Valve (Tube-Based) Amplifi ers 363urged that, for good LF stability, the value of permeability, μ , for low values of B should
be used for primary inductance calculations.
Second, this change in inductance, as a function of current in the windings, is a source of
transformer waveform distortion, as are—especially at high frequencies—the magnetic
hysteresis of the core material and the eddy current losses in the core. These problems are
exacerbated by the inevitable DC resistance of the windings and provide another reason,
in addition to that of improved effi ciency, for keeping the winding resistance as low as
possible.
The third problem is that the permeability of the core material falls dramatically, as
seen in Figure 11.16 , if the magnetization force exceeds some effective core saturation
level. This means that the cross-sectional area of the core (and the size and weight of
the transformer) must be adequate if a distortion-generating collapse in the transformer
output voltage is not to occur at high signal levels. The calculations here are essentially
the same as those made to determine the minimum turns per volt fi gure permissible for
the windings of a power transformer.^1
In practical terms, the requirements of high primary inductance and low leakage
inductance are confl icting and require that primary winding is divided into a number of
sections between which portions of the secondary winding are interleaved. Williamson
1010 K1K10010
100 1 K 10 K
Flux density (B)Permeability (μ)Figure 11.16 : A magnetization curve.