Author Topic: RF transistor amplifier design, any suitable literature?  (Read 22138 times)

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Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #50 on: September 08, 2017, 08:40:35 pm »
Well now I am a bit confused about designing the output match. You write:

Quote
. So a typical L match from 100R to 50R at 1620MHz would have 10nH as the collector choke and a series 2pF cap to your 50 ohm output.

But where did the S22 of the transistor go into that? If I  use 10nH collector choke plus 10ohm in series, combined with S22 of (0.217  -174°, that is 32.2-j1.55ohm)  results in 29.3 + j7.69 ohm, that has to be matched to 50ohm.   Or did I miss something?

 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #51 on: September 08, 2017, 09:09:41 pm »
In reality, the 10nH in the simple L match will need to be tweaked (down?) because the transistor will have some self capacitance. This self capacitance will make a 10nH choke look bigger than it really is at 1620MHz.

Also, the transistor obviously doesn't have perfect isolation from input to output so tweaks to the input match will upset the output match and vice versa. A bit like jelly wrestling. That's why it's important to use a decent simulator that can look at this stuff in real time. So changes at the input can be optimised for the optimal output circuit.

I'll see if I can find time tomorrow to set up a harmonic balance analysis.This is a frequency domain analyser that can display non linear behaviour (in a steady state). It can show real time (small signal) matching and can also show steady state waveforms in the time domain.

Note that your 2SC5773 isn't a good choice to play with here. I've got to go now... I've lots to do tonight... sorry...

 

Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #52 on: September 08, 2017, 11:04:07 pm »
I already got it, that the C5773 is crap at 1.6GHz. But still I'd like to try design the amp with it, and then have a compare with BFP182 I have available also.

Currently sitting in front of the RFSIM99 and toying with it. I will definitely love this thing! The creators definitely took a piss to make a very good UI for their software. It took me just minutes to learn how to use that thing. Quite spectacular compared to the today's complicated anti-intuitive software full of shiny colors and animations...

I put in S params of the BFP182, calculated input match manually and simulated. Well, I wasn't very far off (still the collector circuit was just 50R terminated, need to design the collector circuit somehow).

When adding the 10nH+10R collector load, the automatch in RFSIM99 will generate two component matching network. Same as I thought it should have been used.  :-//
I also toyed a bit with the "emitter whiff" (:) ) and been watching its effect on the gain. The theoretical maximum with the BFP182 being 12dB under ideal conditions, I think one could expect like 8dB from a real design. (looking forward to measure a real prototype, huh!)

Will need to find the touchstone files for the C5773, or to make my own based on the coarse datasheet tables. Just to be able to compare the performance of the two transistors.

//EDIT: I have made the SnP files for the C5773 myself, based on the tables in the datasheet. Haven't found anything more detailed from the manufacturers. Then tried to simulate this transistor at ideal conditions: 8dB of gain. At maybe more real conditions, about 5dB to be expected it seems.
Attaching the files if anyone interested.
« Last Edit: September 08, 2017, 11:59:19 pm by Yansi »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #53 on: September 09, 2017, 02:27:08 pm »
I put together a quick harmonic balance (combined with linear) simulation using the BFP420F and see below for the graphs etc. The gain was good at about 15dB and it also shows that the Vswing of 2Vpk at the collector is as expected at +13dBm (shows little distortion from a sine wave with harmonics at about -20dBc in the spectrum plot) and the input and output match is quite good. The 2pF cap is there at the output but the 10nH inductor at the collector is down to about 6nH because of the capacitance inside the transistor and also the series inductance in the decoupling cap and also inductance/loading effects in the transistor. So it has shrunk from 10nH to about 6nH.

I've used a non linear model of the BFP420F here and this generally isn't as accurate as a decent s2p model when it comes to small signal matching. But when I dropped in an s2p model at 3V 30mA the results for the match were very close. But I think to get the best results for a 'real' circuit the effects of the PCB layout will need to be modelled using Sonnet and the various matching components adjusted accordingly.

