Common gate wideband RF amplifiers

[A.Z.]

Just stumbled upon a paper (Ham Radio Magazine) dating back to 1979 and dealing with the design of common gate JFET RF amplifiers

https://worldradiohistory.com/Archive-DX/Ham%20Radio/70s/Ham-Radio-197911.pdf

the article starts on page #14 of the PDF (page #12 in the magazine), and while it isn't "special", I believe it may be interesting for anyone willing to design the input stage of a receiver fed using 50Ohm, in particular the designs shown in figures #6 and #7 which give unity or low gain, may be suitable as an input buffer and impedance matching stage for homemade receivers

Again, nothing special, but I hope it may be of interest

[RoV]

Thank you.
Unfortunately, it seems that FET CG amplifiers are relatively noisy, with minimum NF=2..3 dB. I remembered having read somewhere they are not good for LNA, but I couldn't locate the book, so I searched the net and I stumbled upon this interesting lecture: http://rfic.eecs.berkeley.edu/~niknejad/ee142_fa05lects/pdf/lect13.pdf: see in particular the result at page 18.

[David Hess]

I would have said that a 2 to 3 dB noise figure is pretty good for a commonly available part and simple circuit, but at HF and maybe 6 meters, that is more than enough performance because of high levels of background noise.

I would like to see more about how to produce high IP3 and compression for an low noise LNA, but the measured performance is pretty good, although power consumption is high as well.  The designs I remember use a differential two transistor common-base amplifier.

A lot can be learned from old articles like this.

[UR5FFR]

Common gate amplifier have constant input impedance in wide frequency range and good isolation. Its other property is that it can be easily reversed. This makes it possible to build fairly simple transceivers. In the attachment there is a schematic of one of my developments - the Raisin TRX.

[TimFox]

Back around 1990, I used premium JFETs (especially the dear departed 2SK152 from Sony) to get very low noise figures around 10 MHz.
As I remember, the optimum source impedance for low noise figure is almost the same for grounded-source and grounded-gate configurations, although the input impedance in grounded-source is a high resistance with non-trivial capacitance and the input impedance for grounded-gate is roughly 1/g_m, which was approximately 100 ohms for this device.
(Apparently, Sony designed this part for use as a low-noise preamplifier for vidicons, but it was quickly recognized for use in HF receivers.  Neither market sufficed to keep the part in production.)

[thundertronics.com]

Microwave LNA FET manufacturers usually provide small signal S-parameters measurement for common source configuration. If you keep source pin well grounded, on a thin substrate with a closely spaced ground vias, stability region will cover large area over center of Smith Chart and provide stable operation with relatively big mismatch margin. I used to think that anything on a source pin is only useful to create series feedback oscillator. Surprisingly, configuration you've noticed is pretty interesting in GHz range too, I've already found few papers on wideband common gate amplifiers, but what interested me most is common gate electronically tunable matching stage using variable bias. Thanks for sharing.

[T3sl4co1l]

There's no shortage of mm-wave circuitry using cascodes and such (often in distributed amplifiers to get the required BW), but those are specifically monolithic (InP and other) constructions, nothing you'll get as board-level components (and, obviously at that frequency range, with good reason!).

Other than that, I think it doesn't matter too much; the board-level components you do find, have incredibly low Crss, as if internally cascoded.  (In the past, you could get dual-gate MOSFETs of exactly this design, but what passes for "RF transistors" these days is whatever variety of HBT or PHEMT with 30GHz+ fT, made with whatever combination of geometry and formulation that makes that possible.)  So, little neutralization is needed even in common source/emitter, and the maximum stable gain and bandwidth are quite high.  Basically, GG would only make things worse.

And still other than that -- with older more pedestrian types like JFETs, BJTs, and the odd MOSFET that's still around that's not just intended for straight up switching (..are there even any?), you'll have, well I guess an easier time using them at modest frequencies (1-100s MHz) heh, not like having to sweat the fT on the GHz ones -- but also the poorer performance that they give in general, and have to address things like by using GG configuration.

So, given that as a limitation, say -- there are still some newer parts, very slightly improved over the classics; CPH3910 or NSVJ3910 are comparable or better than the (more recently-classic, but mostly now obsolete) BF862.

