Amplifiers – With amplifier bypass means
Reexamination Certificate
2001-08-23
2002-07-16
Pascal, Robert (Department: 2817)
Amplifiers
With amplifier bypass means
Reexamination Certificate
active
06420929
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates generally to a feed forward power amplifier, and specifically to improvements in distortion cancellation and improvements in carrier cancellation and efficiency.
To provide high quality voice communication and handle more calls, telecommunication operators use digital modulation technology and multi-channel carriers. Examples of such technologies include CDMA, TDMA, w-CDMA, and EDGE-GSM. A linear power amplifier is needed to transmit digital/multi-carrier waveforms without adding noise, distortions, and adjacent channel interference. Typical RF power amplifiers used for a base station generate −30 dBc distortion levels. A digital/multi-carrier system, however, requires a power amplifier with −45 dBc for CDMA, −60 dBc for TDMA, and −70 dBc for GSM.
Several linearization techniques have been used to linearize or improve the traditional class AB power amplifier distortion performance. Such techniques include RF feedback, Cartesian feedback, analog pre-distortion, digital pre-distortion and feed forward. The RF feedback and Cartesian feedback techniques improve inter-modulation distortion (“IMD”) by 10 db but both techniques are only useful for narrowband. Analog pre-distortion improves IMD up to 8-9 db but is not stable over certain temperatures. Digital pre-distortion uses lookup table A/D and D/A converters to distort the input signal. This technique requires very fast A/D and D/A converters. Successful results have not been reported for work involving this technique.
The feed forward technique improves IMD by almost 30 db with reasonable bandwidth. The feed forward technique is widely recognized and is used by several power amplifier companies. This technique was disclosed in 1930 by H. Black, U.S. Pat. No. 1,686,792. The feed forward technique involves splitting an input signal into two paths, which process along two loops, the first loop producing a distortion component by combining an out of phase signal segment with an amplified signal containing distortion, and the second loop removing the distortion from the amplified signal by using an out of phase distortion component.
Improvements have been disclosed to the feed forward system. Improvements to the first loop have included adding control circuitry to monitor the carrier residue and adjust the gain and phase accordingly to minimize the carrier residue. Improvements to the second loop have included pilot tone injection at the input of a main amplifier which behaves as a distortion. So far, most improvements only aim to solve loop stability. The feed forward amplifier in the existing marketplace, however, does not meet the stringent Global System for Mobile communications (“GSM”) requirements. A dual loop feed forward may meet the GSM requirements, but this approach is very costly and has low efficiency due to using two error amplifiers and two output delay lines. Several power amplifier manufacturers have abandoned this approach.
U.S. Pat. No. 5,412,342 to Sakamoto, et al., for “Power Amplifier Device Comprising a Plurality of Feedforward Distortion Compensating Circuits in Parallel,” discloses using random phase cancellation in parallel feed-forward circuits, and more particularly, discloses a power amplifier device comprising a feed forward distortion compensating circuit comprising N complete feed forward circuits (N being an integer not less than two) that are connected in parallel and are supplied with input radio frequency signals through an N-divider. Component outputs of the feed forward circuits are combined in phase as the output signal by an N-combiner.
The use of N complete feed-forward amplifier circuits, however, is costly. Sakamoto does not disclose random phase cancellation within a stage of a single feed-forward circuit, which would be a less costly alternative.
U.S. Pat. No. 6,037,837 to Miyaji, et al., for “Feed Forward Amplifier” discloses a feed forward power amplifier which cancels distortion by dividing an input signal into two divided signals and employs a vector adjuster to adjust the phase of the distortion components. The use of N power amplifiers connected in parallel coupled with a vector adjuster, is disclosed, but random phase cancellation within a stage of a single feed-forward circuit is not disclosed. The need to employ N power amplifiers and vector adjuster circuitry makes this approach more costly than random phase cancellation within a stage of a single feed-forward circuit.
What is needed is a cost efficient feed forward power amplifier to transmit digital/multi-carrier waveforms with minimum distortion, preferably using random phase cancellation within a stage of a single feed-forward circuit.
SUMMARY OF INVENTION
The present invention meets this need by providing a cost efficient feed forward power amplifier for transmitting digital/multi-carrier waveforms with minimum distortion, using random phase cancellation within a stage of a single feed-forward circuit.
An N way carrier cancellation coupler is provided, N being greater than 1, for use in a feed forward power amplifier, comprising a first N way divider, having an input and N outputs, a second N way divider, having an input and N outputs, an N way combiner having N inputs and an output, N carrier cancellation couplers, each having a first input, a second input and an output, the first input of each of the N carrier cancellation couplers connected to one of the N outputs of the first N way divider, the second input of each of the N carrier cancellation couplers connected to one of the N outputs of the second N way divider, and the output of each of the N carrier cancellation couplers connected to one of the N inputs of the N way combiner.
An M way distortion cancellation coupler is also provided, M being greater than 1, for use in a feed forward power amplifier, comprising a first M way divider, having an input and M outputs, a second M way divider, having an input and M outputs, an M way combiner having M inputs and an output, M distortion cancellation couplers, each having a first input, a second input and an output, the first input of each of the M distortion cancellation couplers connected to one of the M outputs of the first M way divider, the second input of each of the M distortion cancellation couplers connected to one of the M outputs of the second M way divider, and the output of each of the M distortion cancellation couplers connected to one of the M inputs of the M way combiner.
An N way carrier cancellation coupler is described for use in the first loop stage of a feed forward power amplifier, comprising first divider means for dividing a signal N ways and outputting N divided signals, second divider means for dividing a delayed signal N ways and outputting N delayed signals, N carrier cancellation coupler means, each for receiving one of the N divided signals from the first divider means, and one of the N delayed signals from the second divider means, and producing one of N error signals from a coupled form of the received one of the N divided signals and received one of the N delayed signals; and signal combiner means for receiving each one of the N error signals from the N carrier cancellation coupler means and producing a combined error signal from a combined form of the N error signals.
An M way distortion cancellation coupler is described for use in the second loop stage of a feed forward power amplifier, comprising first divider means for dividing an amplified error signal M ways and outputting M divided error signals, second divider means for dividing a delayed signal with distortion M ways and outputting M divided signals, M distortion cancellation coupler means, each for receiving one of the M divided error signals from the first divider means, and one of the M divided signals from the second divider means, and producing one of M main signals from a coupled form of the received one of the M divided error signals and received one of the M divided signals; and signal combiner means for receiving each of the M main signals from ea
Nguyen Khanh Van
Pascal Robert
Rose Robert J.
Sheldon & Mak
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