It looks like you are getting to grips with RFSIM99. It's a shame that the developer stopped working on that software project because it had great potential.
« Last Edit: September 09, 2017, 02:40:50 pm by G0HZU »
 

Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #54 on: September 09, 2017, 09:07:56 pm »
Yes, the RFSIM99 is a very friendly app.

So finally, I cobbled together two experimental amps, one with 2SC5773, the other with BFP182. Neither of them worked as expected, but I did not expect the performance of both to be so piss poor.

The input shunt capacitor for matching may work in theory (in the simulator and on paper), but not in practice.  I could get like ~2dB of gain at 1627MHz from the 2SC5773. After desoldering the 2pF input shunt, gain rose to about 4dB. Well, that at least a bit does what the theory thinks it should do.
However the big disappointment was the BFP182. Couldn't get more like 2dB of gain, even without the input shunt cap.

So something has been done very, very wrong. I checked biases of both amps an they seem to be correct (apart from the slight variation due to hFE dispersion), not far away from the design goal. I am suspicious about two things: 1) Very bad layout, 2) Very bad inductors.  I used some not so known type of inductors I got in a set some years ago and they might have very crap performance at these frequencies. Combined with the bad PCB layout, it is a recipe for a disaster. What do you think?
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #55 on: September 09, 2017, 10:42:26 pm »
The layout doesn't look great and the circuit seems odd because you have put a resistor and a cap in the emitter of the BPF182. However, it might be possible to salvage some performance with the BPF182 version...

Can you make it look like the modified image below? This now has the emitters grounded directly. You have to be careful how you bias it now via the separate Vbias line, start with a volt or two at Vbias and bring it up until the collector current goes up to maybe 20mA.

You need to drill and fit ground vias where there are red dots and use a separate Vbias supply. Also, run Vcc at 5V to start with. Because you can run this device at a higher voltage, the 2pF cap at the output can be smaller and this will reduce the bandwidth but the gain should improve.
Also note that some components have been removed or moved elsewhere. Also a lot of component values have changed. Also the collector choke is now just a bit of very thin wire bent up in the air like a hairpin. You could make this wire from a single strand taken from some 702 hookup wire. This way it will be easy to bend and tune without ripping pads off the PCB. Push it away from vertical towards the PCB to reduce the inductance. This way this inductor can be tuned in circuit. Or you could make a tiny solenoid inductor of about 7nH here instead.

I can't guarantee this will help but you should see some more gain as long as you don't mind taking a scalpel to your PCB and making the changes shown.

Note that the bias method here is very basic and not suitable for a final design as the bias point will be very temperature sensitive and also will vary according the device beta. But you can use this crude method during development as long as you swap over to active bias for the final design.

Thinning down the trace where shown will make this section look like a series inductor. Trim it down if it seems to help with S11. But do this bit last.


« Last Edit: September 09, 2017, 10:51:43 pm by G0HZU »
 

Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #56 on: September 10, 2017, 09:43:52 am »
Oh well. Rather than bastardizing the board, I can make a new one. I base the layout of the BFP182 board on the piece of gear I have reverse engineered. The amp there is working slightly below 1000MHz, so I guessed the layout will be at least a bit good for 1600MHz too. But maybe not. Attaching a detailed photo of the original amp.

So are you telling me, that to get at least some decent performance at 1600MHz, one can not use emitter degeneration (resistor + bypass cap) at all?   If that's the truth, then okay, I get it. But that brings another question: How to bias that little bastard. Having to manually tweak two power supplies may be good enough for prototype but certainly not for a final product.

How to bias a common emitter stage with its emitter directly on GND?  I can see a few ways to do it:

1) Resister in between base and collector (as used commonly in AF amp) , but as there is zero to non DC resistance in the collector load, this is not gonna cut the mustard.

