AM Transmitter Block Diagram
Note: Class C RF output Amplifier is used in High Level Modulation.
Class B RF linear power amplifier is used in Low Level Modulation.
The general block diagram of an AM modulator is shown in the figure. A tuned circuit is used to generate AM. It is probable to create the output current of a class C amplifier relative to the modulating voltage by the application of this voltage in series by means of some of the 'dc' supply voltages for this amplifier.
In an AM transmitter, amplitude modulation can be done at any point after the radio frequency source. If the output stage in a transmitter is collector modulated, the system is known as high level modulation. In high level modulation, the power amplification of the RF Signal takes place before modulation. If modulation is applied to any other point (base or emitter), the system is known as low level modulation. In low level modulation, the power amplification of the RF signal takes place after modulation.
The RF oscillator generates the radio frequency wave for modulation (i.e. the carrier wave). This is generally a crystal oscillator because it can generate highly stable frequency. The oscillator frequency shall not be affected by the loading of the next stages. Hence a buffer amplifier is required at the oscillator output. It will be a class A amplifier. The RF power amplifier raises the power of the RF signal to the required level for modulation. It will be a class-C amplifier.
The audio signal to be transmitted is applied to microphone. Microphone will converts sound wave into electrical audio frequency waves. Microphone is an audio transducer. The output signal from the microphone is very small in amplitude. The signal voltage must be raised to the sufficient level before power amplifications. AF pre-amplifier is used for this purpose. It will be a class A amplifier. The power of the AF signal must be raised to the required level before modulation AF power amplifier raises the power of the audio signal to the required level. It will be a class B amplifier.
The AM modulating amplifiers will modulate the RF signal by the AF signal from the output of the AF power amplifier. The transmitting antenna radiates the RF power from the output of the modulator into space.
