Amplifiers – With semiconductor amplifying device – Including temperature compensation means
Reexamination Certificate
1999-10-29
2001-09-18
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including temperature compensation means
C330S284000
Reexamination Certificate
active
06292059
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to systems, methods, and circuits for amplifying and, more specifically, to systems, methods, and circuits for providing thermal compensation in amplifiers.
BACKGROUND OF THE INVENTION
Communication systems are used to transmit and deliver information to consumers in a variety of ways. These systems include satellite, cellular, and wireline networks and the information can be virtually anything, from analog to digital thereby incorporating telephony, video, data, etc. Due to signal losses inherent in the transmission of signals over great distances, amplifiers are often necessary to boost the signal level. The amplifiers ensure that signals are at the proper power level as they pass through and out of the network.
Maintaining the signals at a constant power level is important for many systems, such as video distribution systems. For cable distribution systems, if the power signal level is either too high or too low, the reception at the customer's premises will be affected. If the power signal level is too high, the signal becomes saturated and the picture is distorted. On the other hand, when the power level is too low, the signal to noise ratio drops and the picture will have poor reception. The cable distribution systems consequently need to carefully monitor the power level of the signals and make appropriate adjustments.
The performance of a communication system, such as a cable distribution network, is affected by temperature. The lines that carry the signals have losses that vary with temperature. In addition to the lines, the performance of amplifiers within a communication system varies with the temperature. A cable distribution system is especially sensitive to temperature changes since the amplifiers and the coaxial cable are at ambient temperature, which can fluctuate greatly during a day or seasonally. Some communication systems consequently have to be operable over a range of temperatures from −40° C. to +60° C.
An example of a communication system
100
is shown in FIG.
1
. The communication system
100
includes headend equipment
105
for generating forward signals that are transmitted in the downstream direction along a communication medium, such as a fiber optic cable
110
, to an optical node
115
that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable
120
, and are amplified, as necessary, by one or more distribution amplifiers
125
positioned along the communication medium. Taps
130
included in the cable television system
100
split off portions of the forward signals for provision to subscriber equipment
135
, such as set-top terminals, computers, and televisions. In a two-way system, the subscriber equipment
135
can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers
125
, converted to optical signals, and provided to the headend equipment
105
.
A simplified diagram of the amplifier
125
is shown in FIG.
2
. As discussed above, the gain of the amplifier
125
is affected by temperature and the impedance of the lines, such as coaxial cable
120
, also varies with temperature. The output of the amplifier
125
should remain at a constant level to provide desired signal levels to the subscriber equipment
135
. In order to provide a constant output of power at the amplifier
125
, the amplifier
125
performs some adjustment based on temperature.
A conventional approach for providing for thermal compensation in an amplifier
125
is shown in FIG.
2
. With reference to this figure, signals from the coaxial cable
120
are first passed through a pre-amplifier
206
to boost the signal level. The amplifier
125
also includes one or more gain stages
210
for amplifying the signals to the desired power level. To adjust for temperature, the amplifier
125
includes a variable attenuator, such as a Bode circuit
208
coupled to a thermal compensation circuit
220
. The Bode circuit
208
is controlled by the thermal compensation circuit
220
so that the output of the Bode circuit
208
, and consequently the output of the gain stages
210
, is at a desired power level.
The operation of the thermal compensation circuit
220
will be described in greater detail with reference to FIG.
3
. The thermal compensation circuit
220
includes a thermistor voltage reference circuit
222
, a resistor
224
, and a current mirror
226
. The thermistor voltage reference circuit
222
outputs a voltage that varies with the temperature. The thermistor voltage reference circuit
222
typically is comprised of a resistor ladder and a thermistor placed in the ladder. The voltage from the thermistor voltage reference circuit
222
is converted into a current by the resistor
224
and this current is input to the current mirror
226
as a control signal. As the temperature changes, the voltage output by the thermistor voltage reference circuit
222
changes as does the current supplied to the current mirror
226
. The magnitude of the currents supplied to the Bode circuit
208
from the current mirror
226
therefore vary with the temperature. The Bode circuit
208
attenuates the signals received from the pre-amplifier
206
to a level determined by the currents received from the current mirror
226
. Thus, changes in temperature detected by the thermistor voltage reference circuit
222
can be used to alter the amount of attenuation provided by the Bode circuit
208
.
Despite the use of the Bode circuit
208
and the thermal compensation circuit
220
, conventional amplifiers, such as amplifier
125
, output at power levels that fluctuate with temperature. For instance, some amplifiers with thermal compensation still have fluctuations of ±1 dB. A need therefore exists for improved methods, systems, and circuits for providing thermal compensation in amplifiers.
REFERENCES:
patent: 4485349 (1984-11-01), Siegle et al.
patent: 5177453 (1993-01-01), Russell et al.
patent: 6052031 (2000-04-01), Ao et al.
Ao Jiening
Kruse Herman A.
Barnhardt III Hubert J.
Couturier Shelley L.
Massaroni Kenneth M.
Mottola Steven J.
Scientific-Atlanta, Inc.
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