Low Phase Noise Amplifiers
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
Am 11.01.20 um 15:36 schrieb Charles Clark:
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.
... and available as cheap SOT343 chip from Infineon: BCR400W
I vaguely remember that its use in an oscillator has been patented. =8-()
cheers, Gerhard
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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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@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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-----Original Message-----
From: time-nuts time-nuts-bounces@lists.febo.com On Behalf Of Richard (Rick) Karlquist
Sent: Saturday, January 11, 2020 7:39 AM
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com; Charles Clark af8z@earthlink.net
Subject: Re: [time-nuts] Low Phase Noise Amplifiers
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
Yes, while I am familiar with active bias and generally like the stabilization I think you're correct that low noise bias will be key. Transistor ft, emitter degeneration (inductive, I'm thinking), minimizing thermal noise from resistors and impedance matching for low noise figure will all be important.
Lifespeed
-----Original Message-----
From: time-nuts time-nuts-bounces@lists.febo.com On Behalf Of Dana Whitlow
Sent: Saturday, January 11, 2020 8:28 AM
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Subject: Re: [time-nuts] Low Phase Noise Amplifiers
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
Opamp stabilization is nice. Do you have any idea of the residual phase
noise or broadband noise floor you were getting with this bias, or was that
not a figure of merit for the cryo LNAs? Even though an opamp circuit can
be designed for low noise, probably large passive filtering components would
be required to tamp down the broadband noise.
Lifespeed
Lifespeed,
The cryogenic amplifiers were used in very broadband situations (often
hundreds of MHz
BW) with "signals" that were basically noise. Most radio astronomy lies in
the art of
measuring very small *changes *in noise level, such as between pointing at
an object of
interest or pointing away from it at a known quiet spot in the nearby sky.
So we are talking
about radiometry, where the "Radiometer Equation" rules. In this game, the
best results
are had by using the widest possible RF/IF BW, then running the noise into
a square law
detector, then passing the output of the detector through a very low BW
filter.
I mention the above because I suspect it means that phase noise in the
predetection part
of the path has little of no effect (unless, of course, there is a strong
signal lurking not
far outside the IF passband). To my knowledge, we've never tried to
measure phase
noise of any of the cryogenic amplifiers.
Our LNAs mostly have noise temperatures in the range of 2K to 3K, which is
roughly
0.03 to 0.04 dB NF.
Dana
On Sun, Jan 12, 2020 at 3:33 PM lifespeed@claybuccellato.com wrote:
-----Original Message-----
From: time-nuts time-nuts-bounces@lists.febo.com On Behalf Of Dana
Whitlow
Sent: Saturday, January 11, 2020 8:28 AM
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Subject: Re: [time-nuts] Low Phase Noise Amplifiers
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
Opamp stabilization is nice. Do you have any idea of the residual phase
noise or broadband noise floor you were getting with this bias, or was that
not a figure of merit for the cryo LNAs? Even though an opamp circuit can
be designed for low noise, probably large passive filtering components
would
be required to tamp down the broadband noise.
Lifespeed
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