[time-nuts] Low Phase Noise Amplifiers

[Dana Whitlow]

FWIW, at the Arecibo Observatory all our cryogenic LNAs had bias stabilized
with active stabilizers based on opamps.  Since the opamps do not work at
~15K,
bias connections to the drain and gate of the RF FETs were brought out
separately
from the RF connections, and the opamp circuitry was at room ambient temp.
This approach would nicely stabilize both drain DC voltage and drain
current over
the whole temperature range from room ambient to 15K, which was handy for
testing and monitoring LNA behavior during cool-down, which takes several
hours
(or more).

Dana


On Sat, Jan 11, 2020 at 9:40 AM Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

> A VERY long time ago, it was discovered that simply
> degenerating a transistor with an emitter resistor
> makes a worthwhile improvement in 1/f noise.  I
> want to say this was published in 1970 by Dick Baugh
> of HP but don't hold me to it.  Note that the resistor
> was NOT bypassed:  it's purpose was RF feedback, and
> any stabilization of bias current was incidental.
> The resistor value was a few dozens of ohms.  That
> is not enough to do anything special in terms of
> stabilizing collector current.
>
> In oscillators, a designer might want to use a high
> performance bias stabilization scheme to minimize
> frequency drift (as opposed to noise).
>
> Various publications out of NIST (Fred Walls, et al)
> recommend using a transistor with high Ft vs the
> operating frequency to get low 1/f noise.  This becomes
> more important when working at 100 MHz vs 10 MHz.
> As far as bias is concerned, the main emphasis seems
> to be on using a bias scheme that doesn't ADD noise
> to the amplifier.
>
> Rick N6RK
>
> On 1/11/2020 6:36 AM, Charles Clark wrote:
> > I wonder if adding active bias feedback around the RF transistor to
> > reduce the low frequency current variations would help.  This is the
> > classic PNP bias scheme which can be applied to BJT's or FET's.  I have
> > used it to successfully improve the phase noise on oscillators.  Details
> > from T.T. Ha, or Gonzales books on Amplifiers.
> >
> > Chuck, AF8Z
> >
> >
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