Bridge Networks Incorporating Active Elements and Application to Network Synthesis

A common structural unit in the synthesis of linear networks is the bridge configuration, which, in certain important cases, is "balanced" at all frequencies. The balanced nature of the bridge may be exploited to realize automatically with considerable accuracy one impedance arm of the bridge, by inserting an amplifier into the network, whose input is the null voltage of the bridge. The general concept of such "active-balancing" is discussed and it is shown how this technique can be applied to the classic Bott-Duffin synthesis of two-terminal impedances and to the constant-resistance bridged-T network, with considerable reduction in the complexity of the resultant structures. In particular, the active Bott-Duffin synthesis becomes a practical synthesis procedure. Practical examples of a constant-resistance bridged-T attenuator, a constant-resistance bridged-T equalizer and the Bott-Duffin synthesis of a fourth-degree positive real (p.r.) function are discussed and in all three cases a comparison of experimental and calculated results is made. An attractive feature of these activelybalanced bridge systems is their virtual independence of amplifier characteristics, provided that the amplifier gains are sufficiently high.