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Common gate wideband RF amplifiers

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


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

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.

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.

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/gm, 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.)


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