HMC980LP4E to bias HMC863ALC4

Question

Hello Community, 
 as mentioned in the subject I am trying to bias the HMC863ALC4 with the HMC980LP4E. I tested the functions of the HMC980 and they seem to work as expected without any amplifier connected. I could measure: V_drain ~= 6V V_gate = +2.8V (if EN high) V_neg = -2.44V V_gate = V_neg (if EN low) V_dd = 6.04V 
 Now even though the HMC863 is connected there are no changes to the voltages. 
 Below you can see the schematics of both HMC980 and HMC863. 
 The diode D1 is a Schottky-diode as on the eval board (BAT54SLT1G). So that's not a problem. I also checked if the current limiter of the power supply kicks in, but that's neither the case. 
 Would really appreciate your help. 
 Regards Jan

Jim · Accepted Answer

Hi Jan, 
 If the current isn't set to 540 mA, you won't make PSAT. The value of current chosen depends on how much Power you need. If you set the current for 350 mA, you will only be able to get ~ 12 dBm Pout. See graph below. 
 The HMC980 holds the drain current at the programmed value so, you need to determine the output power needed and set the resistor values according to that current value. 
 Which means the device will always be consuming the programmed current event at very low signal levels. 
 R5 &R7 are feedback resistors which close the loop allowing the HMC980 to operate. The HMC980 is a closed loop system. 
 Also, you mentioned you tested the functions of the HMC980 without the amp. It doesn't go into operation unless the amp is connected. 
 Regards, 
 Jim B