Grid-Bias Modulation

Author: J.B. Hoag

In this method, audio- and radio-frequency voltages are introduced simultaneously into the grid circuit of an amplifier while the B supply remains fixed. The circuit is indicated in Fig. 16 I, with further details in Fig. 16 J.

Fig. 16 I. Elementary circuit for grid-bias modulation

Fig. 16 J. Further details of grid-bias modulation IP

Fig. 16 K. The principle of grid-bias modulation

At the bottom of Fig. 16 K, we see that the combined low- and high-frequency voltages apply to the grid a wave whose form is that of the r.f. swinging back and forth on an axis which, instead of being straight, shifts back and forth at audio frequency. The resultant plate current is shown at the right in Fig. 16 K. These pulses of current stimulate proportionately strong oscillations in the tank circuit LC (Fig. 16 J) which is tuned to the radio frequency.

The output transformer eliminates the d.c. plate current, delivering a modulated carrier voltage to the next amplifier or to the antenna from which the radio wave is radiated. In a way, one might say that the action of the tank circuit and its output coil (Fig. 16 J) is to "straighten the axis" of the plate current (dotted axis in Fig. 16 K).

The efficiency of operation of the grid-bias-modulation method is quite low as compared with that of the plate-modulation scheme. On the other hand, the power output of the modulator tube of the grid-bias method need only be one or two watts to operate a modulated tube of considerable power.