aaron_do
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Hi,
I think you were right in that your circuit is not starting up. If you show us your schematic it would help but I can understand if you don't want to. You can ensure start-up by either adding some start-up transistors, or by making sure your common-mode feedback loop allows the circuit to start-up.
Out of curiousity, why did you AC couple the mixer core to the op-amp? Are you trying to get a high-pass function? If so, the corner frequency of your HPF depends on the average resistance of the mixer core (for a double balanced mixer core). The smaller the on-resistance, the larger the capacitors you need.
Also, if your mixer core is a double-balanced type (as is normal), you should look out for your TIA loop gain which will depend on the average resistance of the mixer core. Your loop gain will affect the TIA linearity.
cheers, Aaron
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