2) Resistor from base to supply voltage. Will, work, seen that done even in the appnote AN155 from Infineon, but I think this is very "meh", will drift like crazy with temperature, as the hFE changes with temperature and also variations between different transistor's hFE makes this very unfriendly for production of more pieces. Every amp will need to be tweaked separately.

3) The last resort I can think of is to close a feedback loop for the bias. I think of a PNP transistor on a collector current shunt, controlling the base current. This should make the bias current very stable (up to the point of the Vbe tempco of the PNP). But will this work? Isn't that overkill? Of how is it done in real designs?

I will need to try simulate what your change to my circuit does first and to understand it. Now I have to go have a lunch. I'll continue afterwards...
 

Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #57 on: September 10, 2017, 10:35:48 am »
So I tried to replicate your suggestion in the RFSIM99. It seems it does something, but the return loss is not very good, about -10dB.  (Note: Lowering the output cap to 1.2 puff makes it slightly better to -15dB S22, but S11 still -10dB).  Lowering the emitter whiff also makes it better. How did you come up with the input matching circuit?
 

Offline T3sl4co1l

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Re: RF transistor amplifier design, any suitable literature?
« Reply #58 on: September 10, 2017, 10:52:33 am »
Well, the large supply of vias sure stands out to me. ;)

The lack of any connection between connector housings and top side ground also looks rather suspicious, but maybe that's more of a noise problem than a gain or oscillation problem.

Emitter bias: it's fine, as long as you have enough bypass caps and vias to keep the ground return length sufficiently short.

Otherwise, you end up with a significant amount of reactance there, which is degenerating the transistor.  Or often, making it oscillate!

If you can't make the emitter stub length short enough, then you have no choice but to ground it to plane, directly (red dots in marked-up picture above), and use a collector resistor for bias feedback (as discussed earlier).

BTW, regarding port impedances: when 1/s12 << s21, the amplifier has high isolation, and the port impedances are very nearly s11 and s22, respectively.  When s12 is relatively large (which is often the case in common emitter amps and such), the feedback is significant, and this affects the port impedances as well as the stability.  A wideband amp with low dB's gain shouldn't have too much problem, but this is significant in tuned (narrow band) amplifiers, where you're trying to push as much gain as possible (indeed, you may then be trying to push the maximum stable gain, another parameter to be aware of!).

I would hope RFSim is taking this into account, so you should be fine there.  Just to add this as FYI!

Note that inductors on the order of 7nH are just a few mm of thin trace, where "thin" is Zo some ratio above the system impedance (i.e., a reasonable inductor is a length of 100 to 150 ohm trace, in a 50 ohm system).  When specified in these terms -- as transmission line lengths -- you should get even more accurate answers from RFSim.  Hence why you might want to scrape at some traces, or beef them up as the case might be. :)

Indeed, you can express the emitter impedance, and everything else, that way: as transmission lines.  I don't know if you can include all those components around the transistor, in RFSim, to create a model that is well representative of the physical circuit, but if you can, it should provide more insight into how your circuit differs from expectations. :)

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Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #59 on: September 10, 2017, 11:18:19 am »
With transmission lines, it definitely looks more interesting. I had to change the values a lot.

I know (can calculate using tools (SaturnPCB toolkit) and verify with a little help from formulas here) that a line of 10mil width (Which I am able to etch at home with very repeatable results) has ~105ohm Zo on my substrate, which means about 6.1nH per 10mm.

I could use PCB inductors (transmission lines) into the design as well, that is a very good idea! As mentioned above, putting transmission lines into the circuit I had to make a lot of changes in the simulator, to make it fit back where I want. (1627MHz and a decent match).

Here's the result. I think it looks decent (but still may be not corresponding so well with reality).  Match is at -20dB,  S21 being reported 9dB, which would be great if it would work that way in reality.

