Transistor AM modulator

• A circuit which does amplitude modulation is called AM modulator.
• Fig. above shows the circuit of a simple AM modulator. It is essentially a CE amplifier having a voltage gain of A. The carrier signal is the input  to the amplifier. The modulating signal is applied in the emitter resistance circuit.
• The amplifier circuit amplifies the carrier by a factor “A” so that the output is Ae_c. Since the modulating signal is part of the biasing circuit it produces low-frequency variations in the circuit. This in turn causes variations in “A”. The result is that the amplitude of the carrier varies in accordance with the strength of the signal. The amplitude modulated output is obtained across R_L.

Power in AM wave

 

Limitations of Amplitude Modulation

1. Noisy Reception- In an AM wave, the signal is in the amplitude variations of the carrier. Practically all the natural and man made noises consist of electrical amplitude disturbances. As a radio receiver cannot distinguish between amplitude variations that represent noise and those that contain the desired signal. Therefore reception is very noisy.

2. Low efficiency- In AM useful power is in the sidebands as they contain the signal. An AM wave has low sideband power.

For example even if modulation is 100 % ie m=1.

P_S=33% of P_T

Sideband power is only one-third of the total power of AM wave. Hence efficiency of this type of modulation is low.

3- Lack of audio quality- In order to attain high fidelity reception, all audio frequencies upto 15 Khz must be reproduced. This necessitates a bandwidth of 30 KHz since both sidebands must  be reproduced (2f_s ). But AM broadcasting stations are assigned with bandwidth of only 10 KHz to minimize the interference from adjacent broadcasting stations. This means that the highest modulating frequency can be 5 Khz which is hardly sufficient to reproduce the music properly.