NOISE ANALYSIS
Goal:
a) To evaluate the thermal and 1/f noises in a MOSFET, for a given biasing
condition.
b) Effect of Feedback on those Noises.
Set up:
+10 V
r 100k
To Spec. Analyzer
100k
Procedure:
Always use a compensated 10X probe for the noise analysis.
1) Look at the RMS voltage in the Analyzer. Let it be X
2) Now this X = Vr2 +Vfet2
3) Now Vfet2 = X - Vr2
4) The reflection on the Input side =>Vfet2/gain = Y, say
5) Now this Y = Vr2 +4kT(2/3) (1/gm) + k Id (1/f)
6) Obtain the Noise Bandwidth from the Analyzer.
7) Now draw [approximately from the Analyzer output],
i2/f
1/f
Frequency
8) Get the 1/f, from the graph.
9) The thermal noise can thus be obtained by substituting this 1/f in the formula
in 5. [Approximate value]
Now add a feedback of 1k [series–shunt]
+10 V
100k
To Spec. Analyzer
100k
1k
Repeat the procedure, 1 through 7 but here 2 resistors are in series with the
MOSFET.
RESULTS:
Without feedback,
1) The 1/f Noise is ___.
2) The Thermal Noise is ___.
3) The Noise Bandwidth is ___.
With feedback,
1) The 1/f Noise is ___.
2) The Thermal Noise is ___.
3) The Noise Bandwidth is ___.