And yeah, not that incredibly low NF is much of an issue at these frequencies; the main difference is being able to use a smaller antenna -- such that antenna and input stage losses, in terms of Johnson noise, are comparable to received atmospheric noise, thus making the system as small as possible, and requiring just that little bit better NF at the input.  This being most relevant to automotive radios, themselves already a dying breed.

Tim

[David Hess]

Transistors intended for common base and gate circuits use a different pin layout with the emitter/source and base/gate swapped to reduce the impedance at the base/gate as much as possible and increase isolation between the emitter/source and collector/drain.  A TO-39 package would connect the base/gate to the case.  4 pin power packages would have two opposing base/gate pins instead of emitter/source pins.

[G0HZU]

Thanks, that was a well presented article with good info. Dated 1979 it still reads well today. One comment I do have is that it doesn't mention much about RF instability. Common gate (and common base) VHF RF amplifiers are amongst the worst offenders for instability up at UHF unless some care is taken with the design.

[mawyatt]

Here's a very interesting configuration of a type of Cascode that also creates single ended to differential current conversion with a type of class A/B operation. Don't know the exact origin of this configuration but we discovered it in a hand note by Barrie Gilbert as he later used it in early renditions of his MicroMixer.

The design has some unique properties, like handling large input bipolar levels with a unipolar supply, maintaining a controlled input impedance over large input signal ranges, and unlimited current mode dynamic range with ideal transistors. In actual RF usage, select resistors can be replaced with very low Q inductor/resistors to improve the noise figure.

We used this design concept many times with various flavors of IBM SiGe BiCMOS transistors (>400GHz). The intricate details of the controlled input impedance and dynamic range behavior are quite involved and transcendental, however specific values of bias current, and resistor/inductors create very interesting amplifier properties like harmonic "suck out" and gain "expansion" rather than compression.

In the images below note how the collector currents of Q2 and Q4 cross at 0 volts, the input current is somewhat linear from -10 to +10ma and how the difference collector current is also somewhat linear from -10 to +10ma. Also note at zero input level the input impedance is ~ Rinput +[kT/(q*Ibias) + R]/2 and at very high input levels the input impedance asymptotically approaches R1 + R for either input polarity. Of course for RF/MW/MMW use better transistors than 2N3904 would be required.

Edit: Forgot to mention Ibias should be PTAT for temperature compensation of the input impedance.

Anyway, thought this might be of interest to some folks.

Happy Thanksgiving.

Best,

[T3sl4co1l]

Hmm, bears a passing resemblance to this balanced driver circuit I made some time ago:



Seems like the balance (of your simplified / RF-optimized version) would need some tweaking, but also I think I can convince myself that's what the diode strapped part is for?  Impedance would tend to be very low which might not matter for integrated but could be a GBW problem for discrete? 

Tim

[A.Z.]

Thanks, that was a well presented article with good info. Dated 1979 it still reads well today. One comment I do have is that it doesn't mention much about RF instability. Common gate (and common base) VHF RF amplifiers are amongst the worst offenders for instability up at UHF unless some care is taken with the design.

well, the author wrote about applications from VLF up to the "edge" of VHF, and I don't think a proper designed common gate stage could be so problematic at HF and below

[mawyatt]

Hmm, bears a passing resemblance to this balanced driver circuit I made some time ago:



Seems like the balance (of your simplified / RF-optimized version) would need some tweaking, but also I think I can convince myself that's what the diode strapped part is for?  Impedance would tend to be very low which might not matter for integrated but could be a GBW problem for discrete? 

Tim

Yes your circuit does look similar if you just look at the current outputs from the bottom 2 transistors, and seems to be for clock generation and not ultra-linear use due to the Class B outputs?  The circuit we mentioned above (shown below) was from 1980 era, Gilbert used it in his Micromixer later.

However the 3 resistors not shown in your circuit are critical for large signal performance, Rinput, R2 and R4. R5 is just to match R4, but in use should be slightly smaller to match the collector currents from Q2 and Q4 to null the even order harmonics. We used IC traces and in the chips with these resistors to create the effective small difference. Here's an example showing R4 as Rx value of 25.8 vs R4 of 26, note the effect on difference currents between the input and collector Q2 & Q4 differential currents.

Studying the effects of these 3 resistors, or resistor/inductors in actual use for lower noise, is a fascinating undertaking eventually culminating in transcendental solutions. Vaguely remember a post doc student working on this and posting some unique values and ratios between resistors and bias current that had very unique and useful features, like the aforementioned harmonic "suck out" and "gain expansion" characteristics.

