Figure 1a illustrates a typical transformer having a primary to secondary turns ratio of 1:N and a primary inductance of LP. In the real world, however, transformers aren't ideal, and they can become nonlinear at high excitation levels. This produces additional spectral components due to harmonic and intermodulation distortion. The usual solution is to employ physically larger transformers, which have magnetic geometries that saturate at higher flux densities (excitation). But this isn't always convenient or practical, and it may require a custom part.
To achieve the effect of a physically larger transformer, connect individual transformers in series, as shown in Figure 1b. The overall turns ratio is the same (1:N), and the equivalent primary inductance is 3 × LP. However, the LP of each individual transformer can vary significantly, because magnetic core permeabilities differ from unit to unit by as much as 30%, or even more. Consequently, the total primary voltage will not divide equally among the three transformers.
The transformer with the highest LP will dominate and provide a higher voltage drop than the others. Because flux density is proportional to voltage, that particular transformer will operate at the highest flux density and may saturate before the others.
To obtain equal distribution of voltages, a shunt resistor can be added across each primary (Fig. 1c). To ensure that the resistor (and not LP) determines the voltage division, its value should be chosen so that it's slightly less than the reactance of LP at the lowest frequency of interest. A rough approximation would be:
R = 0.8(2πFLLP)
where FL is the lowest frequency of interest.
With this method, the author achieved a 10-dB reduction in harmonic distortion using three transformers, as shown in Figure 1c. This method can be extended to more than three transformers, or reduced to two transformers as well. A limiting factor becomes increased insertion loss from higher dc resistance as the number of transformers rises. On the other hand, the low-frequency response improves with an increasing amount of transformers.