AN749: Broadband Transformers and Power Combining Techniques for RF

The input and output transformers are among the most critical components in the design of a multi-octave amplifier. The total performance of the amplifier (linearity, efficiency, VSWR, gain flatness) will depend on their quality. Transformers with high impedance ratios and for low impedances are more difficult to design in general. In the transmission line transformers very low line impedances are required, which makes them impractical for higher than 16:1 impedance ratios in a 50-Ohm system. Other type transformers require tight coupling coefficients between the primary and secondary, or excessive leakage inductances will reduce the effective bandwidth. Twisted line transformers (Figure 1C, D, F, G) are described in References 1, 2, and 4. Experiments have shown that the dielectric losses in certain types of magnet wire, employed for the twisted lines, can limit the power handling capability of such transformers. This appears as heat generated within the transformer at higher frequencies, although part of this may be caused by the losses in the magnetic core employed to improve the low frequency response. At low frequencies, magnetic coupling between the primary and secondary is predominant. At higher frequencies the leakage inductance increases and the permeability of the magnetic material decreases, limiting the bandwidth unless tight capacitive coupling is provided. In a transmission line transformer this coupling can be clearly defined in the form of a line impedance. The required minimum inductance on the low impedance side is: