EMRFD Message Archive 5817

Message Date From Subject
5817 2011-02-05 19:30:07 g0kla Common Gate FET front end filter
I've built the front end for the R2 "pro" from fig 9.64. I only get about 5dB of gain at 14MHz (rather than the 13dB stated in the text). I'm sure that's not an issue, but it got me analyzing the circuit and trying to understand what is going on.

There seems to be plenty of gain from the source to the drain of the FET, but once I measure the power at the output of the low pass filter network (with a 50ohm power meter), then the gain overall for the circuit is low. I replaced the 27pF capacitor with a trimmer and peaked it, but 5dB is the most I can get. I used -30dBm input in the test. A loud signal I know! But I need to be able to measure it.

I put the output network into ELSIE and was surprised to see that it has about 8-109dB of attenuation, before the low pass characteristic kicks in, then the attenuation is much higher. I used 2k ohm as the input impedance (not sure if that's a good value for a common gate FET output) and 50ohm as the output.

I noticed that the transmission dB of the network is mostly effected by C4 and L3, which must match the input impedance, I suppose. If I increase the value of L3 to 5.3uH then I can get 0dB attenuation at 14.1MHz in Elsie. This is not reproduce on the bench however. Endless playing with the values of C4 and L3 on the bench have not produced better gain.

Am I thinking about this the right way?

5819 2011-02-06 09:15:20 drmail377 Re: Common Gate FET front end filter
Hello Chris,

I have no experience with the R2Pro (and variants) in the shack, but I have followed the subject over time. Maybe some of the following links may prove useful (links below are working as of post time, but YMMV in the future of-course):

* There is an R2Pro Yahoo Group here:


* Gary, WB9JPS has a couple of very interesting papers regarding the performance of the R2Pro LNA, especially for multi-band use. These papers are quite impressive. The WB9JPS homepage is here:



Then go here:


The two-part R2Pro LNA Performance papers are downloadable .pdf's here:



* There is information regarding the IQPro LNA's (closely related to the R2Pro LNA's) here:


* Finally, there's always Todd's (VE7BPO) site with a plethora of info on measuring the characteristics of RF stuff here. Go through the RF-Workbench series:


I hope this informati
5820 2011-02-06 13:57:02 kb1gmx Re: Common Gate FET front end filter
5830 2011-02-08 19:52:44 g0kla Re: Common Gate FET front end filter
Thanks for the links David. Interesting stuff. I need to read through it all.

Thanks Allison. Before I updated the model, let me be sure I understand. I think 1/gm would be the input impedance for the FET, ie 80 ohms ish for the J310. I had modeled that as just 50ohms. I was using 2000ohms as the output impedance of the FET, and as the input to the 7 element filter between the FET and the output.


5831 2011-02-09 08:35:14 drmail377 Re: Common Gate FET front end filter
Hi Again Chris,

Have you tried modeling the R2Pro front-end in the likes of LTSpice (www.linear.com)? I think I have an LTSpice simulati