Digital Signal Processing Reference
In-Depth Information
the open-loop low-frequency hd
2,ol
and im
2,ol
characteristics of the amplifier
(A.58):
hd
3, cl, peak
[dB]
=
hd
2,ol
[dB]
+
im
2,ol
[dB]
−
22
.
8dB
(A.58)
Fortunately, due to the product between hd
2,ol
and im
2,ol
, even the peak mag-
nitude of the feedback-induced third-order distortion remains relatively limited
compared to the second-order distortion level. For a wideband amplifier, this is
a good thing, since the aggregate amount of both second- and third-order dis-
tortion will be reduced. That feedback is not always a good idea, is shown by
the following example: consider the case of a narrowband cmos rf-stage in a
receiver front-end. The amplifier has considerable second-order distortion, but
at the same time a decent third-order linearity. If not carefully designed, em-
ploying local feedback in the amplifier could bring third-order intermodulation
into existence, which generates
in-band
distortion. In contrast to this, second-
order harmonics can be dealt with in the same way as out-of-band blocker
signals, and are effectively removed by the band-select filter in front of the
receiver.
Frequency dependent distortion in a MOS amplifier
This section describes the practical limitations of single-stage and multi-stage
transistor amplifiers in terms of distortion performance versus maximum us-
able frequency. It will become clear that in a real-world implementation, there
is a trade-off between power, gain, speed and linearity. It turns out that any of
those four performance parameters can be improved, but this is always at the
expense of increased power consumption or some other compromise in terms
of gain, speed or linearity.
Common source transistor amplifier
The first, simple circuit that is examined, is the single-stage common source
amplifier. The bias current is provided by a current source with output resis-
tance
r
bias
, while the useful small-signal output current is redirected into a load
capacitor
C
load
. Only the nonlinearities of the voltage-to-current conversion
characteristic of the transistor will be taken into account at this point. The vari-
ation of the output current on the drain voltage of the transistor is ignored for
now. It is for this reason that the load capacitor is supposed to be large enough
so that all of the small signal ac output current is sinked by the load, while
there is only a minimal voltage swing at the drain node of the nmos tran-
sistor (Figure A.17). The second-order distortion components of this circuit
can be immediately reduced to the second-order voltage-current characteristic
(V
gs
−
V
T
)
2
of the transistor.
