Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2000-03-09
2001-09-04
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S086000, C330S277000, C330S294000
Reexamination Certificate
active
06285257
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a feedback type variable gain amplifier, which is specifically suitable to an amplifier used for high frequency band, UHF or higher band.
2. Related Background Art
FIG. 1
shows a circuit diagram of a conventional feedback type variable gain amplifier.
The conventional feedback type variable gain amplifier is provided with an amplifier AMP
31
, and a gain control circuit GC
31
which is a feedback circuit coupled to the amplifier AMP
31
and controls a value of the gain. The amplifier AMP
31
includes a series-connected high frequency cutoff inductor L
31
and amplifying field effect transistor FET
31
across a power-supply potential terminal Vdd and a ground potential terminal GND. The gate of the transistor FET
31
is used as a signal input terminal Vin, and the drain is used as a signal output terminal Vout. The gain control circuit GC
31
includes a series-connected gain control field effect transistor FET
32
and direct-current cutoff capacitor C
31
across the signal output terminal Vout and the signal input terminal Vin. Furthermore, a resistor R
31
connected across the drain and the source of the gain control field effect transistor FET
32
.
In this conventional feedback type variable gain amplifier, the resistor R
31
maintains levels at the drain and the source of the transistor FET
32
to the same potential, the capacitor C
31
isolates levels at the gate of the transistor FET
31
from the power-supply potential Vdd. Furthermore, by varying a voltage Vgc applied to a gain control terminal G
32
, which is the gate of the transistor FET
32
, the transistor FET
32
and the resistor R
31
function as a variable resistor having a resistance value R(Vgc).
FIG. 2
shows a circuit diagram of an equivalent circuit of the gain control circuit GC
31
in the conventional feedback type variable gain amplifier shown in FIG.
1
.
The equivalent circuit of the gain control circuit GC
31
is expressed by a series connection of a variable resistor R
41
and a direct-current cutoff capacitor C
41
. And as described above, since the transistor FET
32
and the resistor R
31
of the gain control circuit GC
31
function as a variable resistor, an impedance Z of the gain control circuit GC
31
is represented as follows:
Z=R
(Vgc)+1/(
j&ohgr;C
)
where, R(Vgc) is a variable resistance value between the drain and the source of the transistor FET
32
, and R(Vgc)<<R
31
.
A phase difference of a voltage to a current flowing through the circuit is represented as follows:
&ohgr;=tan
−1{−1/(&ohgr;CR)}
<0
In the conventional feedback type variable gain amplifier, the direct phase difference between the input voltage and the output voltage is 180°. However, as the frequency increases, the phase difference decreases because of the shifting of the phases of the input voltage and the output voltage. The range of the phase of the impedance Z of the gain control circuit GC
31
is −90°<&thgr;<0°. The phase of the signal fedback to the input terminal Vin of the amplifier AMP
31
via the gain control circuit GC
31
has a value which is obtained by adding the phase difference of the gain control circuit GC
31
to the phase difference of the amplifier AMP
31
.
In order to obtain an ideal negative feedback for the amplifier, it is desirable to feedback the signal to the input terminal of the amplifier with the phase difference 180°. However, in the conventional feedback type variable gain amplifier shown in
FIG. 1
, the decay of the gain is small because the phase difference of the signal fedback to the input terminal of the amplifier can not be 180°, especially in high frequency band, UHF or higher band. Consequently, there is a problem that it is difficult to expand the range of the controllable gain.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a feedback type variable gain amplifier having a wide range of the controllable gain even in high frequency bands.
According to the present invention, a feedback type variable gain amplifier of the present invention is provided with a first inductor and a first field effect transistor connected in series across a power-supply potential terminal and a ground potential terminal, the one end of the first inductor connecting to the power-supply potential terminal, the one end of the first field effect transistor connecting to the ground potential terminal, the gate of the first field effect transistor being used as a signal input terminal, and the other end of the first field effect transistor being used as a signal output terminal, a second inductor, a second field effect transistor and a capacitor connected in series across the drain and the gate of the first field effect transistor, and, a resistor connected across the drain and the gate of said second field effect transistor. Since the second inductor is connected to the capacitor in series between the drain and the gate of the first field effect transistor, the imaginary part of the impedance of the gain control circuit which includes the second inductor, the second field effect transistor, the capacitor and the resistor is controllable, it will become capable to widen the range of the gain control in high frequency bands, especially in UHF or higher frequency bands by controlling the phase of the signal feedback to the signal input terminal of the amplifier.
To achieve the widest range of the gain control, in a feedback type variable gain amplifier according to the present invention, the values of the inductance of said second inductor and the capacitance of said capacitor are set to the proper values so that the phase difference between the signal input to the gate of said first field effect transistor and the signal fedback to the gate of said first field effect transistor via said second inductor, said second field effect transistor and said capacitor is approximately 180° when the value of the resistance between the drain and the source of said second field effect transistor is set to the minimum value by controlling the voltage applied to the gate of said second field effect transistor. In other words, by setting the impedance of the gain control circuit so that the signal supplied to the gate of the first field effect transistor and the signal feedback to the gate of the first field effect transistor via the gain control circuit cancel each other and the decay of the gain comes to larger when the amount of feedback signal of the gain control circuit which is the feedback circuit, it will become capable to obtain the widest range of the gain control by varying the gate potential of the second field effect transistor arranged in the gain control circuit.
REFERENCES:
patent: 4366450 (1982-12-01), Suganuma
patent: 4439741 (1984-03-01), Turner, Jr.
patent: 5051705 (1991-09-01), Moghe et al.
patent: 5570064 (1996-10-01), Sugawara
patent: 5590412 (1996-12-01), Sawai et al.
patent: 11-298268 (1999-10-01), None
K. Nishikawa, et al., Proceedings of the 1995 Electronics Society Conference of IEICE, vol. 2, p. 65, “Thermal Stability of Low-Distortion Variable-Gain Amplifier Using Active Feedback,” Sep. 1995.
Abe Hiroyuki
Nagasawa Hironori
Kabushiki Kaisha Toshiba
Nguyen Patricia T.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Robert
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