Wireless Communication Electronics by Example

Author:Robert Sobot
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

(15.100)

3.25. A simple model developed in solid state physics describes a diode noise manly as a “shot–noise” , which is function of the diode’s biasing current . It is very practical to remember approximate values of diode parameters at biasing current at room temperature over bandwidth, i.e.

At the same time, voltage across a pn junction and dynamic diode resistance are

Therefore, the shot–noise current flowing through the diode resistance generates noise voltage

(15.101)

In order to reduce a diode noise, (15.101) implies that we need to reduce threshold voltage , thus to lover the internal resistance , which is achieved by lowering temperature and by increasing the biasing current, or by decreasing bandwidth.

For example, by doubling the biasing current to , everything else being equal, the dynamic resistance is halved and the diode shot–noise voltage reduced to . Or, equivalently, in order to achieve the same noise reduction by controlling the temperature while keeping the same biasing current we wold have to cool the diode down to c. Concurrent biasing current increase and temperature reduction results in .

We could go even further and create, for example, a plot (15.101) to show how the noise voltage changes relative to the biasing current to , and relative to temperature T = 50–400 K(in steps of ) (Fig. 15.20). (Note that both axis use logarithmic scale.) Within this temperature range for a given biasing current, e.g. , the noise voltage changes over order of magnitude, where the temperature dependance is more pronounced at lower temperatures. Similar plot could be created if we wanted to see how the noise voltage changes with, for instance, bandwidth and the biasing current.

Fig. 15.20Solution 3.25: noise vs. drain current parametric plot

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