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ivers, AM Transmitters and Recelv, , , , , , , , , , Syllabus :, Generation of AM, Block diagram of AM superheterodyne 1, AM signal : Diode detector and practical diode detector, Automatic gain, , lation of, aceiver, its working with wavetorng, Dernodulation, , control and its type. _., , as, , 4.1 Generation of AM (Modulating Circults) :, , - In this section let us see the different methods to generate A.M., - The generating circuits for AM wave are called as amplitude modulator circuits., - The modulator circuits are classified into two categories :, 1. Low level modulation :, - The generation of AM wave takes place at a low power level., - The generated AM signal is then amplified using a chain of linear amplifiers such as class A, B or AB amplifiers., - The linear amplifiers are required in order to avoid any waveform distortion. The efficiency of low level modulator is low, linear amplifiers are not very efficient., 2. High level modulation :, , - In this method, the generation of AM wave takes place at high power levels., - The carrier and the modulating signal both are amplified first to an adequate power level and the modulation takes place in the, last RF amplifier stage of the transmitter., , - — Highly efficient class C amplifiers are used in high level modulation. Hence the efficiency of high level modulators is higher, than that of low level modulation. , 4.1.1 Low Level Modulator :, , A simple low level modulator circuit using OP-AMP and FET is as shown in Fig. 4.1.1(a)., , , , , , , , , , , R,, WN, FET acts asa, variable resistance, Modulating Cy *— AM output, signal — }, input OP-AMP as. noninverting, amplifier A, Carter Gin A=t4 ot, oscillator \, -Vv, (FET blas), , (p-73) Fig. 4.1.1(a) : Low level modulator

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ers, , , , M Transmitters and Receivi, , operation 2, , 4.1.2 Single Transistor Emitter Modulator :, , OP-AMP is connected as a non-invertit i i *, The erting amplifier for the carrier sigrial. The gain of this amplifier is A= 1 +R. The feedback, , i is of fixed val i i, resistor Ry ne but R, is the resistance offered by the N channel JFET. This resistance R, is therefore variable., A negative dc bias keeps the gate source junction of the JFET reverse biased. A modulating signal is applied to the gate through the, coupling capacitor C,. The resistance of JFET i.e. R, will change in accordance with the modulating signal amplitude. This will, change the gain of the amplifier in accordance with the modulating signal., ‘Therefore the carrier signal applied at the non-inverting terminal will get amplified more for the positive going modulating signal., Gain will increase as R, decreases when modulating signal is positive. On the other hand less amplification is provided to the carrier, for negative going modulating signal., , Thus the AM wave is produced at the output of the OP-AMP amplifier., , Fig. 4.1.1(b) shows a circuit which is basically a small signal, class A amplifier. It is called as an emitter modulator circuit and, used as amplitude modulator. s, , c,, The circuit has two inputs namely the RF carrier and the Carrier +, modulating signal., , Vo, , When the modulating signal is absent, only the carrier is, applied, the circuit works only as a class A amplifier and we get, , Modulating, voitage, Vin, , amplified carrier at the output., , When the modulating signal is applied, the transistor operates as, , , , nonlinear device and multiplication of the carrier and, , , , modulating signal will take place., (p-1103) Fig. 4.1.1(b) : Single transistor emitter modulator, , The gain of the amplifier is dependent on the modulating voltage., , Hence the gain varies in a sinusoidal manner at the rate equal to the frequency of the modulating signal, , ‘The modulation index (m) is proportional to the amplitude of modulating signal., The voltage gain is given by,, , A= Ag [I +msin (2% fn 0] (Abd), , where A = Voltage gain with modulation, , Ag, the gain A varies sinusoidally with the modulating signal., , Voltage gain without modulation, , This equation shows that, , Waveforms :, , ‘The waveforms for the emitter modulator are shown in Fig. 4.1.2.

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), , mitters and Receiver,, , , , , , , , 3/MSBTE), , Modulated carrier superimposed, ‘onto modulating signal, , ", , Collector Vo, voltage, (Vo), , , , , , inciples of Electronics Com, , , , , Modulating signal arn, , (Vn), , Modulated camer with modulating, ‘signal frequency removed, , ampssro [In ffl Mh Time, , (p-1104) Fig. 4.1.2 : Waveforms of emitter modulator, toff. This ensures that the amplifier operation, , , , — The modulating signal applied at the emitter of transistor drives it into saturation and cut, will be nonlinear as required for the modulation to take place., - — If we analyze the voltage waveform at the collector, then it includes the following frequency components :, , 1. Cartier, 2. Upper and lower sidebands, , 3. Modulating signal., , — The unwanted modulating signal is removed by the coupling capacitor C, and we get the AM signal at the output (across R,)., , Disadvantages :, 1. Lowefficiency due to class operation., 2. — Itis incapable of producing high output power., , 4.1.3. Medium Power AM Modulator :, , - Fig. 4.1.3(a) shows the circuit diagram of a medium power AM modulator, using transistor., , Modulating, signal, , Carrier, , , , , , (p-1106) Fig. 4.1.3(a) : Simplified medium power transistor modulator

