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Fig. 3-12 (a) An equivalent circuit for computing the h-f response of a pentode stage. (6) The pole-zero diagram for (a), (c) The resulting amplitude response.

of the combination is 0.707/? (or — 3 db in logarithmic units), and the phase angle is 45°.

Note that the gain as co becomes much less than 1 /RlC (the upper band-edge frequency) is the same as that given by Eq. (3-1) for co much greater than 1 /RgCcc (the lower band-edge frequency). Hence, if 1 /RLCy> 1 /RsCcc, as is always the case in a wideband amplifier, the gain is constant over a large region and is given as a limiting case of either Eq. (3-1) or Eq. (3-14). The fact that the high-frequency solution indicates constant gain in the frequency region of the low-frequency cutoff (and vice versa) is actually the proof that the two solutions may be made separately. To find the over-all gain function of the stage, the two individual solutions [Eqs. (3-1) and (3-14)] are multiplied together. (The constant must be such as to give the correct midband gain.)

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