Effects of input routing in switched RF amplifiers

This paper discusses the effects of unequal signal delays in the input network of a multi-finger transistor in a switching RF power amplifier. In a design that employs a distributed transistor, the driving signal must be delivered to each transistor finger by conductor segments that usually are unequal in length. The effects of the consequent non-simultaneous switching are demonstrated with three simulation examples performed with ADS circuit simulator at the frequency of 1.6 GHz, showing significant efficiency degradation when the input trace lengths to the individual transistor fingers differ from each other. An additional cause of highly increased timing differences and non-simultaneous switching is the signal dependent gate capacitance that changes the loading of the signal trace and affects to the signal delays. It is suggested that either tree-like input traces or suitably positioned second harmonic trap should be used to maximize the efficiency and to ensure good driving signal delay match to all fingers of the switching transistor.