AMPLITUDE MODULATED TRANSMITTER

2-3 As you can see in the figure, a key is used to turn the buffer on and off. When the key is closed, the rf carrier passes through the buffer stage; when the key is open (buffer is turned off), the rf carrier is prevented from getting through. The final stage of a transmitter is the power amplifier (referred to as the pa). In chapter 3 of NEETS, Module 1, Introduction to Matter, Energy, and Direct Current, you learned that power is the product of current and voltage (P = IE). In the power amplifier a large amount of rf current and voltage is made available for radiation by the antenna. The power amplifier of a high-power transmitter may require far more driving power than can be supplied by an oscillator and its buffer stage. One or more low-power intermediate amplifiers are used between the buffer and the final amplifier that feeds the antenna. The main difference between many low- and high-power transmitters is in the number of intermediate power-amplifier stages used. Figure 2-2 is a block diagram of the input and output powers for each stage of a typical medium- power transmitter. You should be able to see that the power output of a transmitter can be increased by adding amplifier stages capable of delivering the power required. In our example, the .5 watt output of the buffer is amplified in the first intermediate amplifier by a factor of 10, (this is a times 10 [´ 10] amplifier) giving us an input of 5 watts to the second intermediate amplifier. You can see in this example the second intermediate amplifier multiplies the 5 watt input to it by a factor of 5 (´ 5) and gives us a 25 watt input to our power (final) amplifier. The final amplifier multiplies its input by a factor of 20 (´ 20) and gives us 500 watts of power out to the antenna. Figure 2-2.—Intermediate amplifiers increase transmitter power. Q1. What are the four basic transmitter types? Q2. What is the function of the oscillator in a cw transmitter? Q3. What is the final stage of a transmitter? AMPLITUDE MODULATED TRANSMITTER In AM transmitters, the instantaneous amplitude of the rf output signal is varied in proportion to the modulating signal. The modulating signal may consist of many frequencies of various amplitudes and phases, such as the signals making up your own speech pattern. Figure 2-3 gives you an idea of what the block diagram of a simple AM transmitter looks like. The oscillator, buffer amplifier, and power amplifier serve the same purpose as those in the cw transmitter. The microphone converts the audio frequency (af) input (a person’s voice) into corresponding electrical energy. The driver amplifies the audio, and the modulator further amplifies the audio signal to the amplitude necessary to fully modulate the carrier. The output of the modulator is applied to the power