Step-Down Transformers

The circuits of Figs. 229 and 233 for step-up pulse transformers are essentially the same as those of Figs. 107(e) and 107(c), respectively, for audio transformers. Low-frequency response corresponds to the top of the pulse and high-frequency response to the front edge. In step-down pulse transformers the top is unchanged, but the front edge corresponds to Fig. 113. Step-down transformer analysis shows that the form of equation is similar to that for step-up transformers, except that the damping factor for the sine term is greater by the quantity R₂/L_Sß. Also, the decrement, although still composed of two terms, has the resistances R₁ and R₂ in these two terms reversed with respect to the corresponding terms for the step-up transformer. Except for this, the front-edge curves are little different in shape from those of step-up transformers. Where R₁ = R₂ the curves are virtually the same as in Fig. 231. Pentode amplifiers, with their constant-current characteristics, can be represented by the circuit of Fig. 238.

Fig. 238. Step-down transformer equivalent circuit.

Here I is the current entering the primary winding from the tube, and is constant over most of the voltage range. The transformer is usually step-down for the reasons of impedance mentioned in Pentode Amplifiers. Front-edge response of these transformers is the same as in Fig. 230 if the rise in load current is expressed as a fraction of final current I, and the decrement is changed to R₂/2L_S. It is reproduced in Fig. 239 with this change in constants.

Fig. 239. Pentode amplifier front-edge response.

Flatness of top is approximately that of the curve R₂ = 0.1R₁ in Fig. 234. Trailing edge is the same as in Fig. 235.

It is evident that many practical cases are represented by the figures. If transformer constants are outside the curve values, the pertinent equation should be plotted to obtain the response.