Re: Matching LDMOS device
Thank you Greg, Geoff and Robin for your helpful replies.
The original device was a BLF6G15L-40BR, internally matched for use between 1450-1550MHz. Rated at 40W RF out it was designed for ultra-linear operation at 2.5W output with ACPR of -45dB. It even has an on-die sense FET that looks as though it was used to stabilize the bias, judging by the PCB tracks.
I presume the impedance I’m seeing on theVNA will be substantially modified by the matching for the original device; the substrate is very thin and the tracks wide so I’m guessing it’s very low Z, causing it to be a long way from the 1 ohm Greg suggests that the MRF184 might present at 1.3GHz. I’m not competent to work out the Er of the substrate from the matching of the original device from the track dimensions I can see and the known characteristics of the original device from its datasheet, but somebody with a lot more experience might be able to work it backwards.
Geoff, the Z I’m seeing is with the new MRF184 fitted. It doesn’t make much difference if it’s biased of not (Vds 28v, Ids 300mA)
The original device is only characterized at its intended frequency as here:
Table 9. Typical impedance per section
IDq = 330 mA; main transistor VDS = 28 V
F ZS ZL
1480 3.2- j6.3 4.6- j4.5
1510 4.4 -j6.5 4.6 - j4.5
I’ll have a look at Puff and Robin’s suggestion, SimSmith. It should be possible to run Puff on a guest VM running 32bit Windows in one of the Windows 10 machines.
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Greg - ZL3IX
Sent: 15 February 2021 19:44
Subject: Re: [UKMicrowaves] Matching LDMOS device
I'm not a PA specialist, but have designed one or two in the distant past. I believe that you need to use the impedance characteristics under drive at a typical power level, and these are not usually the same as would be the case under small signal conditions, ie when measured with a VNA. Making impedance measurements under drive is not a simple matter, and you may do well to take the manufacturer's data, here https://www.nxp.com/docs/en/data-sheet/MRF184.pdf. Admittedly it only goes to 1 GHz, so you would need to attempt extrapolation up to 1.3 GHz. Even at 1 GHz the plot looks like around 1 ohm for Zin, which is way different from the 13 ohms you measured on your VNA.