Download 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
1
Outline
• Amplitude Modulation in the amplifier circuit
• Amplitude Modulation in the Oscillator circuit
• Examples of both types of modulation
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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)
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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
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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
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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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