Simultaneous Power and Noise Optimization of Active Circulators

Active circulators offer the advantage of small size and weight, low cost and compatibility with MMIC technology. However, they also introduce excess noise and limit the RF-power handling capacity. In this paper, simple formulas are derived which describe the power and noise behaviour of six active circulator architectures: three 3-way circulators and 3 quasi-circulators. It is shown that the power handling capacity of active 3-way circulators is determined by the required transconductance of the transistors in the circuit while the noise is determined by the drain noise current source of the transistors. The presented formulas give more insight in the operation, the theoretical limits and the differences between active circulators.