Choke-Coupled Plate Modulation

Author: J.B. Hoag

In place of the modulator portion shown at the bottom of Fig. 28 A, it is possible to use the choke-coupled system shown in Fig. 28 B.

Fig. 28 B. Choke-coupled plate-modulation of a r.f. amplifier

Here the choke L must have a high impedance at audio frequencies. In order that there shall be 100 per cent modulation, i.e., in order that the peak a.f. voltage developed across the choke L shall be equal to the d.c. voltage on the amplifier tube, it is necessary that the voltage on the amplifier plate be reduced from that on the modulator tube by means of the resistor R₁. Condenser C₁ is used to bypass audio frequencies around R₁. Condenser C, on the other hand, must not pass appreciable a.f. and should be 0.002 μfd. or less. This type of modulation is rarely used except for very low-power sets of the portable type.