« Last Edit: September 10, 2017, 11:23:36 am by Yansi »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #60 on: September 10, 2017, 12:16:13 pm »
Quote
How did you come up with the input matching circuit?

In a hurry :)

See the very basic sim below.

I spent more time doctoring the PCB image than playing with the sim because the sim is very crude and doesn't model the PCB layout. So I slapped in some standard cap values and I guessed the inductance and the amount of trace thinning required. But these will need to be tweaked to suit the PCB layout.

Note that the transmission lines in the sim below are actually FR4 (0.031" PCB thickness) microstrip models rather than just transmission lines. In an earlier post I did suggest you either use the raw transmission line model in RFSIM99 or better still would be to use Sonnet Lite to model the microstrip. You can probably do the whole thing in Sonnet Lite if you can live with the clunky user interface. ... but I don't know the limitations of Sonnet Lite. Will it allow s2p imports etc?

« Last Edit: September 10, 2017, 12:24:25 pm by G0HZU »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #61 on: September 10, 2017, 12:37:58 pm »
Quote
But that brings another question: How to bias that little bastard?

The last resort I can think of is to close a feedback loop for the bias. I think of a PNP transistor on a collector current shunt, controlling the base current. This should make the bias current very stable (up to the point of the Vbe tempco of the PNP). But will this work? Isn't that overkill? Of how is it done in real designs?

Have a look at the applications section in the datasheet for the classic ATF-54143 PHEMT. Although this is a FET device the active bias circuit can be adapted to suit your requirement. It's worth reading this section in the datasheet because it gives a worked example on active biasing.
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #62 on: September 10, 2017, 01:03:46 pm »
Quote
I base the layout of the BFP182 board on the piece of gear I have reverse engineered. The amp there is working slightly below 1000MHz, so I guessed the layout will be at least a bit good for 1600MHz too. But maybe not. Attaching a detailed photo of the original amp.

So are you telling me, that to get at least some decent performance at 1600MHz, one can not use emitter degeneration (resistor + bypass cap) at all?

I looked at your commercial board a few days ago and I think the key design drivers for those amplifiers are to get modest gain at 1GHz with fairly low P1dB, but to also achieve decent reverse isolation and good stability across a wide frequency range.

I don't think you can adopt the same emitter 0603 R and C in your amplifier at 1620MHz using a BFP182 if you want to achieve a decent gain. eg 10dB or more. But you might be able to use 0402 parts here and a very tight layout with the ground vias fitted very snugly. However, the primary reason I said it was odd that you had used them in your own PCB was because they weren't there in your early RFSIM99 simulations at 1620MHz. You just had an inductor there.... although it did seem to have a lot of inductance.
« Last Edit: September 10, 2017, 01:14:00 pm by G0HZU »
 

Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #63 on: September 10, 2017, 01:20:39 pm »
Quote
But that brings another question: How to bias that little bastard?

The last resort I can think of is to close a feedback loop for the bias. I think of a PNP transistor on a collector current shunt, controlling the base current. This should make the bias current very stable (up to the point of the Vbe tempco of the PNP). But will this work? Isn't that overkill? Of how is it done in real designs?

Have a look at the applications section in the datasheet for the classic ATF-54143 PHEMT. Although this is a FET device the active bias circuit can be adapted to suit your requirement. It's worth reading this section in the datasheet because it gives a worked example on active biasing.

Thanks for the tip, will read it. I have only skimmed through and do not understand why the PNP has the current monitoring shunt in its emitter, instead of base. Guessing so the shunt can be designed with any voltage, not Vbe only.
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #64 on: September 10, 2017, 01:59:41 pm »
It does explain how it works in the text. Best thing to do is study the worked example and then simulate the system in spice over temperature etc.

One classic/amusing aspect of RF design is that if you gave a junior engineer or a student a task to design an RF amplifier using a fast BJT they would often produce an oscillator up at maybe 4GHz. They would then spend considerable time trying to tame it.

