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urgent support required for HMC637LP5E biasing voltage


I made a  6  custom board  based on HMC637 PA and HMC920 bias circuit , two of them are working well but the others  didn't work and have strange behavior 

the first one Vgate  stucked to -1.5V VDRAIN =0.7V  (  when i remove the PA the vdrain =12.2 volts and VGG1=2 volts)

the others Vgate =-2.3V and Vdrain =0.5 V  (PA never removed )

please can some one help me to figure out the problem its very urgent ,below is my schematic.

  please note that  R8 and R22 in the reference schematic of AD are not used here pins are floating   

and  I hope they are not the cause of this behavior   


any help will be appreciated 

  • Hi hotmax2828,

    I would suggest installing R503 in your schematic as it is required for the LDO feedback loop and is essential to the operation of the HMC920.

    Please let us know if this resolves your issue.

    By the way, you might want to consider the HMC980 for future designs. It has a separate VGG2 pin.



  • Hi Dan02

    I didn't understand your feedback, the R503 is already installed, its value =51K as reference design 

    by the way I assembled  new board with 1.6MOHM installed to limit VGG voltage  and I removed R511 and R515  and I get VGate=-2 volt  but   the biasing operation didn't  start 

    Vdrain always 0 V 

    CurrAlrm is high 3.6V

    Trigout High 3.6 volt 

    concerning the HMC980 the first design is based on it but due to availability we moved to HMC920