With good high frequency transistors like the SiGe device we used, very high Dynamic Range, Broadband from ~DC to 10s of GHz are achievable at modest power consumptions, however one can get very useful results with just about any transistor.

This unique amplifier is certainly easy enough to built and play around with. For those interested in just simulations, recall that MEXTRAM or HICUM bipolar models are required if high fidelity results are expected.

Anyway, nice to see similar type concepts being used as we hadn't seen any other uses of this useful circuit topology outside our own use.

Best,

[G0HZU]

Thanks, that was a well presented article with good info. Dated 1979 it still reads well today. One comment I do have is that it doesn't mention much about RF instability. Common gate (and common base) VHF RF amplifiers are amongst the worst offenders for instability up at UHF unless some care is taken with the design.

well, the author wrote about applications from VLF up to the "edge" of VHF, and I don't think a proper designed common gate stage could be so problematic at HF and below

The problem when using a JFET is that it can oscillate up at UHF (eg 800MHz) when the amplifier has been designed and built for use down on the HF bands. I didn't spot anything in the article that warns against this risk. Maybe the author wasn't aware of this instability risk back in 1979?

[A.Z.]

The problem when using a JFET is that it can oscillate up at UHF (eg 800MHz) when the amplifier has been designed and built for use down on the HF bands. I didn't spot anything in the article that warns against this risk. Maybe the author wasn't aware of this instability risk back in 1979?

I doubt it, my take is that the author scope was mainly to exemplify how to design a JFET "common gate" stage; as for oscillation, in my own experience it mostly depends from how one designs the stage and from how one builds it, for sure most RF stages won't perform as expected if one uses a sloppy design or build

[David Hess]

The only added risk that I am aware of with a common base or gate stage is that a higher operating frequency is possible, and the design texts I have read often list greater stability as an advantage.  In my experience, a common emitter or source design is more likely to oscillate.

But great instability might be a problem when parts designed for common emitter or source operation are used in a common base or gate design.  As I pointed out above, the pin layout optimizes a package for one or the other.

[G0HZU]

Yes, it's the introduction of some inductance at the gate connection that can lead to instability up at UHF.

There will be some resistive damping up at higher frequencies due to the lossy ferrite material used in the 4:1 and 9:1 transformers and this ought to be enough to achieve unconditional stability in most if not all of the designs in that article if built with a sensible layout.

[G0HZU]

I had a go at simulating some of the amplifier circuits using some old s2p files of an MMBFJ310 JFET and the gain was generally a bit lower than the results in the Ham radio magazine article. This is because I only have (12Vds) sdata files at 10mA Id and not 17mA.

Note that the author had to use split +/- 12V supplies to get sensible bias resistor values for these JFETs.

I tried paralleling up two s2p models to get more transconductance (to try and mimic the E430 JFET) and again it fell short on transconductance because each model was biased at just 10mA and not 17mA. So my results for gain were a bit low.

The 4:1 transformers are really transmission line transformers (TLT) and this makes them fairly easy to model even up at UHF. My first attempt at a stability analysis showed that only 4nH of gate inductance was needed to make the parallel JFET model lose unconditional stability with the 4:1 TLT model attached. This was with the 10mA s2p model.

I should have some free time tomorrow and I'll try and get an s-parameter model of the sot-23 MMBFJ310 JFET at 17mA Id and 12Vds. I think the simulated gain results will then agree closer with the amplifier designs in the article. It really is a well written article and hopefully the author produced more like this.

[mawyatt]

Added a plot of input resistance vs input voltage for amplifier mentioned a few posts above. This is computed in LTspice as (derivative of input voltage)/(derivative of input current). By adjusting the bias current and mentioned resistors you can change the shape and range of the input resistance.

Best,

[G0HZU]

Sadly I haven't had much free time but see below for a couple of graphs of K factor for a MMBFJ310 (sot-23 SMD) J310 in common gate biased at 10Vds and 10mA.

This is just the JFET in a VNA test fixture biased using VNA bias tees. The first plot shows K when the gate pin of the SMD package is direct to a ground plane. K is above 1 from about 1MHz to 2GHz.

