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Electronic-Circuit II Chap 4. Communication Systems

Electronic-Circuit II Chap 4. Communication Systems. Amplitude Modulation Circuits Instructor: Ajay Kumar Kadel Course Homepage www.courses.esmartdesign.com. Outline. Amplitude Modulation in the amplifier circuit Amplitude Modulation in the Oscillator circuit

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Electronic-Circuit II Chap 4. Communication Systems

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  1. Electronic-Circuit IIChap 4. Communication Systems Amplitude Modulation Circuits Instructor: Ajay Kumar Kadel Course Homepage www.courses.esmartdesign.com

  2. Outline • Amplitude Modulation in the amplifier circuit • Amplitude Modulation in the Oscillator circuit • Examples of both types of modulation

  3. Carrier Signal Modulating Amplifier Modulating signal (Message Signal) Amp. Modulation in the Amplifier Circuit • Carrier signal and Modulating signal are fed to the amplifier • Transistor can be used in any configuration (CE,CB, CC) • Modulating signal can be injected into any terminal (base, collector or emitter) AM Signal

  4. Example of amp. Mod. in the Amplifier Circuit • The above fig. shows amplitude modulated transistor amplifier with base injection in the common emitter configuration. Carrier Signal Message Signal

  5. Amplitude Modulation in the Oscillator Circuit • Amplitude modulation in oscillator circuit • Carrier wave is generated by an LC tank circuit of the oscillator • Modulating signal is directly fed in along with the fed-back carrier wave • Transistor in CE configuration • Parallel tuned L2 & C4 generates the necessary carrier wave

  6. Amplitude Modulation in the Oscillator Circuit (contd.) • Secondary winding L3 acts as coil providing positive feedback for transistor Q1 • Secondary winding L4 couples the modulated output to oscillator load • R1 & R2 provides biasing for the transistor • L1 & C2 are made resonant to carrier frequency thereby providing low impedance path

  7. Amplitude Modulation in the Oscillator Circuit (contd.) • The impedance of L1 & C2 for relatively low frequency modulating signal is very high • This has negligible loading effect to the modulating source • Fixed bias R1 & R2 , supply, the feedback voltage from L3 and the modulating signal voltage varies the bias increasing the gain thereby increasing the amplitude of the carrier signal (couples to output)

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