lable. While the feeder itself may be perfectly matched to the amplifier, it is possible that the antenna or other load connected to the remote end of the feeder may present a poor match. When this happens power is reflected back to the amplifier and high levels of VSWR may be present (see Chapter 4). Under these conditions damage may occur to the amplifier because high levels of current or voltage may be present. If a high voltage occurs, this may cause the maximum voltage for the device to be exceeded with resultant break- down of the device. Alternatively high current levels can cause its current ratings to be exceeded resulting in the burnout of the device. Fortunately it is possible to design low power amplifiers to withstand poor load conditions, but this is not usually possible for high power amplifiers. In view of this many high amplifiers incorporate protection circuitry which reduces the power level as the SWR increases. Other protection systems 188 Newnes Guide to Radio and Communications Technology may remove the power from the amplifier if a high level of SWR is seen. If protection circuitry is required, then it must be very fast acting. Output transistors can be destroyed very quickly, especially if voltage breakdown occurs. Filters and matching networks The networks used to filter and match transmitters are very important to provide the correct matching from an amplifier to the next stage or into the antenna. By providing the correct impedance match, the greatest power transfer is effected. As the matching networks consist of inductors and capacitors, they are usually tuned to ensure that any unwanted signals are reduced to keep them within the required limits. A variety of different coupling methods or networks can be employed. The actual choice is dependent upon a number of considerations including the available driving power, the selectivity required, the impedances being matched and the level of mismatch that can be tolerated. Often the input impedance to the base of a transistor amplifier may 