The second plot shows K when just 4mm of leg inductance is added at the gate and this simulates what you might see from a non-SMD part. You can see that K goes below 1 over much of VHF and UHF. This opens the door to instability. This is just at 10mA Id. It would be a bit worse at 18mA ID and even worse again if two were placed in parallel. This should show that this type of amplifier is really prone to instability up at UHF unless care is take to ensure a short gate connection. If you use the T0 plastic J310 then you really do have to try to cut the gate leg 'really' short and even then I suspect it wont achieve a K over 1 at UHF. However it will help reduce the chance of instability.

The plots below were taken using s-parameter data taken from my VNA in a decent test fixture across LF through to 2GHz.

[G0HZU]

If you explore various transistor types on a VNA like this you can get  some real surprises as to how high up in frequency a part can oscillate. For example the VNA data shows that a jellybean 2N3904 BJT can potentially oscillate above 1GHz when biased at about 15Vce and 10-15mA Ic. Without analysis of the 2N3904 transistor like this using a VNA it would be difficult to know this instability at 1GHz was even possible.

See below for a VNA derived plot of K for a jellybean 2N3904 in common emitter configuration at 10Vce and 10mA Ic. The s-parameter data extracted from my VNA shows that self oscillation of the 2N3904 is possible at over 1GHz as K is <1 to beyond 1GHz.

[G0HZU]

To give an example, if you look closely at the K plot for the MMBFJ310 when there is reactance at the gate it shows K is still below 1 up at about 1500MHz.

I put the s-parameter model into Genesys and designed a negative resistance oscillator at 1450MHz and the idea was to try and exploit the negative resistance generated up at this frequency by the MMBFJ310. See the genesys simulation where it shows there is still a net negative resistance even with lossy resonator components added to make up a 1440MHz oscillator.

I built the oscillator circuit on some copper PCB material and it oscillated at 1410MHz. Pretty close! See the spectrum analyser plot below. It probably is possible to get it to oscillate above 1500MHz but this must be close to the upper limit for the device. It does show how it is possible for these parts to oscillate up at UHF quite readily.

This oscillation at 1410MHz also helps to give confidence that I'm extracting valid s-parameter models (even up at UHF) for the MMBFJ310 using the VNA.

[G0HZU]

I managed to set up the VNA jig to allow me to extract a MMBFJ310 model in common gate at 12Vds and about 18-19mA Id.

I put two of these models in parallel in the Genesys simulator and the first thing of note was that I could only get a gm of 28,000umho compared to the 36,000umho obtained from the E430 in the article. See below for a plot of my simulation of the circuit in fig 4 of the HR magazine article.

This shows just under 7dB gain compared to just over 8dB for the E430 in a similar circuit in the HR magazine article. My result does look to be about right for 28,000umho and it includes a model for the 4:1 transformer with about 0.35-0.45dB loss across 30MHz to 200MHz. My model was of a transformer made from 5 bifilar turns 0.2mm enCu wire on a FT37-43 toroid.

My simple VNA derived models should be very accurate but they can't predict the noise figure or the large signal performance. I'd have to build the model for real to do that. There will also be a fair bit of variation in performance due to differences in Vp and Idss between samples of MMBFj310 and E430 devices.

My MMBFJ310 JFET that I measured on the VNA had Vp and Idss as below.
Vp = -3.58V
Idss 39mA

[A.Z.]

thank you ! you went a loong way, and sincerely, I didn't think that such an old article could raise so much interest, sounds like old tech is starting to be forgotten

As for gain, in own experience, the common gate setup won't give you much gain, but it will offer (at least on HF, where I played with it) good stability, nice impedance matching (for coax feeders) and decent IMD resilience, so in my opinion, it is a nice stage to have at the antenna input of a receiver, imagine having a wide passband filter (say 0.5 to 30 as an example) followed by a common gate whose output will go to a preselector, adjusting the common gate stage we could be able to compensate for signal loss caused by the filters and feed the following stage (probably an adjustable gain preamp) with almost the same signal seen at the antenna input

[G0HZU]

Thanks. It is a very well written article and I do have some interest in circuits like this because I did do a lot of design work with the J310 many years ago.

This evening I built the circuit in figure 4A but I didn't select/match the MMBFJ310 devices beforehand. I only managed to get 30mA Id so the gain was only 6dB. However the noise figure was 2.9dB at 30MHz and I suspect this would dip below 2.5dB (to agree with the HR magazine article) if I could cherry pick a couple of matched J310 devices with higher Idss. The gain would also improve as well.