Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-01-04
2003-09-02
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
Reexamination Certificate
active
06614304
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a variable gain circuit whose gain is externally controllable, and particularly, to a variable gain circuit operable at a low supply voltage.
A configuration using a Gilbert multiplier is common as a variable gain circuit. An example of a variable gain circuit according to a related art is shown in FIG.
3
. In
FIG. 3
, the related-art, variable gain circuit has a differential amplifier circuit
101
, two current divider circuits
102
and
103
, a control voltage (Vc) generating source
104
, and a bias voltage (Vb) generating source
105
.
The differential amplifier circuit
101
is formed by: NPN-type differential pair transistors Q
101
and Q
102
; an emitter resistance R
101
connected between emitter electrodes of the differential pair transistors Q
101
and Q
102
; and constant current sources I
101
and I
102
connected between a ground and the emitter electrodes of the differential pair transistors Q
101
and Q
102
, respectively. Base electrodes of the differential pair transistors Q
101
and Q
102
are connected to circuit input terminals
106
and
107
, respectively.
One current divider circuit
102
has a differential circuit configuration formed by NPN-type differential pair transistors Q
103
and Q
104
, each having an emitter electrode commonly connected to a collector electrode of the transistor Q
101
, and a resistance R
102
connected between a collector electrode of one transistor Q
103
and a power supply Vcc. A collector electrode of the other transistor Q
104
is connected directly to the power supply Vcc.
The other current divider circuit
103
has a differential circuit configuration formed by NPN-type differential pair transistors Q
105
and Q
106
, each having an emitter electrode commonly connected to a collector electrode of the transistor Q
102
, and a resistance R
103
connected between a collector electrode of one transistor Q
105
and the power supply Vcc. A collector electrode of the other transistor Q
106
is connected directly to the power supply Vcc.
The collector electrodes of the transistors Q
103
and Q
105
in the current divider circuits
102
and
103
are connected to circuit output terminals
108
and
109
, respectively. Base electrodes of the transistors Q
104
and Q
106
are commonly connected to a positive electrode side of the control voltage generating source
104
, whereas base electrodes of the transistors Q
103
and Q
105
are commonly connected to a negative electrode side of the control voltage generating source
104
. A positive electrode side of the bias voltage generating source
105
is connected to the base electrodes of the transistors Q
103
and Q
105
, while a negative electrode side of the bias voltage generating source
105
is connected to the ground.
In the thus formed variable gain circuit, letting Ic
1
and Ic
2
be collector currents of the transistors Q
103
and Q
104
and Vc be a control voltage of the control voltage generating source
104
,
Ic
2
/
Ic
1
=exp(
Vc/Vt
) (1)
where Vt=kT/q, k being the Boltzmann constant, T being the absolute temperature, and q being the amount of electron charge.
Letting IA be a current value of the constant current source I
101
, since Ic
1
+Ic
2
=IA,
Ic1
/
IA
=
Ic1
/
(
Ic1
+
Ic2
)
=
1
/
{
1
+
(
Ic2
/
Ic1
)
}
(
2
)
Hence, when substituting the equation (1) into the equation (2),
Ic
1
/
IA
=1/{1+(exp(
Vc/Vt
))} (3)
Letting vi be an input voltage, vo be an output voltage, RA be a resistance value of the resistance R
101
, and RB be resistance values of the resistances R
102
and R
103
, a gain Av of the variable gain circuit according to the related art is given by:
Av
=
vo
vi
=
2
RB
RA
·
1
1
+
exp
Vc
Vt
(
4
)
As is clear from the equation (4), the gain Av can be varied by the control voltage Vc of the control voltage generating source
104
.
However, the thus formed variable gain circuit according to the related art has a circuit configuration with the differential circuits piled in two stages in the direction of the supply voltage. Therefore, supposing that the base-to-emitter voltage of the bipolar transistors is about 0.9 V at a maximum, and supposing that when the constant current source I
101
is formed by a bipolar transistor, the collector-to-emitter voltage of the transistor is about 0.4 V, a voltage of about 2.2 V is required for operation of the differential circuits piled in two stages (differential amplifier circuit
101
and current divider circuit
102
) and the constant current source I
101
.
Furthermore, when device variations and the like are taken into consideration, a supply voltage of at least about 2.5 V is required to prevent saturation of the differential circuits and the constant current source I
101
. In general, when the dynamic range and transient characteristics are taken into consideration, the related-art, variable gain circuit formed as described above is of a circuit type operated under a supply voltage of about 3.3 V. Thus, while there has recently been a tendency toward lower supply voltage in portable terminals, such as portable telephones and PDAs (Personal Digital Assistants), the related-art, variable gain circuit formed as described above cannot meet the need for lower supply voltage.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problem, and it is accordingly an object of the present invention to provide a variable gain circuit operable at a lower supply voltage.
According to an aspect of the present invention, there is provided a variable gain circuit including: a first transistor and a second transistor, each having a control electrode connected to a circuit input terminal; a load connected between a first power supply and a first electrode of at least one of the first transistor and the second transistor; a third transistor and a fourth transistor having second electrodes connected to the first transistor and the second transistor, respectively, and each having a first electrode and a control electrode connected to each other; a first variable current source connected between a second power supply and the second electrodes of the first transistor and the third transistor and having a current value variable according to an external control signal; a second variable current source connected between the second power supply and the second electrodes of the second transistor and the fourth transistor and having a current value variable according to the control signal; a current source connected between the first power supply and a node of the first electrodes and the control electrodes of the third transistor and the fourth transistor; and an impedance component having one end connected to the first electrodes and the control electrodes of the third transistor and the fourth transistor.
Both bipolar transistors and field-effect transistors can be used as the first to fourth transistors. In the case of using a bipolar transistor, the first electrode refers to a collector electrode reached by a carrier (electron or hole), the second electrode refers to an emitter electrode for injecting the carrier, and the control electrode refers to a base electrode supplied with a current for controlling movement of the carrier injected from the emitter electrode. On the other hand, in the case of using a field-effect transistor, the first electrode refers to a drain electrode reached by a carrier, the second electrode refers to a source electrode for supplying the carrier, and the control electrode refers to a gate electrode supplied with a signal for controlling the main current.
REFERENCES:
patent: 5986501 (1999-11-01), Rafati et al.
patent: 2001/0050586 (2001-12-01), Pisati et al.
Shirakawa Tatsuya
Shoji Norio
Kananen Ronald P.
Mottola Steven J.
Rader & Fishman & Grauer, PLLC
LandOfFree
Variable gain circuit having external controls and a low... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Variable gain circuit having external controls and a low..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variable gain circuit having external controls and a low... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3034815