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nciples of Electronics Comm. (E&Tc./, g AS, em S/MSBTE) 4-4 AM Transmitters and Receivers, , modulation takes pl. ., , The * place at the collector. This Stage acts as it, , It is a high level modulator and operates as the class C an output stage of a transmitter, amplifier., , So the efficiency can be upto 80% which i:, pric ‘ is i ier i, modulating signal is applied in series with the emg ee sles ‘A amplifier. The cartier is applied to th, ‘cc to the collector., , e base while the, , The circuit is called as collector modulat, tor, because the modulating signal is applied directly to the collector., , RFC is a radio frequency choke. It acts bec. reactan, - as i, a short circuit for de and open circuit at high frequencies because inductive ice 18, , proportional to frequency., The job of RFC is to isolate the DC power, signal to pass through it as itis, power supply from the carrier and sidebands but to allow the de and low frequency ™, , odulating, , Vp= Voc, , , , Veo, Veg(at), w0V, -1107) Fig. 4.1.3(b) : Collector waveforms without (p-1107) Fig. 4.1.3(c) : Collector waveforms with, modulating signal, , modulating signal, , Waveforms :, , nal are shown in Fig. 4.1.3(a) whereas those in presence of modulanng’ signal are shown, , The waveforms in absence of modulating sig, , in Fig. 4.1.3(C)., , 4.1.4 Principle of High Level Mo, , ulation :, , Property of a tuned circult : Nn Current, pulse, , To understand the high level modulation, we must know an, , , , important property of a tuned circuit., This property states that if we apply a current pulse to a tuned L tr Output voitage, circuit then it generates damped voltage oscillations at its a “ 3, output. . 3, This is as shown in Fig. 4.1.4. The amplitude of oscillations is |, proportional to the size of current pulse and the decay rate 18 meat, asthe eps AM Damped oscillations, , proportional to the time constant. This is called, , flywheel effect in a tuned circuit., (v-78)Fig. 4.1.4 : The property of tuned circuit

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IE ECCS OE O_O, , AM Transmitters and Receivers, J_Principles of Electronics Comm, (E&Tc./Sem 3/MSBTE), , , , Requirements of a high level modulator :, , ee . a stem are :, Keeping in mind the Property of the tuned circuit the essential parts of a high level modulation syst, 1. A tuned Circuit and, , rs), , iona litude, . pace roportional to the amp!, A circuit Supplies current Pulses to the tuned circuit, If the amplitude of current pulses is made prop, , modulating signal then AM wave will be generated at the output of tuned circuit., , , , 4.1.5 High Level Collector Modulator Circuit :, , , , , , , , MSBTE Questions : i, tion., i ( lifler used for AM signal genera\, Q.1 | Sketch the labelled circuit diagram of collector modulated class-C amp ( , 8-05, 4 Marks), (S-15, 4 Marks, Q.2__Drawand explain circuit of AM modulators using BJT. : { ), , , , , , A high level collector modulator circuit is as shown in Fig. 4.1.5,, , ~ This circuit shows the output stage of a transmitter. Itis a high power RF (Radio frequency) class C amplifier., , The class C amplifier conducts for only a portion of the Positive half cycles of the carrier signal applied at the base of transistor Q., Operation of the circuit :, , — The transistor Q, receives the RF Carrier signal at its base. However the transistor will conduct only for a portion of the positive half, , cycles of the carrier signal because it has been biased to operate as class C amplifier., , ~ Thus the collector current of Q, is in the form of Current pulses. This is as shown in Fig. 4.1.5. These current pulses are supplied to, , the tuned circuit., , The high power audio amplifier amplifies the modulating signal to a high power level., , The secondary winding of the modulating transformer T, is connected in series with the de supply Voltage Vio., , — Therefore the modulating voltage will add or subtract from Voc as shown in Fig. 4.1.5., , , , , , , , , , , , , , , , Final class C Transmitting, RF power amplifier antenna, Tuned, Circuit, Carrier, signal, AM output, ae : T, High, Modulating power Modulation, signal audio transformer, ‘amplifier, +Vog, , (D-79) Fig. 4.1.5 : High level collector modulator, , This varying Supply voltage is then applied to the class C amplifier consisting of transistor Q.