LNA not amplifying - Page 1
Forum section: electronics / rf-microwave

author: oldmannewstuff
author_rank: Contributor
timestamp: September 23, 2018, 02:55:35 pm
content: Hello everyone!I am building a low noise amplifier for wx-satellite reception (137MHz). This is the first time I've built an rf circuit or designed a pcb, so it's not surprising that something went wrongThe problem is my circuit is not amplifying. It's actually attenuating at certain frequenciesThe "performance" is best without the metal box. Then I get some kind of amplification, so clearly the transistors are ok. When the circuit is inside the metal box and the lid is on, it practically only acts as an attenuator.My thoughts so far:The pcb has a single layer, which I found out is a big no no in rf circuitsThe transistors have four legs, two of them are emitters. The emitters are internally tied together, but on the pcb the other one is not connected to anything. Could this somehow mess up with the amplification process?Should I use more capacitors on the power input? I think it couldn't hurt..I modified the original circuit by adding a possibility to use parallel resistors on emitter outputs - to be able to get more accurate resistance if needed. This shouldn't be a problem?The caps and resistors are from aliexpress. All other parts are from original manufacturers.Would it be possible to get this amplifier working on 137MHz?

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 23, 2018, 03:41:51 pm
content: bet you it's oscillating

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 23, 2018, 03:44:14 pm
content: BFP740 on a single sided pcb? This is a transistor that has gain up at 10GHz!!!Edit: you also can't wire up two transistors like that at >1GHz; in a MMIC it's fine but with discrete parts think about all the inductance in the leads and bond wires. With this transistor the emitters must be grounded through a low inductance path, which means multiple vias surrounding the pad and a 4-layer board (trace to ground plane distance must be <=0.2mm, or else via inductance is too high). If you only need VHF the bfp740 is also way overkill; a single 2sc3357 will give you decent gain (20dB), low NF, and high linearity (about 30dBm OIP3).

author: oldmannewstuff
author_rank: Contributor
timestamp: September 24, 2018, 09:25:59 am
content: Thank you for your reply!I should have made it more clearer, that this circuit was not designed by me, but I found it here:https://qsl.net/va3iul/High_Frequency_LNA_schematics/High_Frequency_LNA_schematics.htmIf this project can't be salvaged, maybe I can use the pcb as a keyring

author: xaxaxa
author_rank: Regular Contributor
timestamp: September 24, 2018, 01:21:55 pm
content: I simulated that circuit in rfsim99 and it is highly unstable; both S11 and S22 are significantly outside of the smith chart. I think you'll have more luck with the single stage designs. Even just a single bfp740 (common emitter) with some biasing will perform just fine. If you use a 2 layer board choose 1mm thickness or less, and use at least 3 vias per emitter (ground) pin. If you put copper pour on the top layer make sure to stitch it to the bottom ground plane regularly (or just don't put copper pour on top layer).

author: cvanc
author_rank: Frequent Contributor
timestamp: September 24, 2018, 01:57:49 pm
content: If you're forced to redesign, another transistor (actually a dual gate MOSFET) to consider is the BF998. It's often used in single stage RF preamps. Good luck.https://media.digikey.com/pdf/Data%20Sheets/NXP%20PDFs/BF998(R)_Rev_Oct2010.pdf(it's OOP but not that hard to find)

author: oldmannewstuff
author_rank: Contributor
timestamp: September 25, 2018, 05:23:58 pm
content: Thanks for your suggestions and the simulation!At least for now I don't wish to switch transistors as I have bought these..However I might try that single stage design. Would the basic circuit be as simple as this?edit: The output had to be on the collector side, of course

author: dazz1
author_rank: Frequent Contributor
timestamp: September 28, 2018, 03:37:15 am
content: HiI designed and built a 137MHz pre-amp way back in 1995. See the attached photos. It's old so it doesn't look good but it worked very well.It features a tuned input and output to improve off-channel frequency rejection from other RF sources (like FM radio stations).It has 2x BF981 connected in parallel. Each one was connected to the antenna output. I designed a quadrifilar helix antenna to receive the circularly polarized signal from weather satellites.The parallel connection of the BF981 shared the received signal between them and improved the 3rd order intercept performance. It also avoided the need for a balun between the antenna and the amplifier inputs. The amplifier was powered up the coax.If I still have the PCB design, it will be on film, not CAD. I think the circuit was one I got published in Electronics & Wireless World. If you are interested, I can see if I can find my original circuit diagram.Your best option might be to start with an Aliexpress $6 LNA and experiment with that. The problem with these is that they are not tuned. Large off channel signals can swap the weak wanted signal.All metal enclosures can cause problems. The problem is that they don't attenuate the signal inside leading to oscillation. My enclosure on this LNA has open ends to allow rf energy to "escape". A common alternative is to install rf absorbing material (ferrite) inside a the enclosure to soak up rf energy.The enclosure is also quite large compared to the components to provide good separation from the components. There is also a wall between the input/output sides to avoid feedback causing oscillation.Dazz

