Gain control circuit for controlling a gain in a variable...

Details Gain control circuit for controlling a gain in a variable... Gain control circuit for controlling a gain in a variable...

2002-07-03

2004-04-06

Nguyen, Long (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S256000, C330S289000

Reexamination Certificate

active

06717469

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gain control circuit for controlling fluctuations of gain.
2. Description of Related Art
In a variable gain amplifier used for a radio communication system, it is desired that a gain obtained in the variable gain amplifier is exponentially changed with respect to a gain control voltage. Therefore, a conventional variable gain amplifier shown in
FIG. 6
is, for example, used.
FIG. 6
is a constitutional view showing a conventional variable gain amplifier. In
FIG. 6
,
11
indicates a first voltage-to-current converter,
12
indicates a temperature characteristic compensating circuit,
13
indicates an exponentially-changing current producing circuit,
14
indicates a variable gain cell,
15
indicates a second voltage-to-current converter, and
16
indicates a temperature-proportional bias current generating circuit. A combination of the temperature characteristic compensating circuit
12
and the exponentially-changing current producing circuit
13
functions as a gain control circuit, and the exponentially-changing current producing circuit
13
has a differential amplifier
13
a.
Next, an operation of the conventional variable gain amplifier will be described below.
Referring to
FIG. 6
, a gain control voltage V
cont
is applied to the first voltage-to-current converter
11
, and the gain control voltage V
cont
is converted into a gain control current proportional to the gain control voltage V
cont
. Here, a value of the gain control current is expressed by K
1
·V
cont
·K
1
denotes a proportional constant. Thereafter, the gain control current (K
1
·V
cont
) is fed to the temperature characteristic compensating circuit
12
according to a current mirror. Also, a bias current proportional to an absolute temperature T is generated in the temperature-proportional bias current generating circuit
16
, and the bias current is fed to the temperature characteristic compensating circuit
12
according to a current mirror. The value of the bias current is expressed by K
2
·T, and K
2
denotes a proportional constant.
Also, a reference voltage V
ref
fixed with respect to temperature is applied to the second voltage-to-current converter
15
, and a gain reference current corresponding to the reference voltage V
ref
is produced. The value of the gain reference current is constant and is expressed by K
3
. The gain reference current of the value K
3
is fed to the temperature characteristic compensating circuit
12
according to a current mirror.
As is described above, the gain control current (K
1
·V
cont
), the bias current (K
2
·T) and the gain reference current (K
3
) are respectively fed to the temperature characteristic compensating circuit
12
according to the current mirrors. This type of temperature characteristic compensating circuit
12
is equivalent to a temperature characteristic compensating circuit having a plurality of current sources generating the gain control current (K
1
·V
cont
), the bias current (K
2
·T) and the gain reference current (K
3
) respectively.
As shown in
FIG. 6
, the temperature characteristic compensating circuit
12
is composed of a group of a first transistor Q
1
, a second transistor Q
2
, a third transistor Q
3
, a fourth transistor Q
4
and a group of a first current source
12
a
, a second current source
12
b
, a third current source
12
c
and a fourth current source
12
d
. Each of the transistors Q
1
to Q
4
is formed of an n-p-n transistor (or a first conductive type transistor). A differential amplifier is formed of both the second and third transistors Q
2
and Q
3
. Here, the gain control current (K
1
·V
cont
) is generated in each of the first current source
12
a
and the fourth current source
12
d
, the gain reference current (K
3
) is generated in the second current source
12
b
, and the bias current (K
2
·T) is generated in the third current source
12
c.
In the temperature characteristic compensating circuit
12
, a base of the first transistor Q
1
is connected to both a collector and a base of the fourth transistor Q
4
, and the first current source
12
a
is connected to the base of the first transistor Q
1
. The fourth current source
12
d
is connected to both an emitter of the fourth transistor Q
4
and a base of the third transistor Q
3
. The second current source
12
b
is connected to both an emitter of the first transistor Q
1
and a base of the second transistor Q
2
. The third current source
12
c
is connected to both an emitter of the second transistor Q
2
and an emitter of the third transistor Q
3
. A first output current of a value I
L
is output from a collector of the second transistor Q
2
, and a second output current of a value I
R
is output from a collector of the third transistor Q
3
.
Therefore, the value I
L
of the first output current is expressed according to an equation (1).
I
L
=K
1
×K
2
×T×V
cont
/K
3
  (1)
Also, the value I
R
of the second output current is expressed according to an equation (2).
I
R
=K
2
×T
−(
K
1
×K
2
×T×V
cont
/K
3
)   (2)
The first output current of the value I
L
is fed to the exponentially-changing current producing circuit
13
according to a current mirror. Also, the bias current (K
2
·T) is fed to the exponentially-changing current producing circuit
13
according to a current mirror (not shown). Therefore, the exponentially-changing current producing circuit
13
has current sources generating the first output current (I
L
) and the bias current (K
2
·T) respectively. The exponentially-changing current producing circuit
13
is composed of a differential amplifier
13
a
, a fifth current source
13
b
, a sixth current source
13
c
, a seventh current source
13
d
, a first resistor having a value R
1
and a second resistor having the value R
1
. The differential amplifier
13
a
is composed of a pair of fifth transistor Q
5
and sixth transistor Q
6
. Each of the transistors Q
5
and Q
6
is formed of an n-p-n transistor (or a first conductive type transistor). A base voltage is applied to the base of the fifth transistor Q
5
through the first resistor, and a base voltage is applied to the base of the sixth transistor Q
6
through the second resistor. The bias current (K
2
·T) is generated in each of the fifth current source
13
b
and the sixth current source
13
c
, and the first output current (I
L
) is generated in the seventh current source
13
d
. The bias current (K
2
·T) generated in the fifth current source
13
b
is called a reference current of a value Ia, the bias current (K
2
·T) generated in the sixth current source
13
c
is called a fixed current of a value Ie, and the first output current (I
L
) generated in the seventh current source
13
d
is called a control current of a value Ic.
The fifth current source
13
b
of the reference current (Ia) is connected to a base of the fifth transistor Q
5
, the sixth current source
13
c
of the fixed current (Ie) is connected to both an emitter of the fifth transistor Q
5
and an emitter of the sixth transistor Q
6
, and the seventh current source
13
d
of the control current (Ic) is connected to the base of the sixth transistor Q
6
. Also, the first resistor (R
1
) is connected to the base of the fifth transistor Q
5
, and the second resistor (R
1
) is connected to the base of the sixth transistor Q
6
. A third output current having a value I
o
is output from a collector of the fifth transistor Q
5
.
Because the value Ic of the control current is equal to the value I
L
of the first output current, the value Ic of the control current is expressed according to an equation (3) with reference to the equation (1).
Ic
=
 
⁢
K
1
×
K
2
×
T
×
V
cont
/
K
3
=
 
⁢
K
2
×
T
×
K
4
×
V
cont
(
3
)
Here, K
4
=K
1
/K
3
is satisfied.
As shown in
FIG. 6
, in the exponentially-changing current producing circuit
13
, a voltage proportional to a difference between the control current Ic

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