Electronic-Circuit II Chap 4. Communication Systems Amplitude Modulation Circuits Instructor:

Ajay Kumar Kadel

Course Homepage

www.courses.esmartdesign.com

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Outline

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

Amp. Modulation in the Amplifier Circuit

Carrier Signal Modulating Amplifier AM Signal Modulating signal (Message Signal) • • • 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) 3

Example of amp. Mod. in the Amplifier Circuit

Carrier Signal Message Signal • The above fig. shows amplitude modulated transistor amplifier with base injection in the common emitter configuration.

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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 5

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 6

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) 7