Derivation
The basis of a Zobel network is a balanced bridge circuit as shown in the circuit to the right. The condition for balance is that;
If this is expressed in terms of a normalised Z0 = 1 as is conventionally done in filter tables, then the balance condition is simply;
In other words, is simply the inverse, or dual impedance of .
The bridging impedance ZB is across the balance points and hence has no potential across it. Consequently, it will draw no current and its value makes no difference to the function of the circuit. However, its value is often chosen to be Z0 for reasons which will become clear in the discussion of bridged T circuits.
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