Noise
The noise sources of a bipolar transistor can be transformed into two equivalent input noise sources with the following spectral densities:
S(v) = 4kT (Rb + 1⁄2 re + Re)
S(i) = 2q Ib + (K Ib^a) / f
The 1/f noise component is represented by the second term of the noise current source.
In order to measure it, we configure the device as the input of a trans-impedance amplifier,
with a trans-impedance of Rt.
This removes the influence of the input voltage noise,
and amplifies the noise current source so it can be measured.
The parameter i0 is the equivalent noise current of the measurement setup.
The constant Rt is the termination resistor used to convert
the noise current into a voltage to be measured.
Its value must be much larger than the base-resistance of the device.
The parameter af is usually close to 1 for a well-processed device.
Because the parameters af and kf
are almost dependant for most datasets,
fix af at 1 and determine the other parameters first.
Then free af and try to find a better fit.
For most devices an af of 1 will do just fine.