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Common gate wideband RF amplifiers

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I just built the VHF FM LNA circuit this evening but I tweaked it for 50 ohm ports rather than the expected 75 ohm. This makes it easier for me to measure as all my test gear here has 50 ohm ports.

See below for the simulation (based on my s-parameter data of the CPH3910 and some simple circuit models for the R, L and C components) and this is compared to a measurement of the built circuit using the VNA. I think if I had measured the inductors on the VNA and I had modelled the circuit strays better in the simulation the agreement would have been closer. Even so, there will be some spread due to component tolerances.

This does at least show that I can measure s-parameters of SMD components quite well!

I'll have a go at measuring the noise figure and the signal handling tomorrow.

Nice.  That's a tight match!

The 45 mA is for 16 transistors, not for one. In a single FET, the voltage
noise improves only with the 4th root of drain current, so there is no point
to overdo this.

The measurement of Id over Vgs was really to make sure that the 3910 are
close enough to put them simply in parallel. The Interfet IF3601 stray so much
that with a given Vg some may be fully open while others still are closed.
The closed ones would add capacitance while doing no contribution to
voltage noise performance.

For the 3910, it is OK.

I'd like to see if I can extract the parasitics of my layout with ElectroMagnetics 
to predict the instability. Probably a non-starter, for me being just a beginner
with that.

The common gate JFETS with Zin= 50 Ohms remind me of the ring mixer
terminations proposed by DJ7VY and Ulrich Rohde some 35 years ago.

Hi Gerhard, if you can confirm that the Id is 45/16 = 2.8125mA and the Vds is 3V (as in the dc voltage measured across the drain and source rather than just the voltage of a 3V power supply) then I can have a go at measuring the s-parameters in common source. My CPH3910 devices are made by On Semi and purchased at Farnell so should be genuine parts.

--- Quote ---Nice.  That's a tight match!
--- End quote ---
Thanks! I'm lucky to have some nice test gear here so I'll try and measure the noise figure tomorrow. My noise figure setup is about as good as it gets for hot/cold Y factor hardware and I think the overall uncertainty is very low. I think the noise figure will be a bit worse when tested at 50R. I'll also try measuring the NF with a simple L match to step up the source to 75R. I'm expecting to see about a 2dB noise figure with the L match.

I did quickly measure the OIP3 and it was a fairly consistent +20dBm across the whole band. Increasing the supply voltage slightly improved it to just over +20dBm.


--- Quote from: David Hess on November 24, 2021, 09:46:27 pm ---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.

--- End quote ---

I keep hearing about HF and background noise (run into a limit), but what happens to lunar or spaced based HF transmitters?

I thought maybe there is a good reason to investigate this for hobbyists interested in some kind of cubesat etc.


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