Basic Principle of Wide-band Designs
For systems which need to operate over a wide frequency range, such as a power amplifier operating over the range 1 MHz to 30 MHz in some solid state designs, a series of wide band transformers wound on ferrite cores can be used. This design has the advantage of not requiring any tuning when the operating frequency is changed. This type of design can also be used to match an antenna to a transmission line, it has the advantage of not requiring any tuning but it has the disadvantage of not being capable of fine adjustment. These networks can be used to extend the useful range of a conventional narrow band ATU.
In solid state RF power amp design these networks are useful because MOSFETs and bipolar transistors are designed to operate with low resistance loads. Valved RF amplifiers are very different because the load resistance which a valve is designed to operate with is normally much greater, hence for power designs the circuit designs are often very different.
Below is shown for example an autotransformer which has three identical windings on a ferrite core. If the right hand side is connected to a resistive load of 10 ohms, then at each of the left hand terminals a source of different impedance can be attached.
Read more about this topic: Antenna Tuner
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