Figure 4.

Diagram showing the current paths for different decoupling capacitor placements to demonstrate appropriate coupling measurements when using ULOI amplifiers. (a) The decoupling capacitor is placed between the coil feeds and both coil ports are terminated in 50Ω (as is the case for a conventional S₂₁ port measurement). The current path splits at the juncture between the rung and decoupling network and splits again at the connection to the adjacent rung due to the 50Ω termination. (b) Instead of a 50Ω load, the adjacent rung is terminated in the ULOI amplifier, labeled with theoretical infinite impedance. The current paths do not split at the adjacent rung, and the ratio of rung currents is different from the case shown in (a) indicating a difference in coupling. Therefore, if a conventional S₂₁ measurement were used to choose the decoupling capacitor value, the coupling provided by the network would change in the operating case with the ULOI amplifier (or preamplifier). (c) The decoupling capacitor is placed in front of the last distributed capacitor to avoid the difference in current paths for the two terminations cases allowing for direct S₂₁ port measurements to set the decoupling capacitor.