Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2002-12-04
2004-04-06
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S279000, C330S285000
Reexamination Certificate
active
06717471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for automatically adjusting gain variations resulting from a fabrication process for an amplifier to be mounted on an integrated circuit (hereinafter abbreviated as “IC”) and to an amplifier to which the automatic gain adjustment circuit is applied. In particular, it relates to an amplifier effective in a low-noise amplifier (hereinafter abbreviated as “LNA”) for use in a communication transceiver or the like.
2. Description of the Related Art
As shown in
FIG. 18
, an LNA
182
is used in a mobile terminal such as a communication transceiver to amplify an extremely small signal received by an antenna
181
. The received signal amplified by the LNA
182
is subjected to frequency conversion in a mixer
183
, supplied to a programmable gain amplifier
185
through a band pass filter
184
, and subsequently transmitted to a demodulation circuit. A small signal amplifier such as the LNA
182
is normally formed as an IC. In that case, proper setting of the operating points (operating current and operating voltage) and gain of an amplifying element and the stabilization thereof becomes important.
FIG. 19
shows a typical conventional example of an amplifier formed as an IC having a bias circuit for adjusting the operating points. A source-grounded MOS (Metal Oxide Semiconductor) transistor (hereinafter abbreviated as “MOST”)
21
is used for the amplifier. A signal inputted from an input signal source
1
to an input terminal
17
is amplified and an output signal is retrieved from an output terminal
16
formed at the connection point between the drain of the MOST
21
and a load (ZL)
12
. The operating current Id of the MOST
21
is adjusted by a bias circuit
14
via a resistor
15
. The adjustment involves adjusting a DC output voltage at the output terminal
16
such that the dynamic range of an ac output signal is ensured or adjusting an output current flowing in the MOST
21
such that the transconductance of the MOST
21
and the gain determined by the load
12
have respective design values.
On the other hand, there has been known a circuit shown in
FIG. 20
as means for implementing a method for independently controlling the gain without changing the voltage at the output terminal (see, e.g., JP-A No. 308651/2001). In the drawing, a current in a constant current source
205
connected in parallel with a load
204
is adjusted such that a current allowed to flow in a MOST
201
has a specified value, i.e., that the transconductance (hereinafter abbreviated as “gm”) of the MOST
201
has a specified value, whereby the gain of the MOST
201
is controlled to a desired value. At this time, a bias voltage is supplied to the MOST
201
via a resistor
206
but a voltage E
O
at an output terminal
16
hardly changes since it is determined by the gate-to-source voltage V
GS
in the MOST
201
. The operating current of a transistor
203
is determined by a current source
202
.
However, the amplifier shown in
FIG. 19
suffers power supply fluctuations and temperature fluctuations in an actual situation and further undergoes device variations resulting from changes in fabrication conditions even if it has been designed optimally at a given power supply voltage, at a given temperature, and under given manufacturing conditions. Therefore, the operational voltage, i.e., the DC output voltage at the output terminal
16
and the gain are mostly different from design values. In addition, the gain of the amplifier is normally designed to prevent the distortion of an ac output waveform even if such power supply fluctuations, temperature fluctuations, and device variations as to satisfy conditions under which the gain becomes maximum, i.e., maximum gain conditions are encountered. This has caused the problems that, if such power supply fluctuations, temperature fluctuations, and device variations as to satisfy the minimum gain conditions are encountered, an output signal becomes smaller and the DC output voltage at the output terminal
16
greatly changes simultaneously. A change in DC output voltage affects an input bias voltage in an amplifier in the subsequent stage.
In the amplifier shown in
FIG. 20
, a current value in the constant current source
205
is fixed when the amplifier is formed as an IC so that it is impossible to tolerate power supply fluctuations, temperature fluctuations, and device variations. Since the voltage E
O
is determined by the voltage V
GS
, as stated previously, the voltage E
O
cannot be controlled to an arbitrary value.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the problems of the conventional amplifiers described above and provide an automatic gain adjustment circuit for automatically adjusting the gain and DC output voltage of an amplifier against power supply fluctuations, temperature fluctuations, and process variations and an amplifier using the automatic gain adjustment circuit.
In accordance with the present invention, feed back control is performed with respect to a bias circuit for gain adjustment and a variable current source for DC output voltage adjustment, each of which is provided in the automatic gain adjustment circuit, so that the gain and DC output voltage of an amplifier are set automatically. This allows the set gain and DC output voltage to be held constant even if a power supply and a temperature fluctuate after the fabrication of the amplifier as an IC or if the IC fabrication process varies.
To solve the problems, the present invention uses a method illustrated in
FIG. 1
as a gain adjustment method. In the drawing, the adjustment is performed in a bias circuit
14
and a variable current source
13
.
An amplifying element
11
having a control electrode a, a ground electrode b, and an output electrode c is composed of, e.g., a MOST, a bipolar transistor, a metal semiconductor (MES) transistor, a hetero-junction transistor, or the like. A bias voltage is supplied from the bias circuit
14
to the control electrode a of the amplifying element
11
via a resistor
15
, which determines the operating current of the amplifying element
11
. A signal to be amplified is inputted from the input signal source
1
to the input electrode
17
by the control electrode a. The load
12
is connected between the output electrode c of the amplifying element
11
and a power supply Vdd, while the variable current source
13
is connected to the output electrode c of the amplifying element
11
. The output terminal
16
is disposed at the connection point between the load
12
and the output electrode c of the amplifying element
11
.
The operating current of the amplifying element
11
is the sum of the current in the variable current source
13
and a current flowing in the load
12
. A voltage at the output terminal
16
is determined by the load
12
and a current flowing in the load
12
.
If a bias voltage in the bias circuit
14
is changed while a current in the variable current source
13
is held constant, the operating current of the amplifying element
11
changes and the current flowing in the load
12
change so that a DC output voltage at the output terminal
16
changes.
If the current in the variable current source
13
is changed while the bias voltage in the bias circuit
14
is held constant, the operating current of the amplifying element
1
hardly changes, while the current flowing in the load
12
changes, so that the DC output voltage at the output terminal
16
changes. The reason for the operating current of the amplifying element
1
which hardly changes even if the DC output voltage changes is that the internal impedance of the amplifying element
11
is generally extremely high and hence the amplifying element
11
can be regarded as a substantially constant current source.
By adjusting the current value in the variable current source
13
and the bias voltage in the bias circuit
14
, therefore, it becomes possible to change the operating current without changing the DC outpu
Arayashiki Satoshi
Doi Takeshi
Maio Kenji
A. Marquez, Esq. Juan Carlos
Fisher Esq. Stanley P.
Hitachi , Ltd.
Nguyen Khanh Van
Reed Smith LLP
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