R Stiffler wrote:
...
Carbon resistors generate more thermal voltage
noise than Metal film resistors....
This is not really true. We may divide the noise sources in Carbon
composition resistors into two types:
1) True "Thermal noise" (also called "Johnson" or "Nyquist" noise) which
"is the noise generated by the thermal agitation of the charge carriers
(the electrons) inside an electrical conductor in equilibrium, which
happens regardless of any applied voltage." (From
http://en.wikipedia.org/wiki/Thermal_noise). This noise source is
absolutely fundamental and is completely unvarying regardless of type of
resistor, and its power sourcing capability is completely unvarying
regardless of value of resistance, number in parallel/series, size, etc.
It is simply 4kT watts per Hz of bandwidth. (The bandwidth presumably
goes up to some very high limit determined by the mean free path of the
electrons being scattered in the resistive conductor).
2) Excess noise (see http://en.wikipedia.org/wiki/Flicker_noise) - which
is generated by current passing through the resistor and may well be due
to thermal (or thermally induced microphonic) effects, but is not
rightly referred to as thermal noise (at least amongst physicists). It
is readily overcome with better technology. The excess noise present in
a carbon composition resistor is produced by random effects driven by
the power fed in and will only be a very small fraction of this applied
power - ie very far from overunity!
With regard to Johnson noise, if you short or open the resistor, then
the entire 4kT watts generated is simply dissipated back into the
sourcing resistor as heat and there is no net power flow. If you load
it with a matched resistance then you can draw off half of this power,
but if the resistor you load it with is at the same temperature, then it
also generates this same power back in the first resistor and again
there is no net power flow.
Coupling to it via a transformer is no different to using a different
value of resistor as the source - the voltage to current ratio changes
but the power available remains constant. Similarly connecting many
such resistors in series or parallel simply changes the impedance (or
voltage to current ratio) without changing the available power.
A diode is not of course a very good switch and has a gently changing
V/I slope (ie impedance) near zero bias. Thus it must also generate
Johnson noise by the same mechanism (whenever there is a path for
electrical power to be dissipated as heat, then there is the reverse
path in which the heat bath can generate electrical power - this is
called the "fluctuation dissipation theorem" in physics). Presumably
this noise power source/sink will vary slightly in impedance with the
voltage/current fluctuations - but I am sure nature will have organised
it such that no configuration you can dream up will allow net power to
be generated from thermal energy!
If a cold resistor and a hot resistor are connected through electrically
conducting wires which are perfectly thermally isolating (if such things
existed), then thermal energy will flow electrically from the hot
resistor to the cold resistor until they become the same temperature.
However this is no more exciting (and much slower) than simply providing
a thermal conduction path.
What is more interesting is that you can synthesize a "cold" resistor
from a low-noise op-amp and room temperature resistors and actually
"chill" a remote warm resistor (or more usefully a mechanical system
coupled through a transducer) electrically. This is called "cold
damping". Of course the power to refrigerate or pump heat from the warm
system to the synthesised cold one is coming from the op-amp power
supply. With modern op-amps you can synthesise a resistor with a
temperature of less than 1 Kelvin! (if I remember rightly).
Preface: Radiation resistance generates no thermal
noise.
I would guess that the best you could do with any antenna pointing into
deep space would be to pick up the 2.7 K microwave background - which
would probably be indistinguishable from 2.7K thermal noise being
generated in the radiation resistance seen via the antenna.
