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HMC-1131 Evaluation Board Biasing


I recently received the evaluation board for the HMC1131 part, EV1HMC1131LC4. 

Currently, I am just trying to bias the amplifiers. I was following the biasing instructions on page 11 of the datasheet to establish the pinch off condition in the amplifiers. However, the devices are drawing current even when they are supposed to be in the pinch off region.  

Here is the procedure that I am currently following:

  • Terminate RF in and RF out with 50 Ohm terminations
  • Set the current limit on the Vdd lines to 300 mA
  • Turn Vgg1 and Vgg2 to -2 V
  • Turn on Vdd1 to Vdd4 to 5 V

At this point, I am expecting 0 A to be flowing through the channels of the devices, as I am assuming they are depletion mode devices. However, the current limit is already being hit. When I first received the board, the Vgg1,Vgg2 lines measured high impedance and the Vdd1 to Vdd4 lines measured 5 Ohms. After attaching the wires for biasing, the impedance on the lines are the same and there are no shorts to ground. I was wondering if I could get some assistance into why I am not able to establish the pinch off condition.

Thank you for your time