Using HMC981 for biasing an amplifier questions

Dear Sir/Madam,

I am using HMC981 to bias an RF amplifier that operates between 25 to 30 GHz.

The amplifier requires the following

1) Vdrain = 4 volts

2) drain current = 180 mA

3) initial Gate voltage -1 V then it raises for the amplifier to reach the current (typically -0.3 V)


I have several questions regarding my attached schematic

1) In this circuit I am not using VG2 so should I leave pins VG2 and VG2CONT floating ?

2) I found on this forum an attached schematic biasing an amplifier using the DIE version of HMC981 (schematic is attached) , in this schematic a resistor was used to drop the voltage from 5 volts to 4.45 volts by considering the current to be 100 mA , my question is this value of current constant and can I use it to calculate the resistance required to make the required drop voltage as well?

3) In the DIE schematic VDIG was directly connected to VDD can I do the same knowing that my VDD is 4.9 Volts ?

4) Should my "TRIGOUT" pin be floating since I will not use triggering ?

5) in the Data sheet it's written that VREF is used with a capacitor of 0.1 uF for noise filtering in the pin configuration table , however it's always left floating in all application circuits so should I connect it or leave it floating?

6) Regarding VNEG , i added 2 resistors for adjusting the the initial voltage from -2.5 to -1 V but it's written in page 13 in section "VNEG fault detection feature"In depletion mode operation VNEG is continuously monitored against short circuit fault to GND. If VNEG rises above a preset value (typically -1V) the system and the external amplifier are disabled by pulling VDRAIN and VG2 to GND and VGATE to VNEG."

My external amplifier should be biased initially at -1 V and then raises up to -0.3 volts so will this short circuit disabling happen for the configuration in my schematic or will it operate properly?

should I do anything regarding the Pin "DISBLSC" or it's unrelated to this issue?

7) is my attached schematic appropriate for biasing the amplifier using the following sequence or do I need any modifications?

A) Bias the gate to low enough voltage (-1 V)

B) Apply VDS (4V)

C) Raise the Vgate to reach the required current (180mA)


I am very sorry that i have written a lot of questions but it's my first time to use a biasing controller and I really need to find out the answers.

Thank you very much!

Amr Wael


Grammatical mistake
[edited by: Amr Wael at 11:48 PM (GMT 0) on 19 Jun 2020]
  • +1
    •  Analog Employees 
    on Jun 22, 2020 2:28 PM 1 month ago

    Hi Amr,

    1. Yes, VG2_CONT and VG2 can be left floating if not used.
    2. I'm not sure I follow this question, but you shouldn't need to use a resistor to drop the voltage on VDD. VDD should be set such that the drop across the internal FET gives you your desired voltage on the VDRAIN output. It seems as though you've done this.
    3. Don't connect VDD directly to VDIG. VDIG needs to come up before VDD. The voltage is alright though, VDIG must be between 3.3V and 5V.
    4. Yes, you can float TRIGOUT.
    5. I would suggest adding a decoupling cap on VREF.
    6. What's your amplifier part number? It's hard to answer this question without more information, but I would say that if you want VNEG to be -1V, you'll most likely need to disable the short circuit detection by grounding pin 14. 
      1. If you intend to set VNEG to -1V, you can't also set the VGATE threshold to -1V. You need to have some margin to ensure that the circuit will start up reliably. I would suggest having at least 0.3V of margin, meaning that the VGATE threshold should be set to -0.7V. 
      2. If the amplifier can safely tolerate a gate voltage lower than -1V, I would suggest lowering VNEG and the VGATE threshold from the numbers mentioned above. I haven't tried setting these values so high, and I'm not sure if it could present an issue.
    7. See my other comments
    8. Here's the suggested startup sequence. Shutdown should be the reverse order:
      1. VDIG
      2. VDD
      3. VNEG (if applying externally, otherwise ignore)
      4. EN


  • Hi Eng. Weston,

    Thank you very much for your fast reply and for your extremely useful answers.

    1) Regarding (5) in your reply which value do you suggest for the decoupling capacitor on the VREF? should any voltage be applied to this pin or just a decoupling capacitor to GND is enough?

    2)Regarding (6) I am using CHA3395. It's biasing sequence is shown below

    I would like VGATE to be -1V initially so isn't VNEG responsible for setting the initial value of the VGATE before VDRAIN is applied?

    I added the 2 resistors (330k ohms) at the VNEGFB and VGATEFB to make VNEG -1V , do you mean that I should make it -1.3V to be able to get a VGATE of -1 V ?

    3) I would like to ask about the required tolerances and power ratings for the capacitors and resistors used respectively.

    Thank you very much in Advance!


  • +2
    •  Analog Employees 
    on Jun 22, 2020 4:03 PM 1 month ago in reply to Amr Wael

    Hi Amr,

    1. A 0.1µF capacitor to GND should be fine. Do not apply a voltage to this pin.
    2. The suggested powerup/down sequence for the CHA3395 is saying that it will take about -1V to safely start the amplifier. The ABSMAX table shows that the gate voltage can safely be as low as -2V. I would suggest setting VNEG to -2V and the VGATE threshold to -1.7V to ensure safe startup as well as to use the HMC981 in a more "standard" way. To accomplish this, the resistor from VNEG to VNEGFB should be 1580kΩ and the resistor from VGATE to VGATEFB should be 732kΩ.
    3. Please follow the datasheet recommendations for the tolerances and values.


  • Thank you very much! 

    I really appreciate your effort in answering me ! 

    I found the packages for each capacitor and resistor in the evaluation board components List , however , Packages have different power ratings  so I am still curious whether I should stick to a specific power rating to be safe especially that I am not sure of the values of the voltages/currents at most pins ?

    Thank you very much and I apologize for asking too many questions.

  • +1
    •  Analog Employees 
    on Jun 22, 2020 6:36 PM 1 month ago in reply to Amr Wael

    Without going through every external component in detail, just check every component against the expected voltage, current, and power dissipation values and be sure that there is enough margin for safe operation. There isn't anything extraordinary about determining this for the described circuit.