AM Modulator is one which is used for superimposing a low frequency signal on a high frequency carrier signal. In this modulator the amplitude of a carrier is varied in accordance with instantaneous value of message signal.
Types of A.M Modulators are listed below.
It is so named because the message signal (AF) is superimposed on +Vsb and tVsb and then applied to the plate of a triode tube. Now we will study how modulation occurs in this modulator.
It is so named because the input signals RF, AF and negative Vc voltages are applied to the grid of class C amplifier.
The output stage of the transmitter is a high power frequency class C amplifier. Class C amplifiers conduct for only a portion of the positive half cycle of their input signal. The collector current pulses cause the tuned circuit to oscillate or ring at the desired output frequency. The tuned circuit, therefore, reproduces the negative portion of the carrier signal.
The modulator is a linear power amplifier that takes the low level modulating signal and amplifies it to a high power level. The modulating output signal is coupled through modulation transformer T1 to the class C amplifier. The secondary winding of the modulation transformer is connected in series with the collector supply voltage Vcc of the class C amplifier.
With zero modulation input signal. There will be zero modulation voltage across the secondary of T1. Therefore, the collector supply voltage will be applied directly to the class C amplifier, and the output carrier will be a steady sine wave.
When the modulation signal occurs, the AC voltage across the secondary of the modulation transformer will be added to and subtracted from the collector supply voltage.
This varying supply voltage is applied to the class C amplifier. Naturally, the amplitude of the current pulses through transistor Q1 will vary. As a result, the amplitude of the carrier sine wave varies in accordance with the modulated signal. For example, when the modulating signal goes positive, it adds to the collector supply voltage, thereby increasing its value and causing higher current pulses and a higher amplitude carrier. When the modulating signal goes negative, it subtracts from the collector supply voltage making it less. For that reason, the class C amplifier current pulses are smaller, thereby causing a lower amplitude carrier output. Hence amplitude modulated wave is obtained which is then transmitted through antenna.
It is so named because RF carrier and the message signal both are provided to the base of the transistor.