Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
1999-10-11
2001-01-30
Shingleton, Michael B (Department: 2817)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S133000, C455S241100, C455S245200, C455S247100, C455S253200
Reexamination Certificate
active
06181201
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to automatic gain control circuits. More particularly, the present invention relates to a novel and improved automatic gain control circuit capable of independently controlling multiple variable gain amplifier stages while maintaining an estimate of received signal power.
2. Description of the Related Art
In modern communication systems, it is common for a receiver to contain automatic gain control (AGC) circuitry to amplify or attenuate received signals to a desired reference level for further processing by the receiver. An exemplary AGC circuit is described in U.S. Pat. No. 5,099,204, entitled “LINEAR GAIN CONTROL AMPLIFIER”, assigned to the assignee of the present invention and incorporated herein by reference. A communication system using such AGC circuits is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated herein by reference. The foregoing system is also described by EIA/TIA Interim Standard IS-95, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular system” (hereinafter IS-95), incorporated herein by reference.
A mobile station in an IS-95 system, in addition to requiring that incoming signals be gain controlled for further processing, must ensure that its transmitted signals are tightly power controlled so as not to interfere with other mobile stations in the system. Such a power control scheme is described in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference. One element in this power control scheme is the use of a measurement of received signal power, and so in contrast to systems where the only requirement of an AGC circuit is to provide incoming signals at the appropriate reference level, an IS-95 AGC circuit must allow for calculation of received signal strength.
Ideally, amplifiers could be constructed which were perfectly linear, at least over some range. Then the amplifier would be characterized by the equation f(x)=k
1
x, where f(x) is the output, x is the input and k
1
is the gain of the amplifier. In reality, amplifiers are not perfectly linear, and that non-linearity introduces distortion into the signal being amplified. Of all the possible input voltages, an amplifier has what is called its “linear” range and its “non-linear” range. The linear range is where the amplifier most closely approximates a linear amplifier. The distortion introduced can be approximated as a third order component. A more realistic characterization of an amplifier is given by the equation f(x)=k
1
x+k
3
x
3
. Here k
3
is the gain of the third order component. An amplifier with a smaller value for k
3
will be more linear than an amplifier with a higher value.
One type of distortion introduced by non-linear amplifiers that is particularly troublesome comes from intermodulation terms of two frequencies that are outside the band of interest for a mobile station. An example of this can be seen when an IS-95 system is deployed in close proximity to a narrowband system such as AMPS or GSM. The performance of an amplifier with respect to intermodulation is given by its IP3 point. For calculation purposes, it is assumed that the transmitters of the desired band and the source of the undesired frequencies are co-located. This means that as a mobile station moves toward the transmitter, both the desired received power and the intermodulation power increase. The IP3 point is the point where the third order intermodulation power of two equal power tones offset in frequency is equal to the desired first order term. To optimize the IP3 performance of an amplifier, the third order gain, k
3
, should be minimized.
One way to increase IP3 performance is to increase the “linear” range of the amplifier. Supplying more current to the amplifier can do this. However, in typical mobile communication systems, power in a mobile station is at a premium and increasing current is only done when absolutely necessary. Reduced power consumption translates into increased standby and talk time in a mobile station, or alternatively in a reduced battery requirement that leads to smaller and lighter mobile stations. An alternative to increasing the linear range is to reduce the amplitude of the incoming signal so that it stays within the existing linear range of the amplifier.
IS-95 specifies a minimum level of what it describes as intermod rejection.
FIG. 1
shows a typical intermod rejection ratio plot. For a given range of received power, the receiver must be able to tolerate a certain amount of interference, or have a certain intermod rejection ratio (IMR), as shown by the line labeled “spec” between specification points S
1
and S
2
. The intermod rejection ratio of an amplifier with fixed IP3 will increase ⅓ of a dB for every dB increase in received input power. The slope of the spec line may not be ⅓ of a dB per dB, and in fact it is not in IS-95. The IS-95 slope is approximately a 1 dB per dB slope. For a spec as shown, an amplifier must meet the specification at point S
2
. This would yield an IMR given by the line A
1
. To meet the specification requirement at point S
1
, a lower current amplifier could be used which would yield an IMR given by A
2
. As shown, the amplifier that meets point S
2
is overdesigned for point S
1
. This overdesign can equates to an increase in bias current resulting in reduced battery life, or more expensive components being required, or both.
An AGC design that could exhibit the properties attributed to lines A
1
or A
2
is shown in FIG.
2
. Received signals at antenna
100
are directed to ultra high frequency (UHF) low noise amplifier (LNA)
110
. A dashed arrow is shown through amplifier
110
to indicate the option of having it be a variable gain amplifier. That variable gain configuration will be discussed below. The received signal is amplified by LNA
110
and downconverted in mixer
115
via UHF frequency generated by UHF local oscillator
120
. The downconverted signal is passed through band pass filter
130
and amplified by intermediate frequency (IF) variable gain amplifier
140
. This amplified IF signal is then downconverted in mixer
145
via IF frequency generated by IF frequency generator
150
. The received signal is now at baseband, and received signal strength indicator (RSSI)
160
generates an estimate of the received signal power. The difference of this estimate and a reference power stored in power reference
165
is calculated in adder
170
, and RX AGC
180
acts on this error difference to produce the appropriate AGC_VALUE
195
. AGC_VALUE
195
is fed through a linearizer
190
to variable gain amplifier
140
. Linearizer
190
compensates for any non-linear dB/V characteristics of variable gain amplifier
140
. Linearization is described in U.S. Pat. No. 5,627,857, entitled “LINEARIZED DIGITAL AUTOMATIC GAIN CONTROL”, assigned to the assignee of the present invention and incorporated herein by reference. RX AGC
180
could be a variety of circuits as known in the art which alter AGC_VALUE so as to drive the difference calculated in adder
170
to as close to zero as possible. Once this loop is converged, the baseband signal out of mixer
145
is at the appropriate input power level and can be further demodulated (in circuitry not shown). Typically, the IF downconversion is done on the in-phase and quadrature components of the signal, and additional filtering is performed, but those details are not shown for the sake of clarity. The circuit as just described will exhibit the IMR response of line A
1
in
FIG. 1
when designed at a fixed current level. Note that AGC_VALUE can be used to estimate the received power, but only after factoring in the
Brown Charles
Qualcomm Incorporated
Shingleton Michael B
Streeter Tom
Wadsworth Philip
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