The term Amplitude Modulation (AM) refers to the type of modulation in which an audio
signal is combined with a radio frequency carrier in such a way that total wave power is made to
vary in accordance with the modulating audio signal (Bertrand 1).
The amplitude modulation and
transmission of modulated audio signal is carried out through AM Transmitter. Depending upon
the type of application and level of operation, an AM Transmitter consists of following basic
building blocks as shown in fig.1.
Fig. 1 Block diagram of an AM Transmitter
From fig.1 it is obvious that an AM Transmitter has following important building blocks.
1- Oscillator (OSC)
5- Microphone Amplifier
7- Power Amplifier
8- Transmitting Antenna
For detail description of each block, the schematic diagram of a basic level AM Transmitter is given below in fig. 2.
Fig. 2 Schematic circuit diagram of an AM Transmitter
The circuit diagram given in fig. 2 can be broadly divided in two major sections namely;
- The Oscillator section.
- The audio amplifier section and
The oscillator section (that comprises of an oscillator, buffer, pre-driver, driver and microphone amplifier and modulator) is built from the transistors Q1 (BC109) and biasing resistors (R1, R2, and R3). The components L1 and C1 constitutes the tank circuit that can be tuned in RF range of 500kHz to 1600KHz. The Q1 (BC109) that is the key component of oscillator circuit section is provided with regenerative feedback by connecting the base and collector of Q1 to opposite ends of the tank circuit. ...