Feedback effects in an amplifier will be examined in some detail in

4-6. Input Admittance of a Tetrode. The basic equivalent circuit of the tetrode is essentially that of the triode, even though a screen grid exists in the tetrode. A schematic diagram of a simple amplifier circuit employing a tetrode is given in Fig. 4-6. In drawing the equivalent circuit, the rules given in Sec. 3-3 have been appropriately extended and

Fig. 4-6. Schematic and equivalent circuits of a tetrode in an amplifier circuit.

Fig. 4-6. Schematic and equivalent circuits of a tetrode in an amplifier circuit.

employed. This requires the introduction of a point S, the screen terminal, in addition to the points K, G, and P.

Notice that the screen potential is maintained at a fixed d-c potential with respect to cathode and is at zero potential in so far as a-c variations about the Q point is concerned. As indicated in the figure, this effectively places a short circuit across C'/:s and puts Cgh and Cgs in parallel. This parallel combination is denoted C\. The capacitance Cps now appears from plate to cathode and is effectively in parallel with CPk. This parallel combination is denoted C2- Also, from the discussion in Sec. 2-6, the shielding action of the screen is such that the capacitance Cpg between grid and plate is very small. If this capacitance is assumed to be negligible, and it is less than 0.001 ¡xfxi in the average voltage tetrode, then Fig. 4-66 may be redrawn in the form shown in Fig. 4-7. In this figure, the capacitances have the values

Fig. 4-7. The ideal equivalent circuit of a tetrode amplifier.

The input admittance of the tube is then

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