Adaptive linear amplifier without output power loss

Details Adaptive linear amplifier without output power loss Adaptive linear amplifier without output power loss

1998-11-18

2001-03-06

Pascal, Robert (Department: 2817)

Amplifiers

With pilot frequency control means

C330S149000

Reexamination Certificate

active

06198346

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to radio frequency (RF) amplifiers. More particularly, the present invention relates to a multi-tone amplifier and a method for amplifying having adaptive closed-loop control for minimizing intermodulation distortion products, and for minimizing amplifier performance degradation caused by component drift, temperature variation and aging.
2. Description of the Related Art
As is well-known, when a dual or multi-tone input signal is applied to an amplifier that is not perfectly linear, undesirable intermodulation (IM) products are generated at predictable frequencies causing intermodulation distortion (IMD). Amplifiers operating in class AB or class B modes tend to produce high levels of IMD product when multi-frequency signals—that is, multi-tone signals—are amplified. IM product levels on the order of −30 dBc (30 decibels below the fundamental frequency or carrier level) are typical. The undesirable IM products are particularly apparent when the amplifier is operated in saturation or in the gain compression region of the amplifier. The level of the IM products are greater the further into the gain compression region the amplifier is operated.
Harmonic IM products are not of primary concern because they can be removed by a filter. Third order and fifth order IM products, however, fall within the desired communication bandwidth of the amplifier and cannot be removed by a filter. The only way to deal with the third and fifth order intermodulation products is to amplify in a way that does not generate third and fifth order intermodulation products.
A conventional technique for reducing intermodulation distortion (IMD) is to use a correction amplifier that generates correction signals at the same frequencies as the undesirable intermodulation (IM) products, but having phases that are 180° out-of-phase from the phases of the corresponding IM products. When the IM products and the correction signals are applied to an output combiner, the IM products are cancelled by vector summation with the correction signals. As a result, the amplified output signal has substantially only the fundamental input signal frequencies, i.e., the multi-tone components of the input signal.
FIG. 1
shows a schematic block diagram of a conventional low-distortion RF amplifier circuit
10
that includes a correction amplifier. Circuit
10
linearly amplifies an input signal S
IN
to produce an amplified output signal S
OUT
. Input signal S
IN
is a dual-tone high-frequency signal having sinusoidal components at a first fundamental frequency f1 and at a second fundamental frequency f2. For this description, frequency f2 is greater than frequency f1. Both frequencies f1 and f2 are within standard wireless communication frequency bands, such as between 800-960 MHz. The phase of S
IN
is arbitrary. Amplification causes IM products to occur at both higher and lower frequencies than the communication frequency band of interest.
In
FIG. 1
, frequency components f1 and f2 of S
IN
and various other signals are shown vectorally for conveniently showing phase relationships between the same frequency components at specific points within circuit
10
. Power and voltage standing wave ratio (VSWR) losses are ignored in the following description.
Input signal S
IN
is applied to an input port
11
of a first coupler, or power splitter, C
1
. Coupler C
1
splits signal S
IN
into a signal S
1
that is output at a “direct path” output port
12
and a signal S
2
that is output at a “coupled path” output port
13
. Typically, coupler C
1
is a passive device, such as conventional branch line coupler or Wilkinson-type divider, that splits input power unequally between output ports
12
and
13
, with higher power being output at port
12
.
Signal S
1
includes sinusoidal components at frequencies f1 and f2 having respective voltage levels of C
11
V
1
and C
11
V
2
, where C
11
is the coupling coefficient of coupler C
1
. The phases of the f1 and f2 components of signal S
1
are defined to be 0°. Similarly, signal S
2
includes sinusoidal components at frequencies f1 and f2 having respective voltage levels V
1
{square root over (1+L −C
11
2
+L )}, and having respective phases also defined to be 0°.
Signal S
1
is applied to a power amplifier A
1
where it is amplified to produce an amplified signal S
3
output at an amplifier output port
14
. Amplifier A
1
is a conventional high-frequency amplifier operating in class A, AB or B; for example, a power gain on the order of 30 dB to produce RF output power of about 50 W.
Amplified signal S
3
contains amplified frequency components f1 and f2 and undesirable intermodulation distortion products at frequencies f3 and f4. Frequency f3 is at 2f1−f2, and is less than frequency f1. Frequency f4 is at 2f2−f1, and is greater than frequency f2. The components of signal S
3
at frequencies f1 and f2 are G1C
11
V
1
and G1C
11
V
2
, respectively, where G1 is the voltage gain of amplifier A
1
. The phases of the f1 and f2 components of S
3
are −&phgr;
10
and −&phgr;
20
, respectively, where −&phgr;
10
and −&phgr;
20
are the respective insertion phase lags through amplifier A
1
at frequencies f1 and f2. The minus sign indicates a phase lag or delay. The intermodulation distortion components of signal S
3
at frequencies f3 and f4 have respective voltage levels V
3
and V
4
with respective reference phase values of −&phgr;
30
and −&phgr;
40
.
Signal S
3
is applied to an input port
15
of a coupler C
2
, such as a conventional hybrid (e.g., branch line), a backward firing or a Wilkinson coupler. Coupler C
2
has a coupling coefficient of C
22
that is typically in the range of −10 to −20 dB. A coupled-path signal S
4
is output from a coupling port
16
and is, for example, 10 to 20 dB below the level of a direct-path signal S
8
that is output from a direct port
17
. The voltage levels of the frequency components of signal S
4
are each C
22
times the corresponding voltage levels of the signal S
3
frequency components. The voltage levels of the components of signal S
8
are {square root over (1+L −C
22
2
+L )} times the corresponding voltage levels of the components of signal S
3
. The respective phases −&phgr;
11
to −&phgr;
41
of the frequency components f1-f4 of signal S
4
are the same as the phases of the corresponding frequency components of signal S
8
. Specifically, phase values −&phgr;
11
and −&phgr;
12
are the combination of the insertion phase lags −&phgr;
10
and −&phgr;
20
through amplifier A
1
, respectively, plus the respective insertion phase lags at frequencies f1 and f2 through coupler C
2
. Phase values −&phgr;
31
and −&phgr;
41
are the insertion phase lags at the respective frequency f3 and f4 through coupler C
2
, plus the phase lag through amplifier A
1
.
Coupled-path signal S
4
is applied to a phase shifter
18
, such as a variable capacitor-type phase shifter, PIN diode phase shifter or a Shiffman phase shifter, for introducing a 180° phase shift at each of the frequencies f1-f4. A signal S
5
output from phase shifter
18
is input to a coupled port
19
of a coupler C
3
. Signal S
5
contains the same frequency components f1-f4 at the same voltage levels as signal S
4
, but with the phase of each respective component shifted by 180° from the corresponding components of signal S
4
. Specifically, the voltage levels of the f1 and f2 components of signal S
5
are C
22
G1C
11
V
1
and C
22
G1C
11
V
2
, respectively, and the respective phases are −&phgr;
11
−180° and −&phgr;
21
−180°. The voltage levels of the f3 and f4 components of signal S
5
are C
22
V
3
and C
22
V
4
, respectively, and the respective phases are −&phgr;
31
−180° and −&phgr;
41
−180°.
Signal S
2
is input to a delay line DL
1
, which outputs a signal S
6
. Signal S
6
is input to a port
20
of coupler C
3
. Delay line DL
1
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