author: oldmannewstuff
author_rank: Contributor
timestamp: September 28, 2018, 11:17:31 am
content: Quote from: dazz1 on September 28, 2018, 03:37:15 amHi!Nice buildIf it's not too much of a hassle for you, I'd like to see the schematic.I know it would be simpler, cheaper and faster to just buy a pre made LNA, but I'd like to build one myself and maybe learn something in the process

author: jujun
author_rank: Regular Contributor
timestamp: September 28, 2018, 11:34:17 am
content: You can build a quick LNA with a PGA-103 for example.https://ww2.minicircuits.com/pdfs/PGA-103+.pdfBasicaly if you follow the Recommended Application Circuit of the datasheet,and use 50ohm matched traces, it should be quite good

author: oldmannewstuff
author_rank: Contributor
timestamp: September 30, 2018, 03:31:42 pm
content: Small update on the project... I modified the board so that it uses only one transistor, and got about 10db gain on 20MHz - 440MHz. I guess that's the best I can get from this pcb

author: dazz1
author_rank: Frequent Contributor
timestamp: September 30, 2018, 07:57:44 pm
content: HiIf you are planning to use this as a mast head amp, you don't need or want a lot of gain.If you have too much gain any strong signals within the (wide) bandwidth of your amp can over-drive it. You will see this as a drop in gain of your wanted signal.10dB is about right for most applications.What you really want is low noise and high dynamic range.Noise on the first stage amplifier (or noise from cable loss) dominates the noise you see at the output.High dynamic range will amplify weak wanted signals in the presence of nearby strong signals.Dazz

author: Teledog
author_rank: Regular Contributor
timestamp: October 01, 2018, 05:10:08 am
content: We used to use MSA-0xxx or MAR MMICs for quick & dirty RF Rx amp prototyping.I still use them for cheap/fast Rx amps..cut & strip.. on plain 1 or two sided copper boards, easy for VHF, slice/trim the strip as you watch the SA sweep.Can run 2-3 in series, but watch for feedback/oscillation.Easy calc here;https://www.changpuak.ch/electronics/mar_era_bias.php

author: eddiesomesan
author_rank: Newbie
timestamp: March 20, 2019, 05:51:58 pm
content: Quote from: jujun on September 28, 2018, 11:34:17 am@jujun.Can you please give me some more details about the matched traces? I suspect it`s about a coplanar waveguide with ground, but I`m not sure. At this frequency you need a ground plane and a lot of stiches.Quote from: dazz1 on September 30, 2018, 07:57:44 pm@Daz, please give me some details on how can we obtain the low noise and high dynamic range. Also how can we decrease the gain. I want to use the PGA-103+ because of it`s low noise.None of the LNA`s I have found for sell are suited for me, so I think I`ll just build one. But I want to make everything good from start.Thank you.Edy

author: PaulAm
author_rank: Frequent Contributor
timestamp: March 20, 2019, 10:12:45 pm
content: Here's a nice article on using the PGA-103+http://www.w1ghz.org/small_proj/Simple_Cheap_MMIC_Preamps.pdf

author: in3otd
author_rank: Contributor
timestamp: March 21, 2019, 07:44:58 am
content: Quote from: cvanc on September 24, 2018, 01:57:49 pmyes, that was a nice device for VHF preamps, but please try to get it from a reliable source: since it was quite popular you can even now find plenty of them on eBay and such - most of them there sorta work but with low gain and high noise. Don't ask me how I know...Even if xaxaxa has shown that your preamp should be unstable you may try adding a ground plane below the PCB with copper tape under the signal path; make sure to connect it to ground at least at the connectors. If you are lucky the FR4 losses and the phase of the Moon may keep it stable.But a single stagetunedamplifier makes more sense for your application.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 22, 2019, 08:24:08 am
content: Quote from: eddiesomesan on March 20, 2019, 05:51:58 pmAs @in3otd said, a single stage tuned amp is what you need.Google 3rd order intercept to find out how to rate the dynamic range of an amp.Low noise and high 3rd order intercept are generally more important than gain.In general, discrete device amps have better performance because of their higher supply voltage (gross simplification).