Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2000-03-02
2001-11-06
Lee, Benny (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S12400D, C330S002000
Reexamination Certificate
active
06313703
ABSTRACT:
APPENDIX
Attached as an appendix is a state machine specification for a preferred embodiment of the invention. The appendix forms part of the disclosure of the application.
TECHNICAL FIELD OF THE INVENTION
This application relates in general to power amplifiers and in particular to linear amplification of band limited signals using non-linear amplifiers.
BACKGROUND OF THE INVENTION
Radio frequency power amplifiers are widely used to transmit signals in communications systems. Typically a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of amplitude, phase or frequency or some combination of these which causes the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes the carrier itself is not sent since it is not essential to the communication of the information.
When a signal, which contains amplitude variations, is amplified it will suffer distortion if the amplifier does not exhibit a linear amplitude transfer characteristic. This means that the output is not linearly proportional to the input. It will also suffer distortion if the phase shift which the amplifier introduces in not linear over the range of frequencies present in the signal, or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced includes intermodulation of the components of the input signal. The products of the intermodulation appear within the bandwidth of the signal causing undesirable interference, as well as outside the bandwidth originally occupied by the signal. This can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements.
Although filtering can be used to remove the unwanted out of band distortion, this is not always practical, especially if the amplifier is required to operate on several different frequencies. Distortion products which are at multiples of the carrier frequency can also be produced in a nonlinear amplifier, but these can typically be removed by filtering.
Intermodulation is also a problem when multiple signals are amplified in the same amplifier even if individually they do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components beat with each other by adding and subtracting as their phase relationships change.
Amplifiers can introduce some distortion even if they are well designed. Perfect linearity over a wide range of amplitude is difficult to realize in practice. Moreover, as any amplifier nears its maximum output capacity, the output no longer increases as the input increases and thus it becomes nonlinear. A typical amplifier becomes significantly nonlinear at a small fraction of its maximum output capacity. This means that in order to maintain linearity the amplifier is often operated at an input and output amplitude which is low enough such that the signals to be amplified are in the part of its transfer characteristic which is substantially linear. This method of operation is described as “backed off,” in which the amplifier has a low supplied power to transmitted power conversion efficiency. A “Class A” amplifier operated in this mode may be linear enough for transmitting a signal cleanly, but might typically be only 1% efficient. This wastes power and means that the amplifier has to be large and relatively expensive. It also means that the wasted power is dissipated as heat which has to be removed by a cooling system.
Communication schemes using signals which have constant amplitude with frequency and phase modulation can use highly nonlinear amplifiers. These types of signals are unaffected by the distortion and the amplifiers can be smaller, cooler, more power efficient and less expensive. Modulation of this type is used in conventional radio paging systems which use CPFSK modulation.
Many of the newer, bandwidth efficient modulation schemes have both amplitude and phase variations. There is also a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for large, costly high level output signal combining filters which have undesirable power losses.
In the prior art, linearized amplifiers can be made by correcting for the nonlinearities of amplifiers using mechanisms such as cartesian feedback, predistortion and feedforward correction.
Cartesian feedback is a mechanism in which a monitoring system looks at the output of the amplifier and attempts to alter the input of the amplifier so that it produces the intended output. This is arranged as a direct feedback loop. The delay in the feedback path means that the correction can be too late to correct effectively, especially at higher bandwidths.
The predistortion mechanism attempts to correct for the nonlinear transfer characteristic of an amplifier by forming an inverse model of its transfer characteristic. This characteristic is applied to the low level signal at the input of the amplifier in a nonlinear filter, to pre-distort it such that when it passes through the amplifier the signal emerges amplified and substantially undistorted. This method is capable of excellent results over a relatively small bandwidth. The filter has to be updated to account for variations in the amplifier transfer characteristic and this is done by monitoring the output and periodically updating the corrections. The filter also has to change its coefficients as often as every sample using the values stored in memory.
The feedforward mechanism derives a signal which represents the inverse of the distortions produced by the amplifier. This is done by comparing the amplifier input and output. A small linear amplifier is used to amplify the distortion signal. This signal is then subtracted from the main amplifier output. This method operates correctly over a wider bandwidth than the predistortion mechanism. However, balancing the amplitude and delay of the distortion signal so that it cancels the main amplifier errors exactly is complicated to perform.
Both predistortion and feedforward are widely used in commercial products which can amplify multiple signals and work over wide amplitude ranges. Both methods are quite complicated and the power efficiencies are still not excellent. Feedforward amplifiers are typically only 5% efficient. The complicated processing requirements add to the cost and the power used and significant cooling capacity is still required to remove waste heat.
Another prior art amplifier is the LINC (Linear Nonlinear Component) amplifier
10
, as shown in
FIG. 1. A
signal which has amplitude variations can be generated by two signals which vary only in their relative phases. The vector sum of the two signals can represent any amplitude. Thus, it is possible to represent the instantaneous state of any signal or combination of signals. The phase and frequency of the component signals can also be made to represent that of the original so that when combined, the original signal is reconstructed.
In
FIG. 1
, LINC amplifier
10
amplifies tow or more constant amplitude signals, which represent an input signal to be amplified. The LINC amplifier uses a signal separator
11
to split the input
12
into the two components
13
,
14
, which are constant amplitude, phase varying components. The LINC amplifier may be supplied a complex baseband digitally sampled signal
12
. The baseband signals
12
can be a representation of multiple modulated carriers using any modulations. For simplicity, various details such as the need to convert from baseband to a higher frequency and the need to convert from digital into analog have been omitted.
Since amplitude variations do not have to be dealt with, it is possible to build an amplifier which will amplify signals linearly by using the two phase and frequency modulated components. The nonlinearity o
Bennett Steven J.
Wright Andrew S.
Datum Telegraphic, INC
Knobbe Martens Olson & Bear LLP
Lee Benny
Nguyen Khanh Van
LandOfFree
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