d. Temperature measurements that are expressed in the more common temperature scales, fahrenheit and
centigrade, can be converted to the Kelvin scale by use of the appropriate conversion factors given in Table 3-1.
Measurements made in degrees Kelvin can also be converted back to the more common temperature scales.
Table 3-1. Temperature conversion factors.
2. The noise power developed across a resistor is directly proportional to the absolute temperature. This leads
to the concept of noise temperature. Since a given resistor generates a given amount of noise for a given
temperature, it is possible to refer to that amount of power by a noise temperature equivalent. The measuring
system, which will include amplifiers, has some bandwidth. If this measuring system were used to measure a
signal of a fixed bandwidth which is less than the measuring system's bandwidth, then a further increase in the
measuring system's bandwidth would not change the measured power. Such is not the case with noisy resistors.
Doubling the bandwidth of the system doubles the measured power halving the bandwidth halves the power.
This means the power available from the resistor depends on bandwidth as well as temperature. The previous
expression for noise power can be rewritten to include the bandwidth.
Pn = K2TB when Pn
=
noise power,
K2
=
a constant (different from K1),
T
=
absolute temperature, in degrees Kelvin,
B
=
bandwidth in Hz
3. This indicates that specifying the amount of noise power available from a resistor does not mean too much
unless we also know the bandwidth of the measuring system. This is where the noise temperature becomes
valuable, inasmuch as it gives a measure of the noise power available from the transistor. The noise
temperature does not depend on the bandwidth of the measuring system. Further measurements might be made
to determine whether changing the resistance of the resistor while maintaining the same temperature would
yield different noise powers. The results of this experiment would give a negative result; therefore, the power
does not depend on the value of the resistance.
4. Many other sources of noise behave in much the same manner as the resistor discussed. In the case of the
resistor, the thermal noise temperature and noise
SS0031
3-6
Previous Page
