Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
2003-03-19
2004-07-06
Choe, Henry (Department: 2817)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S279000, C455S126000
Reexamination Certificate
active
06759902
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to signal processing, and, in particular, to automatic gain control circuits for amplifiers, such as RF amplifiers.
BACKGROUND OF THE INVENTION
An important parameter associated with radio-frequency (RF) amplifiers is the amplification factor or gain. Numerous methods have been devised to provide automatic gain control (AGC) of RF amplifiers that function to maintain constant gain despite changes in operating parameters, such as temperature, voltage, signal level, and component age, to name a few.
Closed-loop AGC
FIG. 1
is a block diagram of a closed-loop AGC system
100
of the prior art. AGC system
100
has an RF signal generator
102
, closed-loop AGC circuit
104
, and load
106
. The objective of AGC system
100
is to amplify the RF signal produced by signal generator
102
by a fixed amount and deliver the amplified signal to load
106
(e.g., a resistor).
In particular, the RF signal from generator
102
is input to AGC circuit
104
at input terminal
108
. The RF input signal flows through directional coupler
110
and then to voltage-controlled attenuator (VCA)
112
. The purpose of VCA
112
is to vary the level of the RF input signal. The signal is then routed from the output of VCA
112
to the input of RF amplifier
114
. The output of RF amplifier
114
is routed through output coupler
116
and then to output terminal
118
, which is connected to load
106
.
The signal gain G between terminals
108
and
118
of AGC circuit
104
is determined by G=A−B, where A is the gain of amplifier
114
(e.g., in dB) and B is the loss of VCA
112
(e.g., in dB). As the value of gain A of RF amplifier
114
changes in response to various operating parameters, the value of loss B of VCA
112
is adjusted accordingly to maintain the overall gain between terminals
108
and
118
constant.
The control signal to automatically maintain VCA
112
at the proper level of insertion loss is provided by elements within closed-loop AGC circuit
104
. In particular, the input power level of the RF signal sampled by directional coupler
110
is detected by input detector
120
before being routed to the positive input terminal
124
of differential amplifier
122
. The amplified RF output signal is sampled by directional coupler
116
, attenuated (by approximately −A dB) by passive attenuator
128
(e.g., three resistors) before being detected by output detector
130
. The attenuated and detected version of the RF output signal sample is then routed to the negative input terminal
126
of differential amplifier
122
. The output of differential amplifier
122
will be set to a specific DC voltage Vr depending upon the difference between the sampled input power level present on positive input terminal
124
and the sampled output power level present on negative input terminal
126
. The output of differential amplifier
122
is routed to the control voltage input
132
of VCA
112
to control the level of attenuation (i.e., inverse gain) applied by VCA
112
to the RF input signal received from RF generator
102
.
For the following description of AGC action, the power level of the RF input signal received from RF generator
102
is assumed to remain constant. Environmental changes, such as elevated temperature, will cause the gain of RF amplifier
114
to decrease, resulting in a decrease in the power level of the RF output signal at terminal
118
. Accordingly, the attenuated and detected sample of the RF output signal presented to the negative input terminal of differential amplifier
122
will also decrease in value. The detected sample of the RF input signal presented to the positive input terminal of differential amplifier
122
will remain the same since the RF input signal is assumed to be held constant. As such, the output of differential amplifier
122
will increase in voltage, which increases the voltage on the control voltage input of VCA
112
. The transfer characteristics of VCA
112
are designed such that an increase in voltage on the control voltage input results in a decrease in the loss value B. The resulting decrease of attenuation of VCA
112
causes the input power level presented to the input of RF amplifier
114
to increase, which in turn causes the RF power level presented to the RF output signal to increase at output terminal
118
. The RF output power of amplifier
114
will continue to increase until the sampled, attenuated, and detected version of the RF output signal presented to negative input terminal
126
of differential amplifier
122
equals the sampled and detected version of the RF input signal presented to positive input terminal
124
of differential amplifier
122
. The output voltage of differential amplifier
122
will then be set to a value lower than the original value Vr, which restores the original gain between terminals
108
and
118
of AGC circuit
104
.
AGC operation is similar in response to environmental changes or other factors that cause the gain of RF amplifier
114
to increase, such as low-temperature operation. In this case, the sampled, attenuated, and detected version of the RF output signal increases in value even though the RF input signal power level remains constant. The rising value of the sampled, attenuated, and detected version of the RF output signal presented to negative input terminal
126
of differential amplifier
122
causes the output of differential amplifier
122
to decrease in voltage. This decrease in voltage on VCA control voltage input
132
causes VCA
112
to increase its level of attenuation B. This increase in attenuation causes the RF signal power level presented to the input of RF amplifier
114
to decrease, which in turn causes the RF output signal level present at output terminal
118
to decrease in power. The trend continues until the sampled, attenuated, and detected version of the output signal presented to negative input terminal
126
of differential amplifier
122
equals the sampled and detected version of the input signal presented to positive input terminal
124
of differential amplifier
122
. The output voltage of differential amplifier
122
will now be set to a value higher than the original value Vr, which restores the original gain between terminals
108
and
118
of AGC circuit
104
.
Closed-loop AGC circuit
104
can also be used to maintain the gain between terminals
108
and
118
due to change in the gain of RF amplifier
114
resulting from input signal level changes. Large-signal amplifiers implemented with bipolar devices and operating as Class AB devices for improved efficiency typically increase in gain as the input signal level is increased. This gain expansion causes the RF output signal to further increase beyond the expected amplifier amplification factor of A−B. Further increasing the input signal level (beyond the so-called gain compression point) eventually causes the RF amplifier gain to decrease or compress below the expected amplifier factor A as is well known to those skilled in the art.
Large-signal RF amplifiers implemented with new technology devices, such as laterally diffused metal oxide silicon (LDMOS) transistors, exhibit significantly improved linearity over the same dynamic range of input signal. As such, the gain of RF amplifier
114
remains substantially constant regardless of input signal level up until the gain compression point. The issue of gain compression is not of concern for many modern large-signal RF amplifier applications involving digital modulation. In such cases, RF amplifier
114
is sized such that the maximum RF signal output is well below the 1-dB gain compression point. Hence, closed-loop AGC operation as depicted in
FIG. 1
is not necessary to control gain expansion or gain compression of such LDMOS RF amplifiers in many digital modulation applications such as TDMA, CDMA, UMTS, or other well-known digital modulation formats.
On the other hand, employing closed-loop AGC on RF amplifiers in digital modulation applications presents special challe
Andrew Corporation
Choe Henry
Mendelsohn Steve
LandOfFree
Single-detector automatic gain control circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Single-detector automatic gain control circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Single-detector automatic gain control circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3216129