But if you asked another student/engineer to actually design /create an oscillator up at 4GHz they probably wouldn't know how to do it and if they tried they would typically fail to get it to oscillate.

 

Offline cdev

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Re: RF transistor amplifier design, any suitable literature?
« Reply #65 on: September 10, 2017, 02:28:34 pm »
I would like to share my experience making cheap LNAs with commodity off the shelf parts for use with my SDR collection (for receive only)

This was and remains a easy cheap way to learn and make ones self a useful RF gain stage for almost nothing. MMICs simplify the process so much its really a phenomenally easy thing to do to use them. The various lessons I learned were invaluable in jump starting my electronics hobby.

Making my own PCBs I found that making the classic two sided design was asking for trouble even if I made great effort to add lots and lots of vias.

The first ones I made tried to imitate sent away boards but none of them worked very well.

I realized then that having a copper ground plane near to both sides of the trace on the top made them much more prone to problems so I ditched most of the top ground plane. key to success was putting a bunch of drilled through vias literally right underneath the gain device. Also I angled the device by 45 degrees to make the trace a bit shorter and straighter, so the input on one corner and the output on the other were closer to the coaxial connectors.

All of my LNAs ended up being quite tiny.

It was pretty easy and a lot of fun and even though they were small and simple, in the process of doing this I learned essential lessons in RF design fast. My first attempts were really bad as I was trying to use copper and perfboard instead of etching a PCB. They were also much too large for what I was doing. The first ones only worked up to around 500 MHz. The devices I ended up with were not good enough to keep, I redid them. I figured out the following which seems to be a good approach for people who want to make their own boards.

If I confined the signal path to a direct - short - straight trace of appropriate width on the top of the board and ran power to it on the top also, and did not put a ground plane on the sides of the top except where I had to, as part of the bias tee/power -keeping it short and using lots of bypass caps, it worked well.

I used edge mount SMAs and used a ground plane on both sides under them, at both ends but did not have any ground at all along at least one side, and on the other the grounds were used for anchoring bypass caps exclusively and the area was minimal - also I used lots of vias under the MMICs.  I made one attempt at a two stage LNA but it diidnt work well, however I did have success connecting two LNAs together when I used separate power supplies for each of them. (and not the same low voltage post-regulated power) These were very simple tiny boards. Typical size 25 mm long, 17 mm wide. The MMIC handles all the biasing and all I had to do was supply it with clean DC between 3-6 volts, either via a bypass cap through the case or via a bias tee using the coaxial cable, and block DC.

This approach is ridiculously easy and saves a lot of money. I ended up with good LNAs optimized for different uses-otherwise I would have had to spend several times what I paid for all the RF ICs for just one LNA.

The LNAs I made all have extremely decent gain and very low noise. One thing I found is you have to clean them really well, remove all the flux - if you do that and don't connect the LNA right to the antenna, use at least a short pigtail (10 cm will do) on both sides, which I think smooths out the impedance a bit, (putting a ferrite bead there around the coax to reduce common mode signals is good too) they almost always work quite well. Gain was likely consistent with manufacturers claims which are roughly 15 db gain from 25 MHz-4 GHz -

Seems to me gain continued both upward and downward without any radical changes so these devices likely would test out to be functional on pro test equipment.

I have used this same device for an optimized broadcast FM LNA using 75 ohm antenna and connectors and there it works extremely well too. Grounding the device well and simplicity seem to be the key to getting the best out of them.
« Last Edit: September 10, 2017, 02:55:06 pm by cdev »
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Offline T3sl4co1l

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Re: RF transistor amplifier design, any suitable literature?
« Reply #66 on: September 10, 2017, 02:36:26 pm »
Difference being, one is stable and the other isn't.  Before I tamed this thing,



(uses BFR92AWs), it was squealing somewhere up over 2GHz.  Changed with proximity, so, it was also a rather overly sensitive Theremin.  I mean really, my Theremin (proper) only moves 10kHz when I touch it, but a whole GHz? :-DD

(After fixing it up, by the way, it's a solid 20dB gain, apparently pretty flat from a few MHz to over 700MHz.  And has suspiciously low noise of 1.7 nV/rtHz.)

