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Friday, June 10, 2011

Circuits Used in the Amplitude Modulation

Amplitude and Balanced Modulation

Amplitude modulation is a technique which uses a low-frequency signal to control the amplitude of a high- frequency signal. A simple modulator can be constructed using a multiplier as shown in figure 5.2.

Figure 5.2 A simple amplitude modulator circuit.

One input is the high-frequency or carrier signal, and the other input is the modulating signal. A sinusoidal source at a frequency of 10 kHz is used to represent the carrier signal and a second source at a frequency of 1 kHz is used to represent the modulating signal. Notice that the peak amplitude for the carrier is set to 1 volt using the parameter VcarrierPK. The modulating index is the ratio of the peak of the modulating signal to the peak of the carrier. Here, the index is set to 0.8 or 80% modulation. A typical broadcast AM signal includes the carrier as well as the sidebands in the transmission. To get such a double sideband transmitted carrier signal (DSB-TC) we must bias or offset the modulating signal by a value equal to the carrier's peak voltage. The amplitude modulated signal and the modulating signal from this simulation are shown in figure 5.3.
Figure 5.3 An amplitude modulator output signal.

      A balanced modulator produces a double sideband suppressed carrier signal (DSB-SC). By setting the offset of the modulating signal to be zero in the above circuit, we will suppress the carrier. Notice, the output of this modulator shown in figure 5.4; the shape of its upper A balanced modulator output signal envelope resembles a full-wave rectified AC source.
Figure 5.4 A balanced modulator output signal

     By using a specific OTA                       
    ( Operational transconductance amplifier )
      The LM13700 is a typical OTA and serves as a representative device. The LM13700 is a dual-device package containing two OTAs and buffer circuits. Figure 5.5 shows the pin configuration using a single OTA in the package. The maximum dc supply voltages are +18v and  -18v .
 For an LM13700, the bias current is determined by the following formula:

The 1.4 v is due to the internal circuit where a base-emitter junction and a diode connect the external Rbias with the negative supply voltage (-V). The positive bias voltage, +Vbias, may be obtained from the positive supply voltage, +V.

 Fig. 5.5
    Two OTA Applications                   
Amplitude modulator…….Figure 5.6 illustrates an OTA connected as an amplitude modulator. The voltage gain is varied by applying a modulation voltage to the bias input. When a constant-amplitude input signal is applied, the amplitude of the output signal will vary according to the modulation voltage on the bias input. The gain is dependent on bias current, and bias current is related to the modulation voltage by the following relationship:

This modulating action is shown in fig 5.6 for a higher-frequency sinusoidal input voltage and a lower-frequency sinusoidal modulating voltage.

The OTA as an amplitude modulator

Figure 5.6 The OTA as an amplitude modulator.
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