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how to get flat gain of HBT distributed amplifier

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lyxiao653

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Hi Guys,

I modeled a distributed amplifier in ADS with an out-of-date HBT VBIC model. I am just a beginner, so I built the cascoded distributed amplifier by following simple procedures. THe result showed that S22 is very good, less than -20dB from DC to 20GHz and S11 less than -10dB. But the gain is not very flat at all. In fact, the gain drops 6dB from DC to 20Ghz. I want to know how to find what is the reason for that non-flat gain. Besides, in distributed amplifier design, except miller except what else will cause non-flatness?
Xiaolin
 

From DC to 20G with only 6dB drop, that is good.
 

really? I thought the fluctuation should be +/-1dB and S12 starts to roll off after 20GHz.Btw, I designed the gate line at 20GHz.
 

I forgot the theory, the amplifier should rolloff per octave.
But I know someone use parellel RC to compensate the rolloff, you can search the paper.
 

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