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
