Draw the schematic diagram of simplified medium-power transistor AM DSBFC modulator and explain the operation with the help of collector waveforms with no modulating signal and a collector waveforms with a modulating signal.

Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 10M

Year: Dec2014

• The medium-power transistor AM modulator came into existence because of the increased output powers provided by them, which is as high as several thousand watts. The schematic diagram for a simplified medium-power transistor AM DSBFC modulator is shown in Fig1. The modulation takes place in the collector, which is the output element of the transistor.

  Thus if this is the final stage of the transmitter, it is a high level modulator.

• Class C amplifier is generally used by high and medium power AM modulators to achieve high power efficiency.

• The Fig1 shows class C amplifier with two inputs: a carrier $v_c$ and a single frequency modulating signal $v_m$. Class C amplifiers conduct for only a portion of the positive half cycle of the input carrier.

• As the transistor is biased class C, it operates nonlinear and is capable of nonlinear mixing. Thus this circuit is also called as collector modulator because the modulating signal is applied directly to the collector.

• The RFC component is a radio frequency choke that allows dc to pass and blocks high frequencies, thus isolating dc power supply from high frequency carrier and side frequencies, while still allowing low frequency modulating signals to modulate the collector of the transistor.

• The modulator is a linear power amplifier that takes low level modulating signal and amplifies it to a high power level. The modulating output signal is coupled through modulation transformer T2 to class C amplifier. The secondary winding of the modulation transformer is connected in series with the collector supply voltage $V_{cc}$ of the class C amplifier.

• When the amplitude of the carrier exceeds the barrier potential (0.7 V for silicone transistor) of the base-emitter junction, $Q_1$ turns on, and collector current flows. When the amplitude of the carrier drops below 0.7 V, $Q_1$ turns off and collector current stops.

• As a result, $Q_1$ switches between saturation and cut off controlled by the carrier signal, collector current flows for less than 180° of each carrier cycle, and class C operation is achieved.

• Each successive cycle of the carrier turns $Q_1$ on for an instant and allows current to flow for a short time, producing negative-going waveform at the collector. The collector current and voltage waveforms are shown in Fig2 which resembles a repetitive half wave rectified signal.

• With a zero modulation input signal, there is zero modulation voltage across the secondary of T2, the collector supply voltage is directly applied to the class C amplifier, and the output carrier is a steady sine wave.

• When the modulating signal is applied to the collector in series with the dc supply voltage, it adds to and subtracts from $V_{cc}$, causing the amplitude of the current pulses through transistor $Q_1$ to vary. As a result, the amplitude of the carrier sine wave varies in accordance with the modulated signal.

• When the modulation 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 modulation goes negative, it subtracts from the collector supply voltage, decreasing it.

• The waveforms displayed in the Fig3 below are generated when the maximum peak modulating signal amplitude equals $V_{cc}$.

• It can be seen that the output voltage waveform swings from a maximum value of $2V_{cc}$ to approx 0 V $V_{CE(sat)}$. The peak change in collector voltage is equal to $V_{cc}$. Again the waveform resembles a half wave rectified carrier superimposed onto a low frequency ac modulating signal.