I'm trying to simulate a Gilbert cell active mixer using a PHEMT transistor model in ADS. I've referred some articles of Gilbert cell active mixer design but I haven't found any matching circuit or stability circuits we generally use for a amplifier design. So I would like to know if we really require these in mixer design and how to proceed in RF, LO, and IF path.

I've used ATF-36077 transistor model in ADS to realise the circuit. Initially I tried a single-ended amplifier for RF frequency and during the design process I found that we need to add stability resistors to make it unconditionally stable. And also I added matching circuit. Now if I switch to mixer design using same transistor, do I need add the same network? How does it change when I place transistor in differential mode in Gilbert cell configuration?

Also I haven't seen the gate voltage in reference design. Do we need to give this for mixer?

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    \$\begingroup\$ If you assume the LO is a perfect square wave (which it's not), then the RF input just sees a standard differential pair at any given moment. You can treat stability and matching in this case as you would any diff pair, plus the parasitics of the switching transistors that are off for half the LO cycle. This of course ignores the dynamics that the LO induces, where you can have parametric amplification/mixing effects or parametric oscillation in some bizarre scenarios, but I've never seen them arise. \$\endgroup\$ Jul 28, 2023 at 4:03
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    \$\begingroup\$ The Cgd capacitance of the diff pair portion will allow the RF port to "see" the LO switching of course, which is something that will limit isolation, and affect input impedance depending on your frequency of operation. \$\endgroup\$ Jul 28, 2023 at 4:04
  • \$\begingroup\$ @LetterSized Thank you for your insight. I used a matching circuit at inputs of rf and lo section. This improved my isolation. \$\endgroup\$
    – Aparna B
    Aug 27, 2023 at 4:32


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