Amplifier IC needs -8V to start.  Necessary? How to implement - Page 1
Forum section: electronics / rf-microwave

author: Stinson108
author_rank: Newbie
timestamp: April 28, 2022, 04:45:16 pm
content: I'm designing a PCB that includes Analog Devices' HMC1099PM5E, a 40 dB amplifier. The data sheet recommends device start-up and shut down with a pinch voltage of -8 V applied to its ground, and to operate the device with -2.5 V (below the system ground). How crucial is that? Do I need a buck converter just for this? Is there an alternate approach?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: April 28, 2022, 06:21:04 pm
content: Yes, you must follow the power-on and power-down sequence exactly as it is in the datasheet.It's basically an N channel depletion type FET so if you don't negative bias the gate the drain will be effectively a short to GND and the current from the 28V supply will make it go bye-bye in a flash.Moreover for these type of devices it is preferred to have some sort of current sense feedback loop to compensate for variance, aging and temperature. It might be difficult to do that automatically but at least a potentiometer manual setting is required so that the startup quiescent current is as recommended.At my previous job we used TI e-fuses to control the drain voltage to ensure it's never on while the gate is not in a set negative range. (e.g. negative PSU didn't turn on, etc.) It can get pretty complicated with peak current limiting and especially the startup dynamics of that, handling overshoots and that sort of thing.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: April 28, 2022, 06:25:25 pm
content: This gives a good overview:https://www.analog.com/en/app-notes/an-1363.htmlI think ADI doesn't have a ready to use bias controller for GaN as they have for GaAs (HMC980, etc) but the principle is similar.

author: Stinson108
author_rank: Newbie
timestamp: April 28, 2022, 06:56:11 pm
content: Thank you so much Marsupilami --Your citation above put me on the right track. I'd hate to let the magic smoke out of the device; it isn't cheap.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 28, 2022, 07:50:04 pm
content: Ampleon AN11130has another example bias circuit which you could try. The hard part is setting the correct bias voltage. Their strategy is for it to be fixed, but with a temperature compensation circuit. The trouble is that you can only easily set the bias while there is no RF input. When there is an input, the drain current will greatly increase.... so you need some sort of automatic function at startup (perhaps with a microcontroller), or use temperature compensation and manually adjust it periodically.I have not built their circuit, but I've built a similar one with a charge pump and LDO for the negative bias, plus a "power supervisor IC" to verify the negative bias voltage, and when it is established, enable a pass pFET to apply drain bias.The gate current will usually be low enough that you can source it from a charge pump, although if you really overdrive the amplifier (with RF), the gate current can increase. Switching converter modules (e.g. from RECOM or CUI) could also be used to generate the negative bias voltages (but be careful since they usually have a minimum load current).I see there are also some all-in-one GaN RF amplifier controller modules available (like MACOM MABC-001000-DP000L).Applying Vg=0V for a few milliseconds likely will not hurt anything, but may eventually overheat the transistor....

author: Marsupilami
author_rank: Frequent Contributor
timestamp: April 29, 2022, 02:32:25 am
content: Quote from: pigrew on April 28, 2022, 07:50:04 pmOh yes, same same, that's what I meant with the e-fuse. I'm guessing there are a couple of design patterns only for these.@Stinson108I found a public sch from X-Microwave of a board we used before.https://www.xmicrowave.com/product/XM-C121-0810D/(click on the little thumbnails to find the sch)This thing has a TI e-fuse with built in fet switch (U1) to control and monitor VDD and IDD. A non-volatile i2c DAC (U3, this also could be a pot) provides the VGG setpoint that's modified by the D2 diode as temperature compensation. U4 comparator with reference checks for adequate negative supply and only disables the switch turning off VDD if it's not ok. Also the whole thing is gated by the fault output of the efuse so if there's a VDD overcurrent or VDD undervoltage then the gate is pulled back down to the negative rail to turn off the amplifier.I don't really know how good or bad this particular implementation is but I think it's a good illustration of the concept.