Amplifiers – With semiconductor amplifying device – Including distributed parameter-type coupling
Reexamination Certificate
2000-11-30
2002-06-04
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including distributed parameter-type coupling
C330S295000, C330S302000
Reexamination Certificate
active
06400226
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a distributed amplifier, more particularly, to a distributed amplifier as a monolithic microwave integrated circuit (MMIC).
2. Description of the Related Art
The distributed amplifier has been employed at a stage before electric-to-photo conversion or after photoelectric conversion since it has wideband characteristics.
FIG. 7
shows a prior art typical distributed amplifier.
A terminating circuit
29
of an output transmission line
20
having elements
21
to
28
consists of a terminating resistor
291
almost equal to the characteristic impedance of the transmission line
20
and a capacitor
292
for AC grounding, serially connected to each other. The capacitor
292
can reduce power consumption caused by applying a drain bias voltage VDD to the resistor
291
if the capacitor
292
is not connected.
An output voltage signal Vout=0 when an input voltage signal Vin=0. In this state, a DC gate bias voltage VGG and a DC drain bias voltage VDD are respectively applied to the gate and drain of each of FETs
31
to
34
to force a DC bias current to flow through each of the FETs
31
to
34
.
When the input voltage signal Vin is superimposed on the bias voltage VGG, the signal Vin propagates along an input transmission line
10
and a part thereof is applied to the gates of FETs
31
to
34
. In the FET
31
for example, a signal component (i
1
+i
2
)) is superimposed on the bias current, wherein i
1
and i
2
are currents flowing through the output transmission line
20
to the terminating circuit
29
and the output OUT, respectively. Currents flowing from the amplifying FETs
31
to
34
to the output OUT are simply summed at the output OUT since line lengths from the input IN to the output OUT through the respective FETs
31
to
34
are the same as each other and in turn the respective currents therefrom have the same phase at the output OUT.
In order to make a frequency characteristic of the gain wider in bandwidth, it is necessary to employ FETs
31
to
34
having smaller gate capacitances. However, as the gate capacitances are smaller, the gains of the FETs
31
to
34
become lower.
In order to solve this problem, employed is a distributed amplifier configured such that, as shown in
FIG. 8
, capacitors
51
to
54
are connected between the gates of respective FETs
31
to
34
and the input transmission line
10
and thereby, a combined capacitance of each gate capacitance and each capacitor is reduced. In this configuration, the gate bias voltage VGG is applied to the gates of FETs
31
to
34
through resistors
41
to
44
, respectively.
In both of the distributed amplifiers of
FIGS. 7 and 8
, since the impedance of the capacitor
292
can be neglected in regard to the high frequency components of the input voltage Vin, the impedances of the terminating circuit side and the output OUT side viewed from the drain of the FET
31
are almost equal to the characteristic impedance, leading to the relation of i
1
=i
2
. This applies to each case of the FETs
32
to
34
in similar manner. However, since the capacitance of the capacitor
292
cannot be neglected in regard to the low frequency components of the input voltage signal Vin, the relation of i
1
<i
2
holds. This again applies to each case of the FETs
32
to
34
in similar manner. For this reason, as shown in
FIG. 4
, the gain of the distributed amplifier in a low frequency band is higher than that in a high frequency band where the gain stays flat, and it tends to increase as the frequency is lower in the low frequency band.
If the capacitance
292
is omitted in order to prevent the increase in the gain in the low frequency band, power consumed in the distributed amplifier is increased by the drain bias voltage VDD applied to the resistor
291
.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a distributed amplifier capable of improving a flatness of its gain over a low frequency band in a case where a capacitor for ac-grounding is employed in a terminating circuit of an output transmission line.
In one aspect of the present invention, there is provided a distributed amplifier comprising a plurality of series-connected circuits, provided for respective amplifying transistor, each including a capacitor and a resistor connected in series to each other between the control input of the corresponding transistor and a reference potential, each having impedance lower than the input impedance of the transistor in a frequency band lower than a frequency, but higher than this input impedance in a frequency band higher than this frequency.
A current flowing through each series-connected circuit can be neglected in the high frequency band, but cannot be neglected in the low frequency band. As a frequency is lower, an input signal to the transistor decreases and the output signal thereof decreases in the low frequency band.
On the other hand, as a frequency is lower, the impedance of a terminating circuit including a capacitor and a resistor connected in series increases in the low frequency band, thereby a current signal flowing to the output side of an output transmission line from the transistor is larger than a current signal flowing to the terminating circuit located at the opposite side thereof.
Hence, the amplitude of the current signal flowing to the output side can be flattened in magnitude over the low frequency band. That is, the flatness of the gain over the low frequency band can be improved.
REFERENCES:
patent: 4788511 (1988-11-01), Schindler
patent: 6049250 (2000-04-01), Kintis et al.
patent: 6201445 (2001-03-01), Morimoto et al.
Armstrong Westerman & Hattori, LLP
Choe Henry
Fujitsu Limited
Pascal Robert
LandOfFree
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