Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2000-05-12
2001-05-08
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C333S08100R
Reexamination Certificate
active
06229370
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an amplifier that amplifies high frequency signals and is capable of controlling its gain using a control voltage, and more particularly to an amplifier as a semiconductor device in integrated circuit form for use in a high frequency section of a transmitter in a mobile communication terminal.
BACKGROUND ART
In recent years, a technique called CDMA has been becoming a world standard as access means in the mobile communication field. CDMA communication systems, however, have entailed a big problem called the near-far problem in which as the distance between a mobile terminal and a base station decreases, leakage power to adjacent channels increases, increasing the bit error rate and degrading the communication quality.
To overcome this problem, signal output power must be controlled in accordance with the distance between the mobile terminal and the base station. More specifically, in view of the size of the cell range covered by the base station, gain control over a wide range of 70 dB or greater in terms of gain control width must be performed at the mobile terminal transmitter. Moreover, as a feature of CDMA, extremely precise gain control is performed at any given distance from the base station. Therefore, gain control having excellent linearity with a flatness of ±1 dB is essential.
Furthermore, when gain is attenuated at the mobile terminal transmitter, noise figure increases; therefore, if the gain is attenuated over a wide range of 70 dB or greater at intermediate frequencies lower than about 500 MHz where the carrier signal level is low, it would become difficult to distinguish between the level of the carrier signal and the level of noise signals, resulting in a degradation of communication quality. To avoid this problem, the gain control must be performed at high frequencies higher than about 500 MHz where the carrier signal level is high enough that the level of the carrier signal is easily distinguishable from the level of noise signals.
To accomplish the gain control having excellent linearity with a flatness of ±1 dB over a wide gain control range of 70 dB or greater at a mobile terminal transmitter, it has been practiced in the prior art to manufacture two separate semiconductor devices for performing two different kinds of gain control, i.e., the first packaged semiconductor device for performing control in step-like manner using a point in a high frequency range at which the gain changes largely and the second packaged semiconductor device for performing continuous control using an intermediate frequency range where the gain changes linearly, and to connect the two semiconductor devices together using an external circuit. In a mobile terminal, such gain control is performed using separate semiconductor devices with built-in microcomputer logics.
The reason that the two separate kinds of gain control are performed is that gain control having excellent linearity over a wide range, for example, a flatness of ±1 dB over a gain control range of 70 dB or greater, has been difficult to accomplish using a single semiconductor device forming the high frequency section.
A description will be given below of amplifiers as representative semiconductor devices for performing gain control in a prior art mobile communication terminal transmitter.
FIG. 19
is a circuit diagram showing the configuration of an amplifier (semiconductor device) which performs gain control in steplike manner in a high frequency section of the prior art mobile communication terminal transmitter. With this amplifier, the gain is controlled in steplike manner using apoint where the gain changes largely.
As shown in
FIG. 19
, the amplifier includes a signal line
77
containing a series variable resistor
71
and connecting between an input terminal
14
as a signal input part and an output terminal
15
as a signal output part, and shunt (parallel) variable resistors
72
and
73
are connected between a ground line
76
and the input terminal
14
and output terminal
15
, respectively. The ground line
76
is connected to ground GND which is a base potential part. A gain control line
75
is connected to the variable resistors
71
,
72
, and
73
. In this amplifier, a gain control voltage application terminal
4
as a gain control voltage application part is connected to the variable resistors
71
,
72
, and
73
via the gain control line
75
.
The variable resistors
71
,
72
, and
73
are constructed from field effect transistors
6
,
1
, and
9
whose gates are connected to resistors
7
,
5
, and
13
, respectively. The drain of the field effect transistor
6
forming the variable resistor
71
is connected to the input terminal
14
, and the source is connected to the output terminal
15
. On the other hand, the drain of the field effect transistor
1
forming the variable resistor
72
is connected to the input terminal
14
via a capacitor
2
, and the source is connected to the ground GND via a capacitor
3
and ground line
76
. Likewise, the drain of the field effect transistor
9
forming the variable resistor
73
is connected to the output terminal
15
via a capacitor
10
, and the source is connected to the ground GND via a capacitor
11
and ground line
76
.
Further, the gate of the field effect transistor
6
forming the variable resistor
71
is connected to the gain control voltage application terminal
4
via the resistor
7
and gain control line
75
, the source of the field effect transistor
1
forming the variable resistor
72
is connected to the gain control voltage application terminal
4
via the gain control line
75
, and the source of the field effect transistor
9
forming the variable resistor
73
is connected to the gain control voltage application terminal
4
via the gain control line
75
. Supply voltage VDD (about 3 V, a battery voltage itself) from a lithium battery or the like is applied to the source of the field effect transistor
6
forming the variable resistor
71
, while GND potential is applied via the resistors
5
and
13
to the gates of the field effect transistors
1
and
9
forming the respective variable resistors
72
and
73
.
Here, the capacitors
2
,
3
,
10
, and
11
each act to prevent the application of a dc voltage, while the resistors
7
,
5
, and
13
act to block the penetration of high frequency signals.
In this amplifier, the gain control is performed by adjusting the amount of attenuation, and its functional block intended as an amplifier for raising the gain is not shown in the figure. Therefore, as far as the circuit of
FIG. 19
is concerned, the circuit functions as an attenuator.
FIG. 20
shows characteristic diagrams illustrating how the gain control is accomplished in the amplifier of
FIG. 19
when the threshold voltage Vth of each of the field effect transistors
6
,
1
, and
9
is −1.0 V. FIG.
20
(
a
) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the field effect transistor (series FET)
6
forming the series variable resistor
71
. FIG.
20
(
b
) shows the gain control voltage Vc versus gain (amount of attenuation) characteristics of the field effect transistors (shunt FETs)
1
and
9
forming the parallel variable resistors
72
and
73
; the solid line shows the characteristic of the two transistors combined, and the dashed line the characteristic of either alone. FIG.
20
(
c
) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the amplifier of
FIG. 19
, obtained by combining the characteristics of FIGS.
20
(
a
) and
20
(
b
).
When the threshold voltage Vth of each of the field effect transistors
6
,
1
, and
9
is −1.0 V, as stated above, for the field effect transistors
1
and
9
forming the parallel variable resistors
72
and
73
the gain (amount of attenuation) varies over a range of 14 dB with a slope of 46 dB/V in proportion to the applied gain control voltage Vc when the gain control voltage Vc is varied within the range of 0.7
Inamori Masahiko
Motoyoshi Kaname
Tara Katsushi
Matsushita Electric - Industrial Co., Ltd.
Stevens Davis Miller & Mosher LLP
Tran Toan
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