Tim
« Last Edit: September 10, 2017, 02:38:00 pm by T3sl4co1l »
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Offline YansiTopic starter

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Re: RF transistor amplifier design, any suitable literature?
« Reply #67 on: September 10, 2017, 03:27:00 pm »
It does explain how it works in the text. Best thing to do is study the worked example and then simulate the system in spice over temperature etc.

One classic/amusing aspect of RF design is that if you gave a junior engineer or a student a task to design an RF amplifier using a fast BJT they would often produce an oscillator up at maybe 4GHz. They would then spend considerable time trying to tame it.

But if you asked another student/engineer to actually design /create an oscillator up at 4GHz they probably wouldn't know how to do it and if they tried they would typically fail to get it to oscillate.

I understand, how it works. But still need to read it to understand why it was done this way.

Yeah, some years ago, I attempted to make a narrowband VCO for 1627MHz. Yes, the same frequency of interest. You probably can guess, it didn't oscillate, but when it did, it wasn't 1627. It was all over the place and the tuning was severly nonlinear and non-monotonic - which is a very bad thing for a PLL to work with.  Maybe I will try that VCO again, but with a difference of some years more experience in circuit design. :)

I started designing another transistor experiment. Now with "microstrip inductors". Let's see, what it will do.  Not finished yet, but I will make the biasing as simple as a resistor from +5V. I think it is acceptable for a one off experiment, to tweak the Rb value for a proper bias current. (Otherwise I would make the biasing more reliable.) Do you have any suggestions about the layout, what I should change, before I proceed to etch it?



T3sl4co1l: What does that board do (or what should it do)? The layout looks quite "space generous" so to say for RF.

cdev: Fine, I get it. But MMICs are not the only solution to a RF amplifier.  For example, you are not going to build a HF receiver out of MMICs. Therefore I am attacking discrete circuit design.  And for a narrowband fixed frequency amplifier, an adequate discrete transistor is I think suitable solution.  Also, most MMICs suck at delivering power, the OP1dB being mostly well below +15dBm at MW frequencies, while a single transistor (when designed properly into the circuit - which is the reason why I am learning this) can deliver +20dBm from a single SOT143 device. For fraction of the price of a MMIC. 

//EDIT: Changed layout to a completed one.
« Last Edit: September 10, 2017, 03:35:01 pm by Yansi »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #68 on: September 10, 2017, 03:40:05 pm »
Difference being, one is stable and the other isn't.  Before I tamed this thing,



(uses BFR92AWs), it was squealing somewhere up over 2GHz.  Changed with proximity, so, it was also a rather overly sensitive Theremin.  I mean really, my Theremin (proper) only moves 10kHz when I touch it, but a whole GHz? :-DD

(After fixing it up, by the way, it's a solid 20dB gain, apparently pretty flat from a few MHz to over 700MHz.  And has suspiciously low noise of 1.7 nV/rtHz.)

Tim

I only skimmed over that PCB but that first stage looks like a common base amplifier. The poor decoupling on the base (via a few mm of microstrip) is going to cause that first stage to lose any unconditional stability in a big way in the upper UHF region if that first device is a BFR92. Common base amps using fast BJTs are notorious for instability and it looks like there are more of them there in cascode amps.

The placement of the RF decoupling caps does seem odd as does the length of some of the hot traces.

You would never get that amplifier PCB through a critical design review where I work :) I can tell you why it would raise alarm bells but maybe you are happy with it if you have tamed it enough already.

