Hi is it possible to set Vgate to -0.45V and Vneg to -0.6V? As written in DS a preset value of Vneg=-0.6V would cause it to stay in stand-by mode.
I set R5 200Kohm and R7 166Kohm. Is this correct? ref to cct below
Active bias controller is meant to bias the DUT like an externally biased amplifier or FET. Rsense value is calculated for desired Idd, then HMC980 monitors that current and adjusts Vgate.
Those resistors will not make HMC980 to supply constant Vgate level. Operating the bias controller for constant Vgate is not really feasible and kills one of the main purposes of this device.
During power up Vgate output goes to a default value of approximately -2.5 V. Then starts to increase to set the desired Idd. Some DUT`s absolute min rating for Vgg input is above -2.5 V, e.g. -2 or -1.5 V. To operate the DUT is a safer region, you can use those R5-R8 on the SCH you shared, and set that default min value to something above -2.5 V, i.e. -2 or -1.5 V. The logic is similar with Vneg. Having an offest between Vneg threshold and Vgate threshold would be better to compansate the part to part variations effects of the resistors. e.g. If Vgate threshold is -2 V, then Vneg threshold can be set to -2.2 or -2.1 V.
See below content from an upcoming App Note on bias controllers.
ADJUSTING DEFAULT VNEG and VGATE THRESHOLD VALUES
The typical VNEG value is -2.46 V, as seen on Figure-15. Due to the internal logic that exists inside HMC980LP4E, this default value limits Vgate output voltage swing capabilities of HMC980LP4E.
Figure 15. Default VNEG value
With default configuration, typical Vgate output swing is in between -2 to 0 V.
Adjusting default values of VNEG and VGATE by external resistors solves both of the listed problems. Resistors R5, R6, R7 and R8, shown on Figure-16 are used for this purpose.
Figure 16. External resistors for adjusting default VNEG and VGATE values
If desired Vneg < -2.46 V,
If desired Vneg > -2.46 V,
If desired Vgate < -2.46 V,
If desired Vgate > -2.46 V,
Where, E1=50, E2=262, E3=1.44 and E5=0.815.
During power up, VNEG is enabled, if VNEG reaches default value of -2.46 V; hence VNEG value should be lower than VGATE.
It is recommended to configure HMC980LP4 for VNEG values higher than -3.5 V.
then by above formulas, R5=631 kOhm, R7= 477 kOhm, R6=R8=Open.
then by above formulas, R6=221 kOhm, R8= 303 kOhm, R5=R7=Open.
Is it possible then to set Vneg as -0.6V as my DUT required Vgate to be -0.4V.
Therefore: Vneg :R6 open ; R5=200Kohm
& Vgate: R8 open ; R7=156Kohm
By this using the above values, would Vneg be adjusted to -0.6V? (I’m unsure if Vneg can be set to -0.6V) As it is written in the datasheet that a -0.6V would set the active bias controller into standby mode.
I had tried setting the values as such, but I’m unable to obtain the required Vgate for my DUT.
I’m aware that HMC980 do not supply a constant Vgate current however I should be able to measure a initial value of -0.4V at the Vgate pin of the HMC980 before loading it with my DUT right?
Thanks in advance
As you know, those external resistors are used to set the threshold value of the Vneg and Vgate pins.
We have not tested the Vneg threshold value at lower than -0.8 V.
Typical GaAs amplifier can handle -1 V Vgate voltage. Is there a reason why that you are not setting the threshold to -1 instead -0.6 V? If your DUT can handle -1 V, and there are not any requirement that limits you, I suggest to go with -1 V threshold value.
When you test the bias controller without a DUT, the feedback loop for Vgate control is not closed. Therefore, the Vgate output should hit its limits given at the Electrical specifications table of the datasheet. So It can even be a positive voltage value.
Does these formulas work for HMC920 also?
Yes, they do work for HMC920.
The constants for HMC980 and HMC920 have minor differences. We use HMC980`s for both, and tune further if needed.
Retrieving data ...