  Classical ways of cooling require some of these elements: phase transition,
compressor, non-linearity, valve, and/or switch. A recent example is the 2018
patent of Linear Technology Corporation; they utilize the shot noise of a diode
to produce a standalone nonlinear resistor that has T/2 noise temperature
(about 150 Kelvin). While such "resistor" can cool its environment when it is
AC coupled to a resistor, the thermally cooling effect is only academically
interesting. The importance of the invention is of another nature: In low-noise
electronics, it is essential to have resistors with low noise temperature to
improve the signal-to-noise ratio. A natural question is raised: can we use a
linear system with feedback to cool and, most importantly, to show reduced
noise temperature? Exploring this problem, were able to produce standalone
linear resistors showing strongly reduced thermal noise. Our must successful
test shows T/100 (about 3 Kelvin) noise temperature, as if the resistor would
have been immersed in liquid helium. We also found that there is an old
solution offering similar results utilizing the virtual ground of an inverting
amplifier at negative feedback. There the "cold" resistor is generated at the
input of an amplifier. On the other hand, our system generates the "cold"
resistance at the output, which can have practical advantages.
