Amplifiers – With semiconductor amplifying device – Including field effect transistor
Reexamination Certificate
2001-12-05
2003-05-20
Tokar, Michael (Department: 2819)
Amplifiers
With semiconductor amplifying device
Including field effect transistor
C330S302000, C330S305000, C331S11700R, C455S118000, C455S183100, C455S196100, C334S015000
Reexamination Certificate
active
06566953
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high-frequency amplifier circuits for UHF television tuners.
2. Description of the Related Art
Hereinafter, a known art is described with reference to
FIGS. 3
,
6
, and
7
.
FIG. 6
shows a known high-frequency amplifier circuit for a UHF television tuner (hereinafter referred to as a high-frequency amplifier circuit). UHF band television signals are input via an input tuning circuit
21
to a first gate G
1
of a dual gate FET
22
. Connected to the first gate G
1
is a peaking coil
23
for compensating gain reduction of a low UHF band, and a bias voltage is applied from a power supply B to the first gate G
1
via the peaking coil
23
. An AGC voltage A for varying the gain of the FET
22
is applied to a second gate G
2
via a feed resistor
24
. The second gate G
2
is grounded in a high-frequency state via a grounding capacitor
25
, which has a capacitance of several nF. A source S is also grounded in a high-frequency state.
The amplified UHF band television signals are output from a drain D and input to an interstage tuning circuit
26
. Then, UHF band television signals to be received are selected there and the selected signals are input to a mixer (not shown) in the next stage.
FIG. 7
shows a circuit diagram in which the FET
22
is equivalently represented with two single-gate FETs each having a grounded source or a grounded gate. Herein, it is difficult to securely ground the second gate G
2
in a high-frequency state due to the residual inductance of the grounding capacitor
25
and the inductance of a wiring conductor for connecting the grounding capacitor
25
. Actually, the second gate G
2
is grounded via an inductance component L having a low inductance. The impedance of the inductance component L is not negligible in the UHF band, because it generates distortion in an operation of the FET
22
and deteriorates a cross modulation characteristic.
The cross modulation characteristic is defined by a level of a disturbing signal which causes 1% of cross modulation. Accordingly, the higher the level is, the better cross modulation characteristic is. Since the cross modulation occurs in the high-frequency amplifier circuit, a variation in the level to attenuation in the gain of the high-frequency amplifier circuit is measured in general. In
FIG. 3
, X
1
represents a cross modulation characteristic in the known art. The cross modulation characteristic is deteriorated when attenuation in the gain is 25 dB or more, and the level of the disturbing signal is lowered to 96 dB&mgr; at 45 dB, thus easily causing cross modulation. Although the reason for this is not clear, a possible reason is a phase when the FET at the grounded gate is fed back negatively by the inductance component L.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a high-frequency amplifier circuit for a UHF television tuner, in which a dual gate FET of the circuit reduces distortion generated when a UHF band television signal is amplified.
To this end, according to the present invention, a high-frequency amplifier circuit for a UHF television tuner comprises an FET having a first gate for inputting a UHF band television signal and a second gate for applying an AGC voltage which varies the gain. The second gate is grounded via a series circuit including a resistor and a DC-cutting capacitor. The impedance of the DC-cutting capacitor is sufficiently lower than the resistance of the resistor in the UHF band.
With these features, the FET is fed back by the resistance, and distortion is improved.
Preferably, the resistor is connected to the second gate, the DC-cutting capacitor is grounded, and the AGC voltage is applied to the node between the resistor and the DC-cutting capacitor.
Accordingly, the voltage source impedance of the AGC voltage is not coupled to the second gate and the second gate is grounded securely via only the resistor.
Further, the resistance of the resistor may be in the range of 25 to 100&OHgr;. In such a case, the gain and noise figure are slightly degraded while the cross modulation and distortion of PCS beat are remarkably improved.
REFERENCES:
patent: 4048598 (1977-09-01), Knight
patent: 4091419 (1978-05-01), Rhee et al.
patent: 4442548 (1984-04-01), Lehmann
patent: 4590613 (1986-05-01), Tannery, IV
patent: 4703286 (1987-10-01), Muterspaugh
patent: 5576662 (1996-11-01), Price et al.
patent: 2001-156567 (2001-06-01), None
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Nguyen Linh V
Tokar Michael
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