Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2002-07-24
2004-02-24
Le, Dinh (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S062000, C330S308000
Reexamination Certificate
active
06696866
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication devices and in particular to a wideband envelope tracking amplification system of a wireless communication device.
BACKGROUND OF THE INVENTION
Power amplifiers for wireless transmission applications, such as radio frequency (RF) power amplifiers, are utilized in a wide variety of communications and other electronic applications. Ideally, the input-output transfer function of a power amplifier should be linear, that is, should implement a constant gain adjustment and phase adjustment of an input signal, wherein a perfect replica of the input signal, increased in amplitude, appears at the output of the power amplifier.
In addition, for greater efficiency, various RF systems, such as cellular systems, attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the amplifier is just below its maximum rated power output level. This power output level is generally related to the supply voltage (or supply power) to the power amplifier, such that a greater supply voltage will produce a correspondingly greater output power from the amplifier; for higher power input signals, a correspondingly greater actual power output is required to maintain the amplifier at or near saturation. In various prior art amplifiers, however, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range of several orders of magnitude, use of a fixed supply voltage is highly inefficient, as output power is often an order of magnitude or more below its maximum, and the power amplifier is not maintained at or near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is power supply modulation (PSM) that varies, or modulates, the supply voltage to the power amplifier in order to maintain the power amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks the input signal variations, typically utilizing a signal detector in conjunction with a tracking power supply. In the prior art, however, the various PSM techniques have generally been limited to narrowband applications, or have poor efficiency characteristics.
For example, one prior art PSM technique, known as “envelope elimination and restoration” (EER), utilizes a limiter to provide an essentially constant drive level to the power amplifier to maintain the amplifier in a hard saturation state and increase efficiency. Use of the limiter, however, greatly expands the bandwidth of the RF signal input to the amplifier and requires very accurate tracking of the input signal envelope, with a power supply switching frequency approximately ten times greater than the bandwidth of the RF input signal. As these switching frequencies increase, the transistors within the tracking power supply become less efficient, resulting in excessive power losses. The resulting bandwidth expansion of the limiter also requires the bandwidth capability of the amplifier to be significantly greater than the input signal bandwidth, limiting the EER configuration to narrow bandwidth applications, such as amplitude modulation (AM) RF broadcasts.
Another prior art PSM technique, known as “envelope tracking,” does not utilize the limiter of EER. Consequently, envelope tracking power amplification systems may be more suitable for higher bandwidth applications.
FIG. 1
is a block diagram of an exemplary envelope tracking power amplification system
100
. A radio frequency (RF) signal
101
is coupled to an input
102
of amplification system
100
. A signal coupler
104
samples input signal
101
to produce a sampled input signal
105
and routes sampled input signal
105
to an envelope tracking power supply (ETPS)
106
. ETPS
106
tracks or detects an envelope of sampled input signal
105
to produce an envelope detector signal, typically a voltage, and produces a variable supply voltage
107
based on the detected envelope of input signal
101
. ETPS typically includes a switching power supply whose switching pulse width or frequency is varied in order to track the envelope of input signal
101
and produce variable supply voltage
107
.
ETPS
106
sources variable supply voltage
107
to an RF power amplifier
108
. Variable supply voltage
107
is designed to maintain RF power amplifier
108
at or near saturation and to increase the efficiency of power amplification system
100
over a wide range of variation in input signal
101
. When input signal
101
is a wideband RF signal, the switching power supply of ETPS
106
must have a very rapid response in order to track RF input signal
101
. However, if variable supply voltage
107
is to accurately reproduce the envelope of RF input signal
101
, then the switching frequency of ETPS
106
should be 5-10 times the bandwidth of input signal
101
. For example, if input signal
101
has a bandwidth of 20 MHz, as is common in multi-carrier amplification systems, then ETPS
106
should have a prohibitively high switching frequency of 100-200 MHz.
In order to resolve the requirement for a prohibitively high switching frequency power supply, schemes have been proposed for utilizing multiple voltage supplies in implementing the ETPS, such as in U.S. Pat. No. 5,239,275, entitled “Amplitude Modulator Circuit Having Multiple Power Supplies,” and U.S. Pat. No. 5,736,906, entitled “Power Supply Modulator Circuit for Transmitter.” Such schemes typically involve selecting a voltage supply of the multiple voltage supplies, or serially connecting one or more voltage supplies of the multiple voltage supplies, based on a detected instantaneous magnitude, or amplitude, of the input signal. However, multi-carrier input signals typically have a wide dynamic range, often in the range of 10-20 dB, due to high short-term peak-to-average power ratios caused by the multiple carriers and due to long term fluctuations in average power due to variations in traffic loading. In order to track a multi-carrier input signal, the ETPS may require a subdivision of an input signal voltage range into as many as 10 to 12 input signal amplitude steps and correspondingly may require as many as 10-12 voltage supplies. The use of such a large number of voltage supplies is both prohibitively expensive and complex.
Therefore, there is a need for a high efficiency, low cost method and apparatus for tracking a wideband RF signal under high dynamic range conditions.
REFERENCES:
patent: 5239275 (1993-08-01), Leitch
patent: 5736906 (1998-04-01), Jacobson
patent: 5886572 (1999-03-01), Myers et al.
patent: 6157253 (2000-12-01), Sigmon et al.
patent: 6324135 (2001-11-01), Kim et al.
Le Dinh
May Steven A.
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