Electronics > RF, Microwave, Ham Radio

Common gate wideband RF amplifiers

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David Hess:

--- Quote from: G0HZU on December 04, 2021, 10:55:26 pm ---I've not tested many PNP BJTs like the 2N3906 so I can't really comment other than to ask what operating point are they testing at?
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The test had to have been at 20 volts and 10 milliamps because that is what the Ft (min) was specified at, and the real circuits operated at that kind of current.  They gave the grading criteria but no details on exactly what the test was.

I can probably find the exact circuit the transistors were used in if you think it would be helpful; I think it was a 100 MHz PNP cascode so common base which is why I mentioned it here.

--- Quote ---I can see a significant variation in Ft for the same BJT in the same test fixture when tested at a lower operating point. See below for the very same 2N3904 tested at just 0.5mA and 3Vce. The Ft drops from 300MHz to just 133MHz.

I usually measure s-parameters for a BJT or JFET across 3V, 5V, 7V, 10V, 12V and 15V and sometimes 20V. I also do this across 500uA, 1mA, 2mA, 3mA, 5mA ,10mA and 15mA for each voltage. Usually I do a log sweep across 300kHz to 2GHz. It's important to turn the VNA source power really low to prevent compression problems. I also use a carefully corrected test fixture that sets the reference plane very accurately at the device pins. I use the built in 'fixture simulator' feature in a 4 port E5071 VNA for this and I use a N4431B 4 port Ecal to calibrate everything.


I can try and create a classic VCCS transistor model to try and 'fit' the s-parameter data but I'd have to guess Rbb and see if it fits the s-parameter data at a chosen operating point.
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There is an additional notation of NF being 6dB which made me wonder if Tektronix measured the noise figure to get an estimate of the Rbb (1), but the 250 MHz 2N3906 is marked as a 4dB noise figure which is the opposite of what I would expect, so I do not think they were measuring noise figure to calculate Rbb to get to rb'Cc to get to Ft, and they listed rb'Cc as the grading criteria and not Ft.

(1) That is the kind of weird thing I would do, but only after verifying that there was a real correspondence between them.  I never found a useful explanation about how to measure base transit time except estimating from Ft.


--- Quote from: G0HZU on December 04, 2021, 10:19:12 pm ---I had a rummage in my s2p library of old autotransformer models and found an s2p file of a basic 9:1 autotransformer wound on a 61 material toroid. This was not a great transformer when it was tested as you can see in the second plot below.

However, I tried simulating it along with a pair of MMBFJ310 JFETs in parallel. The performance was much better than I was expecting. It managed just over 10dB gain up to almost 75MHz and the results were close to that of the HR magazine for the amplifier in figure 5 of the magazine article.
Note that I did struggle a bit with the 1.4uH inductor. Up at 75MHz the behaviour of this inductor will depend on the way it is designed and wound and also on its Q up at 75MHz. I found I had to put a 4k7 damping resistor across the inductor model to prevent excessive peaking up at 75MHz.

See the simulation plots below. Of all the amplifiers I've looked at so far this one is the most interesting. I think it deserves a better transformer than the autotransformer model I used in the simulation. This autotransformer was not wound the same way as the 9:1 transformer in figure 5 of the HR magazine. I didn't use three twisted wires. I just used a classic tapped autotransformer with 1 main winding wire tapped a third of the way along.

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The inductor can be dispensed with if you are interested in HF.  It has no effect on NF or anything else besides VHF gain (and I suppose S22.)

I tried the figure-5 amp just now with a CPH6904 (dual CPH3910) at its max Id of 40 mA in place of the E430.  It worked fine.  Best performance I saw was 12.7 dB of gain at 50 MHz with NF in the 1.5 dB range.  At currents less than 40 mA, the VHF gain fell off, the HF gain came up, and the NF was largely unaffected.  The input impedance varied quite a bit with Id. 

I didn't see any peaking, but I also didn't try any inductors other than the 910 nH / ADT9-1T combination in green.

Reverse isolation is more like 30 dB rather than 36 dB predicted by the article, but I didn't characterize it versus Id.  I also didn't check IP1dB but it was +6 dBm with a single CPH3910 configured for 10 dB of gain at 10 mA, so likely better now. 

Of course 40 mA at +/- 12V is almost a watt, so you wouldn't normally run the part this hard.  (Edit: there is also some saturation in the Mini-Circuits transformer at this current level, so AC-coupling the transformer buys another dB or so of gain.  NF is unaffected as is the oddly-poor LF performance.)

Good part.  Everybody buy lots of them, so they keep making them.


--- Quote from: G0HZU on November 26, 2021, 10:38:18 pm ---Yes, it's the introduction of some inductance at the gate connection that can lead to instability up at UHF.

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--- Quote from: G0HZU on November 29, 2021, 10:10:31 pm ---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.

--- End quote ---

I share G0HZU concern over common gate's sensitivity to ground inductance. In a 435 MHz amp, a dual gate mosfet (which is a cascode of CS & CG) BF988 / 998 developed an unintended gain peak at ~1.6 GHz (black trace in gain graph) when the gate 2's bypass capacitor (C4 in the attached photo) is located 5 mm away from the transistor package. Replacing C4 with a 0805 chip at ~1mm away, eliminated the gain peak (blue trace in gain graph).

--- Quote ---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.

--- End quote ---

Noise matching the J310 at 98 MHz resulted in 0.4 dB NF whereas conjugate matching achieved 3 dB NF (attached image)

Some good results there from KE5FX and biastee! I'll have to check out the CPH6904 and CPH3910 JFETs on the VNA. They do seem to have a very high transconductance compared to the J310.

I don't think I've ever managed to get a NF below about 1.5dB with a U310/J310 at VHF but usually I aim for a compromise between input match and noise figure.

I had some free time today and managed to take a suite of s-parameters for the CPH3910 JFET at various operating points.

I haven't built any amplifiers but I did try to marry the s2p file up with various transformer models I have here.

One interesting result was with an S2p model of a 49:1 transformer with some compensation. The simulation predicts about 14dB gain across 1 to 14MHz with fairly good input and output match across this range. This was with a pair of CPH3910 JFETs in parallel each biased at 10V and 15mA.

I'm not sure what practical application this amp has but it was interesting to see the result. The gate inductance needs to be sub 1nH to maintain K above 1 up at UHF with this simulation.


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