Radio Frequency Integrated Circuit Design - Webs

Impedance Matching
Note that dots in Figure 4.17 are placed such that if current flows in the
indicated direction, then fluxes will be added [4]. Equivalently, if Ip is applied
and Vs is 0V, current will be induced opposite to Is to minimize the flux.
For transformers, it is necessary to determine where to place the dots. We
illustrate this point in Figure 4.18, where voltages V1, V2, and V3 generate flux
through a transformer core. The currents are drawn so that the flux is reinforced.
The dots are placed appropriately to agree with Figure 4.17.
An equivalent model for the transformer that uses mutual inductance is
shown in Figure 4.19. This model can be shown to be valid if two of the ports
are connected together as shown in the figure by writing the equations in terms
of Ip and Is and using the mutual inductance M.
Figure 4.18 Flux lines and determining correct dot placement.
Figure 4.19 Two equivalent models for a transformer.
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