Amplifiers – With semiconductor amplifying device – Including differential amplifier
Utility Patent
1999-01-07
2001-01-02
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C327S359000
Utility Patent
active
06169452
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a gain control amplifier, a variable gain amplifier and an automatic gain control amplifier using the variable gain amplifier.
BACKGROUND INFORMATION
Automatic gain control (AGC) amplifiers are commonly used in receivers. The overall performance of receivers depend on the input dynamic ranges and noise factors of the AGC amplifiers. The input dynamic range is defined by the ratio of the maximum input signal amplitude for linear operation of the AGC amplifier and the minimum input signal for which the output amplitude is the nominal output amplitude of the amplifier. The output dynamic range is defined as the ratio of the maximum output signal and the minimum output signal for a given input dynamic range. For most of the applications, the input dynamic range is 100 to 300 (40 to 50 dB) and the output dynamic range is 1.2 to 1.5 (1.5 to 3.5 dB). The noise factor is a measure of the amplifier equivalent input noise and is defined as the degradation of the signal to noise ratio due to the AGC noise. In general, the input dynamic range for linear operations must be large to accommodate without degradation different applications and the equivalent input noise must be low to minimize the signal to nose ratio degradation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved gain control amplifier, variable gain amplifier and AGC amplifier.
According to one aspect of the present invention, there is provided a gain control amplifier for amplifying an input voltage and providing an amplified output voltage, comprising first, second and third differential circuits and current split means.
The first differential circuit comprises first and second transistors, the emitters of which are coupled, and a first load element connected to the collector of the first transistor.
The second differential circuit comprises third and fourth transistors, the emitters of which are coupled, and a second load element connected to the collector of the fourth transistor, the bases of the third and fourth transistors being coupled to the bases of the second and first transistors, respectively, the amplified output voltage being provided from the collectors of the first and fourth transistors.
The third differential circuit comprises fifth and sixth transistors, the emitters of which are coupled through a pair of resistance elements, the junction of the resistance elements being connected to a first current circuit, the collector of the fifth transistor being connected to the coupled emitters of the first and second transistors, the collector of the sixth transistor being connected to the coupled emitters of the third and fourth transistors, the input voltage being fed to the bases of the fifth and sixth transistors.
In the gain control amplifier, the current split means splits current flowing in the first and second load elements from the respective transistors. The current flowing in the fifth transistor of the third differential circuit is proportional to the difference between the current flowing in the first load element and the split current. Similarly, the current flowing in the sixth transistor of the third differential circuit is proportional to the difference between the current flowing in the second load element and the split current. Due to current splitting, relatively small currents drive the fifth and sixth transistors which amplify the input voltage. Since the relatively small currents flow in the resistance elements coupled to the emitters of the fifth and sixth transistors, noise caused by the transistors is relatively low. Thus, it provides a wide input dynamic range with low noise.
For example, the current split means comprises fourth and fifth differential circuits. The fourth differential circuit comprises seventh and eighth transistors, the emitters of which are coupled, the coupled emitters being connected to a second current circuit, the collector of the seventh transistor being connected to the collector of the first transistor, the bases of the seventh and eight transistors being connected to the bases of the second and first transistors, respectively. The fifth differential circuit comprises ninth and tenth transistors, the emitters of which are coupled, the coupled emitters being connected to a third current circuit, the collector of the tenth transistor being connected to the collector of the fourth transistor, the bases of the ninth and tenth transistors being connected to the bases of the fourth and third transistors, respectively. The current split means further comprises split control means for controlling currents flowing in the transistors of the differential circuits in response to a voltage difference between the voltages at the collectors of the first and fourth transistors. The split control means comprises base voltage control means for generating a variable base voltage in response to the voltage difference, the variable base voltage being fed to the bases of the transistors of the first, second, fourth and fifth differential circuits. In response to the variable base voltage, the currents flowing in the transistors of the first, second, fourth and fifth differential circuits are varied to vary the amplifier's gain.
According to another aspect of the present invention, there is provided a variable gain amplifier comprising: an input stage amplifier for amplifying an input voltage; and a main amplifier for further amplifying an input stage amplified voltage and providing an output voltage, the input stage amplifier comprising the gain control amplifier.
According to another aspect of the present invention, there is provided an automatic gain control amplifier comprising: the variable gain amplifier, the variable gain amplifier amplifying an input voltage and providing an amplified voltage; detection means for detecting variations of the output voltage of the variable gain amplifier; and means for comparing the detected output to a reference voltage and providing again control voltage to the variable gain amplifier.
REFERENCES:
patent: 5896063 (1999-04-01), Marsh
Howlett Kathryn Louise
Popescu Petre
Wight Mark S.
de Wilton Angela C.
Nortel Networks Corporation
Shingleton Michael B
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