« Last Edit: September 10, 2017, 04:45:56 pm by G0HZU »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #69 on: September 10, 2017, 04:26:47 pm »
Quote
Do you have any suggestions about the layout, what I should change, before I proceed to etch it?
If you gang the supplies together you will have to 'select on test' the bias resistor going to the base if you want to set the bias current. You might have to swap the resistor several times to get this right so be careful not to damage/lift the copper pads for this resistor with the iron.

Otherwise your latest PCB doesn't look to be easily tweakable for the printed inductors. Maybe add some extra artwork to make them easier to trim? Otherwise you could end up having to cut them out and use sticky copper tape in their place to make alternatives.
 

Offline T3sl4co1l

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Re: RF transistor amplifier design, any suitable literature?
« Reply #70 on: September 10, 2017, 04:49:06 pm »
T3sl4co1l: What does that board do (or what should it do)? The layout looks quite "space generous" so to say for RF.

Sorry, didn't make that very clear -- 20dB wideband amp.  Idea was to use relatively many stages, at relatively low gain, to maximize bandwidth, without cranking up the bias (BFR92s aren't very powerful) or resorting to MMICs or what have you.

Wideband, mainly because I have more interest in time domain and EMC, though it does a fine job as a preamp for my spectrum analyzer, with a 1m bowtie antenna, to see much of what's floating around the airwaves here. :)

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//EDIT: Changed layout to a completed one.

I like the emitter vias. ;)  Don't like the suicide bias, but if you're okay with that, then that's fine I guess.  You should consider being able to hack in a trimpot, though.  Drift isn't something you can deal with by soldering in different chip resistors.  :scared:

I only skimmed over your PCB but that first stage looks like a common base amplifier. The poor decoupling on the base (via a few mm of microstrip) is going to cause that first stage to lose any unconditional stability in a big way in the upper UHF region if that first device is a BFR92. Common base amps using fast BJTs are notorious for instability and it looks like you have more of them there in cascode amps.

The placement of the RF decoupling caps does seem odd as does the length of some of the hot traces.

Yeah, not sure what the layout guy was thinking when he did that board... :-DD

Looking at one here that I "fixed", it's studded with extra resistors, and I think 100pF bypass caps, in strategic areas.  Several cases of scraped-off soldermask for desperately-needed ground pads near important connections (like the input's common base).

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You would never get that amplifier PCB through a critical design review where I work :) I can tell you why it would raise alarm bells but maybe you are happy with it if you have tamed it enough already.

I'm still a bit unsure about a couple of the variants I've made (one seems to have dubious peaks around 500MHz, like it's nearly oscillating, or that I'm seeing conversion products against an unseen GHz tone; but anomalous response is not seen in the noise-source test), but they certainly cleaned up from what you see there.  Well, functionally clean.  The boards don't look so good...  :-/O

Tim
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Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #71 on: September 10, 2017, 05:38:39 pm »
Are you sure you get a flat bandwidth up to 700MHz out of that module?

It scares the bejeezus out of me. Have you ever looked into it with a VNA? I know you have cured the instability but I'd expect to see spikes of negative resistance up above 2GHz from what I see in the version on screen. Even if you grounded the base really well the BFR92 itself has parasitics and the output signal path looks quite long before it reaches the next BJT stage. There isn't much ESR here and I'm not sure what the input Z of the next stage is at 2GHz but there's enough trace length to do a decent trip around a smith chart at UHF so I'd expect to see negative resistance at the board input 'somewhere' in the upper UHF region and maybe beyond this. But I can only look and guess. I think other engineers at my place of work would also be scared of that module in a review :)

I can suggest some 'alternative MMIC' circuits using BJTs if you want broadband 50R gain up into UHF with high reverse isolation. The difficulty is achieving unconditional stability as well as low noise figure and decent signal handling.
I haven't (properly) played with stuff like this for many years because a MMIC would almost always be a better/smaller/lower risk alternative.

