Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2003-01-21
2004-09-07
Nguyen, Khanh Van (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S151000
Reexamination Certificate
active
06788140
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to electronic systems and in particular it relates to an amplifier architecture using actively phase-matched feed-forward linearization.
BACKGROUND OF THE INVENTION
The performance of broadband and wireless communication systems is becoming increasingly limited by the analog signal chain because the analog portion is required to operate at higher frequencies with larger dynamic range, decreased distortion, and smaller power supply voltages. Communications receivers in particular have pushed the performance of analog components. Such base station designs currently demand the highest performance analog-to-digital converters available. Driving the received signal to these converters requires an amplifier that is characterized by dynamic range and distortion performance levels greater than the converter itself. Many receiver architectures also employ subsampling techniques that require the amplifier to maintain this linearity at frequencies approaching (and soon to surpass) 100 MHz. Attaining −106 dBc of distortion performance at 100 MHz (supporting a full 16-bits of resolution into the digital signal processor) is the current goal in high-performance analog design.
Future trends in communications systems will make the linearity of amplifiers even more paramount. The much discussed 3G standards for wireless communications employ a modulation methodology known as Orthogonal Frequency Division Multiplexing (OFDM). This multi-channel modulation scheme provides the key benefit of immunity to inter-symbol interference that plagues many current communications systems. The cost of this immunity is increased linearity requirements. In such systems the intermodulation distortion of an amplifier plays a particularly important role in dictating performance. Filtering in most systems serves to reduce the contribution of harmonic distortion products to the degradation of system performance. However, the intermodulation distortion products (primarily the third-order intermodulation distortion) can fall directly within band in such multi-channel systems, often corrupting adjacent channels in the spectrum. Thus the trend towards increased linearity requirements and high frequency operation combine to form a powerful motivation for developing amplifier architectures that maintain ultra-low distortion across wide frequency bands.
Many previous works have explored “feed-forward” as a means of removing an amplifier's distortion products from its output. These techniques involved complex and precisely tuned delay paths and attenuation/gain paths in an attempt to remove all distortion products from the amplifier's output signal.
The technique described in the prior art shown in
FIG. 1
is the first to specifically target third-order distortion products. The technique drastically simplifies the analog signal processing required for effective distortion cancellation. The removal of third-order distortion is highly desirable. The magnitude of harmonic components typically decreases with increasing order. Even-order harmonic products (of which the 2
nd
is most significant) are readily suppressed through the use of differential signal paths. This has been done in several recent operational amplifier designs. Third-order harmonic distortion and its related intermodulation distortion are typically the most significant terms remaining. However, no recent amplifier designs, except the prior art of
FIG. 1
, have made anything more than incremental improvements in third-order distortion performance. This is especially true of the third-order intermodulation distortion which appears at a frequency approximately that of the input signal. While higher odd-order harmonic products also produce related intermodulation products near the input signal frequency, they are typically less significant than the third-order products. For these reasons, the technique cited in the prior art shown in
FIG. 1
is an important development in analog signal processing.
The prior art technique of
FIG. 1
is a feed-forward linearization technique published in the 2001 IEEE ISSCC Proceedings, Ding, Y., “A +18 dBm IIP3 LNA in 0.35 &mgr;m CMOS,” IEEE ISSCC Digest of Technical Papers, pp. 162-163, 2001. The prior art amplifier shown in
FIG. 1
includes main amplifier
20
in the main signal path; scaling amplifier
22
, replica amplifier
24
, and correcting amplifier
26
in the feed-forward signal path; summing node
28
which combines the main signal and the feed-forward signal; input node x; and output node Y
out
. The equations that describe the operation of the prior art of
FIG. 1
are shown below:
out
20
=Ax+A&agr;
1
x
3
out22=&bgr;x
out
24
=A
(&bgr;
x
)+
A&agr;
1
(&bgr;
x
)
3
=A&bgr;x+A&agr;
1
&bgr;
3
x
3
out26
=
A
β
2
x
+
A
α
1
x
3
y
out
=
Ax
+
A
α
1
x
3
-
(
A
β
2
x
+
A
α
1
x
3
)
=
A
(
1
-
1
β
2
)
x
where A is the gain of amplifiers
20
and
24
, x is the input signal, &agr;
1
is the coefficient for the third order term, &bgr; is the gain for scaling amplifier
22
, 1/&bgr;
3
is the gain for correcting amplifier
26
, out20 is the output of amplifier
20
, out22 is the output of amplifier
22
, out24 is the output of amplifier
24
, and out26 is the output of amplifier
26
.
The underlying principle behind the linearization technique introduced in the prior art of
FIG. 1
is that an amplifier's transfer function can be expressed as a Taylor series expansion of the form y=a
1
x
2
+a
3
x
3
+ . . . +a
n
x
n
. Limiting the number of terms in the expansion reduces the model's accuracy, but is often acceptable under certain circumstances. The amplifier transfer function in the above analysis includes only the fundamental and third-order terms as shown by out20 in the above equations. This is a reasonable approximation because a differential signal path suppresses the even-order harmonics (the terms with even powers of x). Furthermore, many systems include filters that attenuate the higher-order harmonics. The third-order term remains because it generates distortion products near the fundamental through intermodulation.
Another important point to note is that the third-order term varies as the cube of the input. Thus an amplifier with twice the input signal will produce eight times the third-order harmonic distortion. The system illustrated in
FIG. 1
exploits this relationship.
The system includes two signal paths, the main path through main amplifier
20
, and the feed-forward path through scaling amplifier
22
, replica amplifier
24
, and correction amplifier
26
. The feed-forward path begins by scaling the input by a factor of &bgr;. By using a replica amplifier
24
that is identical to the main amplifier
20
, the scaled input signal produces a third-order harmonic that is &bgr;
3
times bigger than that produced at the output of the main amplifier
20
. The correction amplifier
26
reduces the level of this distortion by a factor of 1/&bgr;. This equalizes the third harmonic distortion with the third harmonic component of the main amplifier output, while attenuating the fundamental component in the feed-forward signal path. When these two signal paths are combined at the summing node
28
, the third harmonic component is eliminated while the fundamental is only slightly attenuated as described by the equation of y
out
. This is an important result because the same mechanism that produces the third-order harmonic distortion (which can often be filtered) also produces the third-order intermodulation distortion (which cannot be filtered). Note that the addition of feed-forward paths tailored to address higher-order distortion products is possible.
The prior art technique of
FIG. 1
has several problems, which include:
1. The unbalanced nature of the main and feed-forward signal paths results in phase mismatch at the summing node
28
. Phase mismatch c
Cameron Jay K.
Stetson Philip S.
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