Digital Signal Processing Reference
In-Depth Information
the interferer power level at which the distortion power becomes equal
to the value of the signal-of-interest in the spectral density plot of
the amplification chain of the receiver. If the interferer power is re-
ferred to the input of the system, this power level is called the input-
referred third-order intermodulation intercept point (iip
3
). It is called
the output-referred ip
3
if the interferer power is referred to the output
node of the signal chain. The minimal value of the corresponding in-
termodulation intercept point iip
3
of a mobile gsm receiver (ms) can
now be obtained by extrapolating the blocker level until im
3
becomes
equal to 0 dB, which is exactly the point where the third-order distor-
tion beat power at the output of the amplifier becomes equal to the
power of the signal-of-interest (A.26):
1
2
im
3
@ blocker level
[dB]
=
−
iip
3
[dBW]
blocker level [dBW]
1
2
=−
49dBm
+
58dB
=−
20dBm
(A.27)
Distortion in a single-stage MOS amplifier
To illustrate the usefulness of the formulas that were introduced in the previous
section, the distortion components of a single mos transistor amplifier with re-
sistive source degeneration are calculated. The quadratic transistor model of
(A.3) is used, but the calculations can easily be expanded to any other transis-
tor model. It will also turn out that degeneration causes third-order harmonic
distortion components, although only second-order coefficients are taken into
account in this simplified transistor model. The transistor schematic with re-
sistive degeneration in the feedback path and the corresponding small-signal
model are represented by Figure A.7.
I
DS
i
D
S
v
G
V
G
v
GS
g
m
v
GS
V
S
R
S
R
S
Figure A.7.
A mos transistor amplification stage with resistive degeneration in the
feedback path. The voltage drop over the resistor reduced the overdrive
voltage
V
gst
over the transistor. This reduces the overall gain of the
circuit, but will also suppress second-order distortion components.
