Amplifiers – With pilot frequency control means
Reexamination Certificate
2001-01-05
2003-02-25
Pascal, Robert (Department: 2817)
Amplifiers
With pilot frequency control means
C330S151000
Reexamination Certificate
active
06525603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to radio frequency amplifiers and, more particularly, to radio frequency amplifiers having feedforward for reduction of distortion.
2. Description of the Related Art
Radio frequency (RF) amplifiers are used in a wide variety of applications, including communications. Ideally the transfer function of an RF amplifier is linear, with the output of the amplifier being an amplified replica of the input to the amplifier. However, RF amplifiers typically have some degree of non-linearity in their transfer function, particularly at high power levels. This non-linearity in an RF amplifier produces distortion in the RF amplifier output.
One approach used to reduce the distortion during RF power amplification is the feedforward amplifier. In a feedforward amplifier the instantaneous difference between a sample of the amplifier input signal and a sample of the amplifier output signal is measured. The difference signal is then amplified and subtracted from the RF power amplifier output, the amount of amplification being such as to cancel the existing distortion of the RF power amplifier.
FIG. 1
is a block diagram of a conventional feedforward amplifier
100
. An input signal is coupled to an input directional coupler
101
. The input directional coupler
101
divides the input signal sending a main portion of the input signal to a main amplifier
102
via a first loop modulator
103
, and a sample, or sense, portion to a first loop delay
104
. The main amplifier
102
amplifies the portion of the input signal routed to it. Amplification of the input signal by the main amplifier
102
produces distortion in the signal output from the main amplifier
102
. The output of the main amplifier
102
is routed through a first loop sampling directional coupler
106
to a second loop delay
108
.
The first loop directional coupler
106
extracts a sample of the output of the main amplifier
102
and routes the sample to a power combiner
110
. The other input to the power combiner
110
is the sense portion of the input signal output from the first loop delay
104
. The delay the sense portion of the input signal experiences as it passes through the first loop delay
104
is selected to match the delay experienced by the main portion of the input signal as it passes through the first loop modulator, the main amplifier
102
and the distortion sampling directional coupled
106
. The power combiner
110
destructively sums the two signals at its inputs. Thus, because the two inputs to the power combiner
110
are a sample of the original input signal and a sample of the input signal plus the distortion introduced by the main amplifier
102
, the power combiner
110
output is the distortion of the main amplifier
102
. The first loop modulator
103
may be used to adjust the amplitude and phase of the signal routed to the main amplifier
102
to minimize the amount of input signal (carrier) at the power combiner
110
output.
The output of the power combiner
110
is coupled to a second loop modulator
111
. The second loop modulator
111
adjusts the amplitude to a level to match the distortion level out of the main amplifier
102
, and the phase is adjusted to be 180° out of phase with the main amplifier
102
output, when combined in a distortion canceling directional coupler
114
. The output of the second loop modulator
111
is routed to an error amplifier
112
. The error amplifier
112
buffers the output of the second loop modulator
111
. The error amplifier
112
output is combined with the main amplifier
102
output from the second loop delay
108
in a distortion canceling directional coupler
114
. The delay experienced by the signal passing through the second loop delay
108
is selected to match the delay experienced by the signal passing through the power combiner
110
and the error amplifier
112
. Thus, the output of the distortion canceling directional coupler
114
is a substantially non-distorted replica of the input signal.
Typical problems in conventional feedforward amplifiers, as illustrated in
FIG. 1
, are that the amplifier performance is dependent on component characteristics and tolerances that affect the gain and phase of the device. In particular, proper operation of the feedforward amplifier requires that proper gain and phase be maintained for the feedforward signal to effectively cancel the distortion present in the main amplifier output. Thus, variations due to, for example, component age, component tolerance, or operating conditions such as temperature, power supply voltage, operating frequency, and humidity, may adversely affect the performance of the feedforward amplifier. In addition, there is no feedback to allow adjustments to correct for errors introduced by the components.
FIG. 2
is a block diagram of an improved feedforward amplifier
115
incorporating a pilot signal. Incorporating a pilot signal into the conventional feedforward amplifier
100
addresses several of the problems associated with the conventional feedforward amplifier
100
. In the improved feedforward amplifier
115
, a test signal, or pilot signal, is inserted into the signal path before the first loop modulator
103
and the main amplifier
102
via a pilot signal directional coupler
116
. In the pilot signal directional coupler
116
the pilot signal is mixed with the main portion of the input signal before delivery to the first loop modulator
103
and the main amplifier
102
.
The main amplifier
102
output, containing the pilot signal, is routed through the second loop delay
108
and is then sampled by an output sampling directional coupler
118
. The output sampling directional coupler is connected to a pilot detector
120
. The pilot detector
120
selectively detects the pilot signal present in the improved feed forward amplifier
115
output. For example, the detector may be a bandpass filter with a center frequency at the pilot signal frequency followed by an envelope detector. The output of the pilot detector
120
represents the amplitude of the pilot signal present in the output of the improved feedforward amplifier
115
. The pilot detector
120
output is routed to a second loop controller
122
.
As in the conventional feedforward amplifier illustrated in
FIG. 1
, the output of the main amplifier
102
is sampled by the first loop directional coupler
106
and routed to the power combiner
110
. In the improved feedforward amplifier
115
, the two inputs to the power combiner
110
are a sample of the main amplifier
102
output, with distortion and the pilot signal, and a sample of the original input signal. Thus the power combiner
110
output is the distortion of the main amplifier
102
plus the pilot signal. The first loop delay
104
is selected to match the delay introduced by the pilot signal directional coupler
116
, the main amplifier
102
, and the first loop directional coupler
106
. As described above the first loop modulator
103
may be used to adjust the amplitude and phase of the signal routed to the main amplifier
102
to minimize the distortion of the main amplifier
102
.
The output of the power combiner
110
is routed to a second loop modulator
124
. The second loop modulator
124
modifies the amplitude and phase of the output of the power combiner
110
as commanded by the second loop controller
122
. The output of the second loop modulator
124
is routed to the error amplifier
112
. As discussed in relation to the conventional feedforward amplifier
100
illustrated in
FIG. 1
, the error amplifier output is routed to a distortion canceling directional coupler
114
and is combined with the main amplifier
102
output.
The second loop controller
122
commands the second loop modulator
124
to adjust the amplitude and phase of the power combiner
110
output to minimize the amplitude of the pilot signal present in the improved feedforward amplifier
115
output, as measured by the detector
120
. The pilot signal behaves the same as the distortio
Heller Ehrman White & McAuliffe
Nguyen Khanh Van
Pascal Robert
Remec, Inc.
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