Amplification a V2 _ ngmRQ2

Equation (6-6) displays the basic property of the distributed amplifier, namely, that the amplification increases linearly with the number of "stages"; i.e., each tube contributes a gain of gmRQ2/2, and the total gain is the sum of the individual contributions. Indeed, each tube may contribute a gain of less than unity, and yet the total gain can be made as large as

desired by adding a sufficient number of tubes. This situation is not possible with cascade amplification.

6-2. Cascading Distributed Amplifier Stages. It is feasible—and frequently advantageous—to cascade whole distributed stages of the type of Fig. 6-1. This is done by connecting the grid line of the second stage as the load on the first stage (with a blocking capacitor for d-c isolation, of course). Should R0i and R02 not be the same, a transformer is in principle required to join the two stages in order to prevent reflection. If very low frequencies as well as the high frequencies must be amplified, a suitable transformer cannot be obtained; if, however, only the band from a few megacycles to the upper limit of the amplifier is required, a suitable coaxial transformer may be constructed. In the former case, it is usual to make Box and R02 the same by adding to the smaller of Ci or C2 in order to equalize them. The effect is the same, but the gain is reduced by the added capacitance. With the transformer, Eq. (6-6) becomes ngm\/R^iRQ2 .

0 0

Post a comment