The NRE costs of discrete design are a real killer where I work. 15-25+ years ago I could design discrete VCOs and also lots of variants of discrete small signal amplifiers because there were few alternatives. But the NRE costs of putting a VCO or exotic amp through formal testing are too prohibitive these days unless you are making stuff in huge volume on very low margins.

But even in those days I don't think a wideband (UHF) MMIC alternative would have been tolerated in a design review because of the risk of instability up at many, many GHz. i.e. at frequencies beyond the manufacturer's s parameter tables. There's nothing like putting a lid over a marginal amplifier design and then cooling it to -40degC to make it wake it up into oscillation :)

« Last Edit: September 10, 2017, 05:52:32 pm by G0HZU »
 

Offline G0HZU

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Re: RF transistor amplifier design, any suitable literature?
« Reply #72 on: September 10, 2017, 05:49:34 pm »
Some time ago I analysed a simple BC547B test board on a VNA to get s parameter data for the transistor and the VNA data (post processed on a PC) suggested this transistor could (in theory) oscillate up at 600MHz+ if the conditions were right. The faster RF transistors can hoot at frequencies way beyond what we are looking at here.
 

Offline CD4007UB

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Re: RF transistor amplifier design, any suitable literature?
« Reply #73 on: September 10, 2017, 05:56:01 pm »

(After fixing it up, by the way, it's a solid 20dB gain, apparently pretty flat from a few MHz to over 700MHz.  And has suspiciously low noise of 1.7 nV/rtHz.)

Tim

Just to pick up briefly on the point about noise, 1.7nV/rtHz is indeed a very small voltage by everyday standards. However, the noise from a 50ohm resistor at 290K (the standard reference temperature) is only about 0.9nV/rtHz. So, the noise figure (NF) for your amplifier is about 4.6dB (a noise temperature of about 550K).

LNAs usually have a NF <~1dB (a noise temperature of about 75K). For example, in our radiotelescope, we're looking at 1420MHz hydrogen emission from the galaxy, which has a noise temperature of ~100K. The receiver needs to have a comparable (or lower) noise temperature. That translates to a NF of about 1.3dB, and the LNA at the front end aims for a lower NF (which may not quite be realized in practice).

BJTs are not generally used in LNAs because of their shot noise (due to random fluctuations in base and collector current). In contrast, the main RF noise source in FETs is Johnson noise from the drain-source channel resistance. The high transconductance (gm) of pHEMTs means they have a low resistance (~1/gm), and hence low noise, combined with a high gain. They are, therefore, the device of choice for low-noise applications, including measuring the cosmic microwave background from the Big Bang (which has a temperature of about 3K), as described in https://arxiv.org/abs/1310.3088 .

It can be tricky to convert between the different ways of measuring noise. Some useful online calculators are at http://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtml and http://www.rfcafe.com/references/calculators/noise-figure-temperature-calculator.htm .
 

Offline cdev

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Re: RF transistor amplifier design, any suitable literature?
« Reply #74 on: September 10, 2017, 06:41:12 pm »
Lots lots more vias. move two vias around each ground leg closer to legs. Need many more of them. Right under ground pins.

Need vias all along sides, Go with big vias along trace and smaller vias behind it. (so as to avoid resonance) The closer to your device, the more via density.

Also if your software allows your doing it, maybe angle the chip - I dont know if the design - which has a pretty high via density in the red PCB below would even be enough but it might be. If it wasnt you could drill a few more holes so you could have even more right under the legs.
Best if your pins could go right onto or next to vias on both sides. The little chip would straddle a break in the signal path, the trace would be cut. Has to be a certain width to reduce the capacitance. Ideally there both side ground areas get connected. If you cant do anything like that consider leaving the side ground planes out and doing something like the mini-circuits suggested layout below.

(The pattern is for a PSA4-5043+ MMIC which, if you look around you seem now to be able to buy for as little as $0.75 sometimes.)
« Last Edit: September 10, 2017, 08:07:54 pm by cdev »
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