A similar arrangement to that of Fig 4.9 can be used for single band low-power work on the HF bands, but in general the requirements here tend to be a great deal more severe, ie higher powers are generally called for with operation over all bands.
The lower input and output impedances of transistors which were referred to earlier mean that in a PA having an output of 100W, these impedances will be of the order of 1- 10Ω . Conventional pi-network designs at such low impedances lead to impractical values of inductance and capacitance.
The solution adopted at high power is to transform the impedance of the RF input to a transistor PA down from 50Ω and then transform the output impedance of the transistor back up to the normal 50Ω . The basic arrangement is shown in Fig 4.10. The transformers used have ratios of about 4:1 and two such transformers may be used in cascade to provide a higher ratio.
Fig 4.10. Basic circuit diagram of PA suitable for use on the HF bands
These transformers are known as 'transmission line' or 'broadband' transformers and typically consist of relatively few turns of wire on a toroidal core. The primary and secondary windings are wound together as a pair of wires (a bifilar winding) which may be twisted together.
Because this type of transformer is broadband, it can operate over a wide frequency range, ie from 3-30MHz. If the drive circuits are similarly designed, the whole transmitter becomes broadband, ie apart from the VFO no band switching or retuning is required. This is obviously highly satisfactory from the operational point of view; however, the design of such transformers is complex and the constructional work involved is not for the inexperienced amateur.
The fact that the RF circuits are broadband means that any harmonic or spurious frequency which may be generated is also amplified and appears at the output. Consequently the output must be then filtered to avoid the radiation of any unwanted frequencies. A separate (switched) filter for each band is often used.
