Gain variation and noise figure degradation in balanced amplifiers
Coşkun, Akif Alperen
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Although using balanced amplifiers in modern wireless communication systems has many advantages, the output and the noise response of the balanced ampli- fiers may vary from the expected value considerably due to the imperfections in the amplifier and the divider/combiner sections. First, we analyze the variation in the total gain of the balanced amplifiers using 2-way 0ff power dividers and 90ff couplers. In this context, mathematical analyses are made and analytical expressions are obtained to compare the two topologies. Analytical expressions show that the uncertainty in the total gain is more if the 90ff coupler is used in the balanced structure as opposed to a 2-way 0ff power divider for the same return loss/isolation values. Then, we present exact and approximate analytical results for the noise figure and noise parameters of the balanced amplifier in divider topology by using the noise waves. By the help of the noise wave approach, the output noise powers generated by each element in the balanced amplifier are derived. Y-parameters are used to determine the noise waves and the correlation matrix of the divider whereas the noise parameters are used to write the noise waves of the amplifier. Balanced amplifiers suffer in the noise figure performance in comparison to a stand-alone amplifier even with an ideal input divider. The noise parameters degrade further with an imperfect divider. Not only the nonzero return loss and the isolation but also the ohmic loss, amplitude and phase imbalances are considered. Besides, non-zero optimal source impedance affects the noise parameters of the balanced amplifier. Measurement results are also presented as a verification.
Phase and amplitude imbalance