Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1999-09-30
2001-09-04
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S151000
Reexamination Certificate
active
06285252
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a circuit for improving the distortion performance of amplifiers, and more particularly, to a linearization circuit to improve the distortion performance of broadband hybrid RF amplifiers.
2. Description of Related Art
In transferring and distributing signals, most cable television (CATV) networks employ hybrid fiber/coax (HFC) networks. A typical HFC network includes optical transmitter and nodes, and distribution amplifiers to provide optical and electrical paths for the signals, respectively. (An optical node converts between the optical and RF domains.) Effective transfer of the signals along the optical and electrical paths requires output stage signal amplification at the optical transmitter and nodes, and the distribution amplifiers. For this amplification, hybrid RF amplifiers, which typically employ push-pull and power-doubling techniques to improve their distortion performance, are commonly used. Such hybrid RF amplifiers are commercially available from vendors such as Philips and Motorola.
When the coaxial cable portion of an HFC network is passive, that is, there is no distribution amplifier in the network, the distortion from the node output stage amplifier may dominate the total system distortion performance. At such a passive coaxial cable portion, it is desirable to drive the output stage amplifier in the node at maximum power level in order to transmit the signal further. However, higher output power can produce worse distortion performance. Therefore, the optical nodes need an enhanced output stage amplifier to improve the distortion performance at high output power.
There are several known linearization techniques that improve distortion performance of output stage amplifiers. Among them are predistortion and feedforward circuits which are schematically shown in
FIGS. 1A and 1B
, respectively. Even though both techniques can improve the distortion performance, they have their own limitations.
Referring to
FIG. 1A
, the feedforward circuit includes an error amplifier
20
to cancel the distortion produced by an output amplifier
10
. A first directional coupler
1
splits an input signal onto two paths
12
and
14
. On path
12
, output amplifier
10
amplifies the signal, and then a second directional coupler
2
splits the amplified signal onto paths
12
and
16
. On path
16
, an attenuator
3
attenuates the signal, and a phase-shifter
4
phase-shifts the signal by 180°. At a third directional coupler
7
, the signal from path
16
and the signal from path
14
through a first delay line
5
combine together. As a result, fundamental portions of the signals from paths
14
and
16
cancel each other, and distortion portions of the signals are coupled at the output of third directional coupler
7
. Then, error amplifier
20
amplifies the combined signal which is a distortion signal.
The signal from second directional coupler
2
on path
12
passes through a second delay line
6
, and is combined with the amplified signal from error amplifier
20
at a third directional coupler
30
. When the output signals combine together at a combiner
30
, the distortions in both output are equal in magnitude but opposite in phase, so that the distortions cancel each other. This feedforward technique is known for its high distortion cancellation capability. For example, this feedforward technique can cancel all orders of distortions whereas other linearization techniques only cancel one type of distortion. Thus, the feedforward technique is widely used in broadband applications where reactive components have different effects at different frequencies. However, the feedforward technique has several shortcomings. For example, multiple output amplifiers cannot be coupled to a single feedforward circuit, and there can be signal loss at the output stage. Driving the output amplifier at a higher level to overcome the loss at the output stage may introduce more distortions.
The predistortion circuit shown in
FIG. 1B
does not introduce any loss at the output stage of output amplifier
10
. This circuit employs a distortion generator
40
to produce predistortion from a signal that was split from an input signal through a first directional coupler
22
, and provides the predistortion into the input stage of error amplifier
20
. At a second directional coupler
24
, the other signal that was split from an input signal through a first directional coupler
22
passes through a delay line
25
combines with the signal from error amplifier
20
. Then, the combined signal is inputted into output amplifier
10
.
The predistortion from distortion generator
40
through error amplifier
20
cancels the distortion from amplifier
10
at the output stage of output amplifier
10
. Typically, distortion generator
40
does not have similar distortion characteristics as does amplifier
10
, limiting the circuit's cancellation capability.
Diode-based distortion generators, which are widely used for predistortion, have disadvantages when employed in hybrid RF amplifier linearization. First, different orders of distortions may require separate, different distortion generators. That is, separate distortion generators are required to cancel second and third order distortions. This increases cost and circuit complexity. Second, when the hybrid RF amplifiers are driven at high output power, higher (fourth and fifth) order distortions become more pronounced, and overlap on lower order distortion frequencies. Such diode distortion generators cannot effectively cancel the high order distortions.
U.S. Pat. No. 5,258,722, incorporated herein by reference in its entirety, discloses a distortion cancellation circuit including a distortion generating circuit
50
, as shown in present FIG.
2
. Distortion generator
50
includes a first intermediate amplifier
36
, which is ideally identical to each of four output amplifiers
60
-
1
to
60
-
4
, and a second intermediate amplifier
42
. In distortion generator
50
, a first splitter
53
splits an input signal onto two paths
32
and
34
. The signal on path
32
passes through amplifier
36
, a first attenuator
54
, and a phase-shifter
57
, and the signal on path
34
passes through amplifier
42
and a second attenuator
55
. The signals on both paths
32
and
34
combine together at a combiner
56
, and the combined signal is inputted into output amplifiers
60
-
1
to
60
-
4
through a second splitter
58
.
Amplifiers
36
and
60
-
1
to
60
-
4
are intended to be driven at the same output power. Theoretically, distortion generating circuit
50
may generate a predistortion that can cancel the distortion generated from output amplifiers
60
-
1
to
60
-
4
. However, distortion generating circuit
50
may have several shortcomings in practice.
For effective distortion cancellation, output amplifiers
60
-
1
to
60
-
4
must be of high gain (30 dB or more) in order to have amplifier
42
driven at far lower power level than are output amplifiers
60
-
1
to
60
-
4
, so as to compensate for the excessive signal loss in distortion generating circuit
50
. Otherwise, amplifier
42
must be driven at much higher power level. The distortion generated from amplifier
42
will become dominant, and the magnitude of the predistortion generated at combining point
56
will not be proper to cancel the distortion generated from output amplifiers
60
-
1
to
60
-
4
.
Commercially available hybrid RF amplifiers used for forward transmission typically have gain of 18 to 21 dB. If such amplifiers are used for amplifier
36
, amplifier
42
will be driven at only a few dB below the output power level of output amplifiers
60
-
1
to
60
-
4
. In addition, higher gain amplifiers are very difficult to make, and not commercially available.
Another shortcoming is that the fundamental signals from paths
32
and
34
must be subtracted from each other at combining point
56
to obtain the out-of-phase distortion component. The fundamental signal power difference at combining point
Harmonic Inc.
Klivans Norman R.
Nguyen Khanh Van
Pascal Robert
Skjerven Morrill & MacPherson LLP
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