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matching problem for cascode lna

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wizardyhnr

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cascode lna

i am designing a lna whose operating frequency is 60GHz, the topology of the circuit is cascode, but i find it's quite difficult to match the ouput impedance to 50 Ω. As shown in the schematice below, there is a parasitic pad capacitor (about 35 fF, which is quite large in 60GHz), which influence the matching and gain seriously.

The output impedance looking into the drain of CB transistor is (42.5-j190)Ω (not including pad parasitic capacitor), so LC network may require a quite small capacitor which is about 20fF and the capacitor has to be very accurate.

If a serial inductor is first attached to drain of CB transistor and then a parallel inductor and the pad parasitic capacitor, the lna may be not unconditional stable in all frequency range.

i tried to put a indutor of about 200pH parallel with the pad parasitic capacitor to avoid the signal to be grounded, it seems not acting well.

can someone advise a good matching network for this situation? thanks
 

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