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TX port baluns and impedances

Question asked by JeffGoos on Aug 13, 2014
Latest reply on Aug 14, 2014 by tlili

Hi tlili


I'm designing for an application that covers about 200MHz to 3GHz.  My application has a very limited maximum component height of 2mm (80mils).  From all the part searches I've done this prevents me from using any ferrite core baluns; the lowest profile I found was about 3mm.  This has pushed me toward using Anaren baluns and breaking up the frequency coverage into two bands.


My question is relating to the impedance presented to the differential TX ports for F < 1GHz.  My desired balun selection is not a 50:50 ohm impedance transformation, it is 50:200.  There is a 50:50 ohm part available but its much larger and has other undesirable characteristics.


The TX port is a medium power match, therefore its closer to a loadline approach rather than a small signal conjugate match.  My thoughts are that using a 50:200 balun would reduce the maximum TX power by 10LOG(50/200) = -6dB.  By presenting a higher impedance it makes the output transistors hit the voltage rails sooner.  I'm also thinking this would reduce the TX DC current for the same RF power, unless the output stage is closer to class A than class AB.


So it seems that as long as I'm OK with about a 6dB lower max TX RF power that this will work OK and provide a more efficient transmitter.


Can you please comment on these two assumed results?  Is there anything else negative that may result from this 4x higher impedance on the TX ports?  I'm wondering if reactive parasitics could dominate the higher real impedance and ruin everything.


Thanks in advance.