminal of the rectifier circuit here. So, here only positive cycles are passed through the rectifier circuit and it appears across the output terminal ok. Once we get the half waveform at the output terminal of the rectifier circuit this signal is given to as a input to the envelope detector. Envelope detector detects the envelope present on the high frequency signal we will see in detail in the next slide. Here you can see there are current pulses which are only positive cycle here negative cycle is not present in this waveform ok. So, now in this circuit as I said the envelope detector consists of a capacitor and a resistor ok. So, here when the positive cycle is applied across the diode conducts and that current appears across this capacitor and resistor ok. Now when the voltage appears across the capacitor the capacitor gets charged to the peak value of that current pulse ok as well as that voltage also appears across the resistor here ok. As I said in the reverse bias diode do not conduct ok. Now when the level input level of the signal goes below the capacitor level the diode is reverse bias we will see that in this waveform here ok. In the first current pulse you can see when the current pulse is positive diode conducts and the capacitor starts charging here up to the peak value. Once the input signal decreases below the level of capacitor the capacitor starts discharging through the load resistance ok. This process continues till the next positive cycle. Again when the input signal increases the more than the capacitor level again capacitor starts charging up to the peak value. When the inputs level decreases below the capacitor value it starts discharging here. In this way this process continues till the input signal is present and the RC filter circuit goes on detecting the envelope of the AM wave here ok. This is the entire waveforms here as you say as you can see the first waveform is the AM output AM input waveform and second one is the output of the diode circuit and this one is the output of the RC filter circuit. Second receiver is the TRF receiver this is the block diagram of the tuned radio frequency receiver. Here you can see there are multiple stages first this is the antenna, first stage RF amplifier, second stage RF amplifier, detector, audio amplifier, power amplifier and this is the loudspeaker. Now as we can see between the antenna and the detector there are n number of RF amplifiers. For our convenience we have used only two stage RF amplifiers here. Now here below that you can see there are dashed lines which are connected first RF amplifier, second RF amplifier and detector circuit to this ganged tuning knob ok. Now these three blocks are connected mechanically to a ganged knob here which is used to tune these blocks to a particular or selected frequency ok. Once antenna receives the electromagnetic signals which are present in the atmosphere and then gives those signals to the first stage RF amplifier. The ganged we use this ganged tune to select a desired frequency and this RF amplifier amplifies that desired frequency and rejects all the remaining signals here. Once it is amplified then it will be fed to the second stage amplifier. Second stage amplifier again it is used to amplify the desired signal and reject the remaining signals. Once the desired signal is amplified to a desired level then it will be fed to the detector. As we have seen in the earlier in this video session regarding the detector detector detects the envelope present on the AM wave. So this block or this stage detects the envelope present in the AM wave and the it recovers the original signal which is present in the AM wave and it will be fed to the audio amplifier. Audio amplifier increases the power level of the signal again it will be fed to the power amplifier. This power amplifier also increases the power level of the recovered signal and then it will be fed to the loudspeaker. Now this TRF receiver has some problems like tracking of tune circuit instability variable bandwidth. What is this tracking of tune circuit? As I said ganged tuning knob is used to vary the frequency connected to the variable capacitor. It is expected that all the capacitor value used in the first stage RF amplifier, second stage RF amplifier and detector are equal but practically it is not the case that is why this problem occurs tracking of the tune circuit. Second thing is the instability instability in the sense it is the case of selectivity and sensitivity. Selectivity in the sense it is the capability of the signal to select the desired signal and sensitivity in the sense to capture the weakest signal present in the atmosphere. Now third one is the variable bandwidth. Variable bandwidth increases the bandwidth of this TRF receiver increases has the frequency increases. So these are the three basic problems which are present in the receivers. So to overcome these three problems we will study the super heterodyne receiver in the next video session and this is the reference thank you.