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Phase Relations in Amplifiers

The phase relations for an amplifier working into a pure resistance load are shown in Fig. 18. The grid of the tube is biased negative an amount Eg by the grid bias source Ecc. When the positive half cycle is impressed, the grid is driven less negative and more plate current flows. This increase in plate current causes an increase in the ibRL voltage drop in load resistor RL, and, thus, the voltage from plate to cathode falls. When the negative half cycle is impressed, the grid is driven more negative and less plate current flows. This decrease in plate current causes a decrease in the ibRL voltage drop across RL, and the voltage from plate to cathode rises. The instantaneous phase relations are as shown in Fig. 18.

amplifier with resistive load
Figure 18. For an amplifier with resistance load the plate current is in phase with the grid signal voltage, but the plate-to-cathode voltage is 180° out of phase with the grid signal voltage.



Last Update: 2011-05-18