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.

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