The amplitude radio transmitter using high frequency power level of carrier has the following constituent stages namely master oscillator, buffer amplifier or isolating amplifier, harmonic generators, class C power amplifiers, modulated amplifier and the modulating amplifier. All high frequency power level of carrier requires all there stages.
The master oscillator generates oscillations of desired frequency with high constancy of frequency. The generated frequency is required to remain constant within close limits in spite of variations in the supply voltage, ambient temperature and temperature of components of load. Further frequency variations with time and with age of the transistor should be avoided.
If the master oscillator directly drives a harmonic generator or class C power amplifier which draw input current then power is drawn from the master oscillator. This results in loading of master oscillator which in turn causes variation of effective resistance of the tank circuit of the oscillator and hence results in frequency variation. Accordingly a buffer amplifier or isolating amplifier is placed between the master oscillator and the harmonic generators. This buffer amplifier does not draw any input current and hence causes no loading of the master oscillator. Changes in carrier frequency due to variations in loading are thus avoided.
Usually the master oscillators generate voltage at a frequency which is a sub multiple of the carrier frequency that is the frequency of the radiated power. Basically these harmonic generators are class C tuned amplifiers in which the output radio frequency voltage is first distorted through class C operation and then the tuned circuit in the output circuit of amplifier selects the desired harmonic frequency.
The radio frequency voltage generated by the master oscillator has usually very small power of the order of few watts. The power level is required to be raised to the final high value in a chain of class C amplifier having high output circuit efficiency of the order of seventy percent.
The modulated amplifier is a class C amplifier usually of the push pull type and is modulated by audio modulating voltage from modulating amplifier. High efficiency series plate modulation is most popularly used in high power radio broadcast and radio telephone transmitters. Grid bias modulation and suppressor grid modulation are used particularly for modulation at low power levels.
The modulating amplifier is usually a class B push pull amplifier and feeds audio power into the modulating amplifier in the plate circuit, the control grid circuit or the suppressor grid circuit depending upon the method of modulation used. Class B operation is generally used because of high plate circuit efficiency. While the Class A modulating amplifier are also sometimes used particularly in low power transmitters.
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