[time-nuts] how to find low noise transistors

KA2WEU at aol.com KA2WEU at aol.com
Sun Jul 19 23:57:14 UTC 2015


Good evening, this turns out to be a good discussion... 
 
Any more inputs ?  73 de Ulrich 
 
 
In a message dated 7/19/2015 7:13:39 P.M. Eastern Daylight Time,  
richard at karlquist.com writes:

On  7/18/2015 2:16 AM, Attila Kinali wrote:
>
> I always wonder how  you figure out whether a transistor is low noise
> or not. What part of  the datasheet hints at which transistors have low
> noise and which have  not? Even if it's just try and measure, how
> do you find good  candidates to measure?
>
>            Attila Kinali
>

For a BJT operating above the 1/f noise  corner, and at non-microwave
frequencies, the noise properties depend only  on RF current gain and
base spreading resistance.  See "Low noise  electronic design" by
Motchenbacher and Fitchen.  RF (not DC) current  gain can be measured the
usual ways, but base spreading resistance has to  be inferred from
noise figure measurements made with low source  resistance.  The
RF current gain is the real fundamental noise  property of the device
that you cannot change.  Fortunately, it can be  determined from the
data sheet, if not directly, then by calculating it  from DC current
gain and F-sub-t, based on the operating frequency.   The low frequency
current noise (above the 1/f corner) is simply equal to  the shot
noise of the DC base current.  The low frequency voltage  noise is
the sum of the Johnson noise that a resistor would have if  its
value were the sum of the base spreading resistance and half  of
r-sub-e.  Where r-sub-e is the "emitter resistance",IE the  effective on 
resistance of the transistor.  Base spreading resistance  can be
overcome by using a sufficiently high source impedance  and/or
paralleling devices (if you can tolerate the additional  capacitance).

At frequencies such as 100 kHz and 10 MHz, it is very  easy to get
a noise figure well below 1 dB with a BJT, so it should be no  great
problem to find a suitable device.

Even lower noise figures  are available with JFET's, which have
noise current equal to the shot noise  of gate current, which is
specified.  The resulting noise current is  negligible for most
devices.  This leaves the noise voltage, which is  just the
Johnson noise of a resistor equal to the channel resistance.
By  scaling to larger devices and/or paralleling devices, this
can be reduced  to arbitrarily low values.  The limiting factor
is the substantial  capacitance of JFET's.  This limits them
to about 1 to 10 MHz, before  high beta BJT's dominate.  I have
observed noise figure of below 0.2  dB in JFET's at 2 MHz.

Below 50 to 100 MHz, MOSFET's and ePHEMT's have  excessive 1/f
noise and are a non starter.  Above the 1/f corner, it  is easy
to get noise figures of a few tenths of a dB with  ePHEMT's.

All of this discussion doesn't address 1/f noise, which  could
be an issue in oscillators and low phase noise amplifiers.
For  that purpose, you are back to characterizing devices yourself.
Putting  negative feedback around the transistor can alleviate
this by reducing  upconversion of noise.

Rick Karlquist  N6RK
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