Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
1999-10-22
2001-01-30
Lee, Benny (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S252000, C330S260000, C330S305000, C455S150100
Reexamination Certificate
active
06181207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a current amplifier having an input terminal and an output terminal intended to receive and supply an input current and an output current, respectively, comprising a first and a second transistor each having a bias terminal, a transfer terminal and a reference terminal, the reference terminals of the first and second transistors being interconnected via a first resistor.
2. Description of the Related Art
Among multiple possible applications, such an amplifier is often used for forming an input stage, called preamplifier, within a low-noise amplifier intended to amplify a signal coming from a frequency converter in an apparatus for receiving electromagnetic signals. In the current low-noise amplifiers, the preamplifier is constituted by the first and second transistors arranged in a differential pair configuration, in which a first resistor which connects their reference terminals, then forms a degeneration resistor. If, for example, the first and second transistors are bipolar transistors, their bias terminals, transfer terminals and reference terminals will be constituted by their bases, collectors and emitters, respectively. The advantage of the known preamplifier is that its gain, defined as the ratio between the value of the AC component of the output current and that of the input current, and, consequently, the gain of the overall low-noise amplifier, can be easily controlled by adjusting the value of the first resistor. However, the known preamplifier has a high input impedance because its input terminal is constituted by the bias terminal of one of the first or second transistors. In conformity with Ohm's law, this strong input impedance induces the necessity for the frequency converter to generate a signal having a considerable voltage swing, and thus involves a strong energy consumption, which is a considerable drawback in an era in which one of the major aims of research and development in the manufacture of integrated circuits is precisely the reduction of the energy consumption in such circuits. Moreover, a high input impedance of the preamplifier induces constraints in the dimensioning of passive elements, such as inductances or capacitances, included in the frequency converter. Indeed, the nominal values of these elements should be lower as the input impedance of the amplifier is higher. The real values of these passive elements will inevitably exhibit differences with respect to their nominal values, due to imperfections related to the employed manufacturing process, which differences will influence the behavior of the frequency converter to a larger extent as the nominal values are lower. Thus, of a too high input impedance of the preamplifier may introduce unpredictabilities in the functioning of the frequency converter, which is to be prohibited.
SUMMARY OF THE INVENTION
It is an object of the invention to remedy these drawbacks to a large extent by proposing a current amplifier having an input impedance which is low in comparison with that of a known preamplifier, and whose gain can be easily controlled.
To this end, according to the invention is, a current amplifier as described in the opening paragraph characterized in that, the bias terminal of the first transistor to recieve a voltage of a predetermined value, the transfer terminal of the first transistor being connected to the output terminal of the amplifier via a second resistor, the reference terminal of the first transistor constituting the input terminal of the amplifier, the bias and transfer terminals of the second transistor being connected to the transfer terminal of the first transistor and to a positive power supply terminal, respectively.
In this current amplifier, the input terminal is constituted by the reference terminal of the first transistor, which intrinsically has an impedance which is lower than that at the bias terminal. It will hereinafter be shown that the gain of such an amplifier can be easily controlled by adjusting the values of the first and second resistors. This structure is remarkable because of its simplicity, which enables to remedy the drawbacks as described above at low manufacturing costs. Moreover, such a simplicity renders the amplifier particularly suitable for applications where the power supply voltage has a low value.
In a particular embodiment of the invention, the nominal values of the first and second resistors are substantially equal. A very considerable gain can be obtained with such a choice.
The amplifier described above has the function of amplifying current signals which are asymmetrical by nature. For reasons essentially related to the necessity of suppressing parasitic signals which may appear during frequency conversion, the frequency converter often has a symmetrical structure, generating a signal of a differential nature. The known preamplifier, constituted by a degenerated differential pair, is capable of processing such a signal. This preamplifier has then a first and a second input terminal formed by the bias terminals of the transistors constituting the differential pair, and a first and a second output terminal formed by the transfer terminals of these transistors. The input impedance of such a preamplifier, which is the impedance viewed from the bias terminals of the transistors, is large, which induces drawbacks similar to those described above.
A variant of the invention provides a current amplifier intended to amplify signals of a differential nature, this amplifier having an input impedance which is low as compared with that of the known preamplifier, and whose gain can be easily controlled.
A current amplifier in accordance with this variant of the invention has a first and a second input terminal for receiving an input current of a differential nature, and a first and a second output terminal for supplying an output current of a differential nature, comprising a first, a second, a third and a fourth transistor, each having a bias terminal, a transfer terminal and a reference terminal, the reference terminals of the first and second transistors being interconnected via a first resistor, the reference terminals of the third and fourth transistors being interconnected via a second resistor, the bias terminals of the first and third transistors to receiving a voltage of a predetermined value, the transfer terminals of the first and third transistors being, respectively, connected to the reference terminals of the fourth and second transistors, whose transfer terminals are connected to the first and second output terminals of the amplifier via a third and a fourth resistor, respectively, the reference terminals of the first and third transistors constituting the first and second input terminals of the amplifier, and the bias and transfer terminals of the second and fourth transistors being connected to the transfer and bias terminals of the fourth and second transistors, respectively.
In this current amplifier, the first and second input terminals are constituted by the reference terminals of the first and third transistors, which intrinsically have a lower impedance than that of their bias terminals. It will also be shown hereinafter that the gain of such an amplifier can be easily controlled by adjusting the values of the first, second, third and fourth resistors.
In a particular embodiment of the invention, the nominal values of the first and second resistors are substantially equal to a first nominal value, and the nominal valves of the third and fourth resistors are substantially equal to a second nominal value which is half the first nominal value. With such a choice, a very considerable gain can be obtained.
While the present invention can be used in all types of applications necessitating a current amplification, it will be advantageously used for amplifying signals which are representative of radio signals supplied by a frequency converter. The invention thus also relates to an apparatus for receiving radio signals, comprising:
an antenna and filte
Chevallier Gilles
Stikvoort Eduard F.
Biren Steven R.
Lee Benny
Nguyen Patricia T.
U.S. Philips Corporation
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