Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-11-21
2003-07-29
Tokar, Michael (Department: 2819)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S149000, C330S254000, C330S257000, C330S260000, C330S261000, C330S270000, C330S271000, C330S273000, C330S279000
Reexamination Certificate
active
06600371
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention refers to a low noise amplifier, particularly to a radio frequency low noise amplifier.
2. Description of Related Art
In the field of telecommunication integrated devices a low noise amplifier is typically utilized as input stage for a receiver. Such amplifiers must assure besides a low noise in working frequency band thereof, even an efficient reduction of the distortion of the signals that are to be amplified.
Many problems of the same circuit of the amplifier are linked both to the optimisation of the noise and distortion performances and the control of the input impedance of the amplifier. Such problems have, above all in the radio frequency field, a basilar importance because the input impedance of the amplifier fixes, with a given source resistor, the adaptation conditions of the amplifier circuit at its interface with the input signal, thereby causing the ROS (Stationary Wave Ratio) of the receiver.
Open loop or feedback technologies can be utilized to control the input impedance of the low noise amplifiers. A typical circuit typology of a low noise amplifier that uses a feedback technology is shown in
FIG. 1
where a differential stage is described which is constituted by two bipolar transistors Q
10
and Q
20
which have the emitter terminals connected with each other and with a current generator Iee, connected in turn with the voltage Vee, and the respective collector terminals connected with each emitter terminal of two bipolar transistors Q
30
and Q
40
. Such transistors have each base terminal connected with a bias voltage VB and each collector terminal connected both with a resistor Rc, connected in turn with a supply voltage Vcc, and with a base terminal of the transistors Q
50
and Q
60
which have in turn the collector terminals connected with the supply voltage Vcc and the emitter terminals connected with current generators connected with the voltage Vee. The base terminals of the transistors Q
10
and Q
20
, which represent the input terminals of the amplifier (which has in input a voltage VIN), are coupled with the emitter terminals of the transistors Q
50
and Q
60
, which represent the output terminals OUT− and OUT+ of the amplifier, by means of a resistor RF. In such amplifier the input impedance is fixed by the feedback resistor RF and it is equal to:
Ri
≈
2
RF
Av
=
2
RF
g
m1
,
2
Rc
wherein Av is the voltage gain of the amplifier circuit. Since the input impedance in such circuit configuration of the amplifier is fixed by a physical resistor and by a voltage gain, it results constant in a very large frequency band and therefore it makes this circuit configuration adapted to the large band appliances.
However a circuit configuration as that in
FIG. 1
does not allow to solve the distortion problem. An added specification required to a low noise amplifier is therefore the gain variability, in fact an adjustable gain allows to optimise the distortion performances of the amplifier. The gain variation can be controlled statically by a regulation with an external trimming or a software programming, or it can be controlled by means of a closed loop technology with an AGC (Automatic Gain Control) circuit. An amplifier employing a technology of the above mentioned type is shown in
FIG. 2
where a differential stage is shown which is constituted by two bipolar transistors Q
11
and Q
21
the emitter terminals of which are connected with each other and with a current generator Iee, connected in turn with the voltage Vee, and the respective collector terminals connected with other two differential stages Q
31
, Q
41
and Q
51
, Q
61
and specifically with their emitter terminals. The collector terminals of the transistors Q
31
and Q
51
are connected with a resistor Rc, connected in turn with a supply voltage Vcc, and with a base terminal of the transistor Q
70
, while the collector terminals of the transistors Q
41
and Q
61
are connected with a resistor Rc, connected in turn with the supply voltage Vcc, and with the base terminal of the transistor Q
80
. The collector terminals of the transistors Q
70
and Q
80
are connected with the supply voltage Vcc while each one of the emitter terminals, which constitute respectively the output terminals OUT− and OUT+, is connected with a current generator connected with the voltage Vee. The base terminals of the transistors Q
11
and Q
21
constitute the input terminals of the amplifier (which has in input a voltage VIN), while between the common base terminal of the transistors Q
41
and Q
51
and the common base terminal of the transistors Q
31
and Q
61
a voltage Vc is applied which serves to regulate the gain of the low noise amplifier. By such a circuit configuration a maximum gain is obtained equal to:
Av
MAX
=g
m1,2
Rc
for example with a voltage Vc≧0.2V, while the minimum gain is obtained for example with Vc=0 and it is limited only by the incorrect coupling (mismatch) of the transistor couples belonging to the three differential stages.
However in such circuit configuration the input impedance cannot be correctly controlled; in fact the insertion of the variable gain does not allows to achieve a constant input impedance.
SUMMARY OF THE INVENTION
In view of the art described, it is an object of the present invention to provide a low noise amplifier that overcomes the aforementioned problem.
According to the present invention, such object is obtained by means of a low noise amplifier comprising a first circuit block suitable for converting a first amplifier input voltage signal into current, a second circuit block adapted to divide the current coming from said first block, said second block being controlled by a second voltage signal, said first and second blocks conferring a variable voltage gain to the amplifier, characterised by comprising at least one first and at least one second resistors and a feedback network said at least one first resistor connected with one first output terminal of said second block and with a supply voltage, and said at least one second resistor being connected between said at least one first and at least one second output terminals of said second block, and said feedback network being coupled with said at least one first terminal and with said first circuit block, and said at least one second terminal being coupled with at least one output terminal of said low noise amplifier.
Thanks to the present invention it is possible to provide a low noise amplifier having the variable gain and fixed input impedance features.
REFERENCES:
patent: 4380739 (1983-04-01), Velo
patent: 4560947 (1985-12-01), Frey
patent: 5327097 (1994-07-01), Igarashi et al.
patent: 5532637 (1996-07-01), Khoury et al.
patent: 5587689 (1996-12-01), Bowers
patent: 5884154 (1999-03-01), Sano et al.
patent: 5999056 (1999-12-01), Fong
patent: 6100759 (2000-08-01), Sirna et al.
patent: 6124761 (2000-09-01), Robinson et al.
Bongini Stephen
Fleit Kain Gibbons Gutman & Bongini P.L.
Jorgenson Lisa K.
Nguyen Linh Van
STMIcroelectronics S.r.l.
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