[time-nuts] Low Phase Noise Amplifiers

[Richard (Rick) Karlquist]

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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