The principal point of contrast with the inductively coupled case is that here there are three zeros at the origin, as indicated by p3 in the numerator,

Fig. 11-12 Double-tuned capacitively coupled interstage.

Fig. 11-12 Double-tuned capacitively coupled interstage.

instead of only one in Eq. (11-4). This has a pronounced effect on the shape of the amplitude-response curve in the wideband case.

First, however, in the narrow-band case, which we distinguish by ignoring the zeros at the origin and also the conjugate poles in the third quadrant (Fig. 11-13), there is essentially no difference between capacitance and inductance coupling. The variation of pole locations with k and Q, the gain-bandwidth factor, etc., are all the same in the narrow-band case.

But, in the wideband case, matters are far from equivalent. Speaking in terms of the potential analogy, the three negative charges at the origin p2x p,x

P2x f

Fig. 11-13 Pole positions for a double- Fig. 11-14 Response of a double-tuned tuned capacitively coupled interstage. capacitively coupled interstage, (a) Wideband. (I) Narrow band.

cause the potential on the low-frequency side to fall much more rapidly with frequency than it does on the high-frequency side of band center. The amplitude response thus appears as in Fig. 11-14.

Moreover, the gain-bandwidth factor of the capacitance-coupled circuit is highly unfavorable in wideband situations. For comparison with the inductively coupled case, there are plotted in Fig. 11-15 the curves of gain-bandwidth factor for = <x>.

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