On Sun, 25 Feb 2007 18:55:29 -0800, Frank Gilliland
wrote:

+++On Mon, 26 Feb 2007 02:03:09 GMT, james wrote
+++in :
+++
+++On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland
wrote:
+++
++++++This method has been used in the real world for many years, and it is still
++++++being used. Better ways?
++++++
++++++
++++++Several.
++++++
++++++Long story short, the power-to-voltage ratio of a signal is always
++++++higher than the power-to-voltage ratio of noise. Most RF front ends
++++++are voltage amps. But a -power- amp on the left can dig the signal out
++++++of the noise on the order of 2-4dB, sometimes more. I like using a
++++++common-base for the 1st RF, but you can re-bias a common emitter and
++++++make pretty good improvements. And, as I stated before, a low input
++++++impedance will reduce or eliminate the impedance transformation prior
++++++to amplification.
+++************
+++
+++That is true in most cases. Most of my RF work in the front end dealt
+++around using small loop antenna( less than 1/8 wave) for paging
+++recievers and those puppies have very low radiation resistance. You
+++need some impedance transformation even if you do use common base.
+++
+++
+++Well, yeah, with a 1/8 wave loop? LOL!
+++
*********

Actually not that difficult. Definitely the frontend transistor were
bipolar. Often configured in cascode and operating at 0.95VDC and
narrow band operation (5 MHz wide) anywhere between 30 and 1000 MHz.


+++Anyway, a common base with a single transistor can get you in the
+++neighborhood of 100 to 500 ohms, depending on the transistor. With a
+++50 ohm input that leaves you with a transformation ratio from 2:1 to
+++10:1, which is -way- better than the typical 1000:1 to 10000:1 range
+++needed for a bipolar voltage amp (I won't even mention FET's). The
+++lower the ratio the better. Put two or three transistors in parallel
+++and you can divide that ratio down even further.
+++
+++Take a half-hour or so and sift through your pile of schematics. I'm
+++sure you'll find a few radios that do this. Even some HF tube radios
+++used a grounded-grid triode on the front end -- not for stability as
+++might be assumed, but for performance.
+++
***********

true.

I still like depletion mode MOSFETs as they operate more like vaccum
tubes than bipolar transistor do. Ever tried a common gate depletion
mode MOSFET amp in any RF AMP?

james