Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2003-03-21
2004-11-30
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000
Reexamination Certificate
active
06825722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a mixer for mixing an AC signal with a reference signal having a particular frequency, and also to a differential amplifier for amplifying the difference between two signals and outputting a resultant amplified differential signal.
2. Description of Related Art
In an RF (Radio Frequency) receiving circuit, a received RF signal is mixed by a mixer with an LO (Local Oscillator) signal and the RF signal is down-converted into an IF (Intermediate Frequency) signal.
FIG. 7
illustrates a mixer in an RF receiving circuit.
FIG. 8
illustrates an exemplary process in which the mixer shown in
FIG. 7
down-converts an RF signal into an IF signal.
FIG. 7
illustrates an RF signal serving as a carrier signal and an LO signal supplied from a local oscillator (not shown) applied to the mixer
101
. The mixer
101
mixes the RF signal and the LO signal and outputs an IF signal as shown in FIG.
8
. Thus, the RF signal is down-converted into the IF signal.
When it is required to remove undesirable signal components in frequency bands other than the IF frequency band from the IF signal obtained via the down conversion, a bandpass filter is generally positioned at a stage following the mixer.
FIG. 9
illustrates a mixer and a bandpass filter.
FIG. 10
illustrates an exemplary process in which undesirable signal components are removed from an IF signal by the bandpass filter.
As shown in
FIG. 10
, when there are signals A
1
and B
1
at both sides of an RF signal, the RF signal including the signals A
1
and B
1
and the LO signal are applied to the mixer
101
shown in FIG.
9
. As a result, in addition to the IF signal, signals A
2
and B
2
are output from the mixer
101
. If signals A
2
and B
2
are passed through the bandpass filter, the signals A
2
and B
2
are attenuated into signals A
3
and B
3
. Thus, their influence on the IF signal is reduced.
FIG. 11
illustrates a circuit configuration of the bandpass filter shown in FIG.
9
.
FIG. 12
illustrates the frequency characteristic of the bandpass filter shown in FIG.
11
.
As shown in
FIG. 11
, the bandpass filter
102
is formed of passive elements including capacitors
102
_
1
and
102
_
4
and resistors
102
_
2
and
102
_
3
. As shown in
FIG. 12
, the bandpass filter
102
has cutoff frequencies f
1
and f
2
determined by the values of the passive elements. When the capacitors
102
_
1
and
102
_
4
have capacitance C
1
and C
2
, and the resistors
102
_
2
and
102
_
3
have resistance R
1
and R
2
, the cutoff frequency f
1
is given by the equation:
f1
=
1
2
π
C1
·
R1
(
1
)
and the cutoff frequency f
2
is given by the equation:
f2
=
1
2
π
C2
·
R2
(
2
)
The bandpass filter
102
passes frequency components within a particular band determined by the cutoff frequencies f
1
and f
2
.
FIG. 13
illustrates a circuit configuration of a bandpass filter, configured differently from the bandpass filter shown in FIG.
11
.
FIG. 14
illustrates the frequency characteristic of the bandpass filter shown in FIG.
13
.
The bandpass filter
103
shown in
FIG. 13
is an active bandpass filter including capacitors
103
_
1
and
103
_
4
, resistors
103
_
2
and
103
_
3
, and an operational amplifier
103
_
5
. As with the bandpass filter
102
shown in
FIG. 11
, the bandpass filter
103
also has cutoff frequencies f
3
and f
4
determined by values of the passive elements, and the bandpass filter
103
passes frequency components within a particular band determined by the cutoff frequencies f
3
and f
4
.
FIG. 15
illustrates a biquad bandpass filter.
FIG. 16
illustrates a circuit configuration of a transconductor amplifier used in the biquad bandpass filter.
The biquad bandpass filter
104
shown in
FIG. 15
is a bandpass filter using the Gm-C technology comprising transconductor amplifiers (OTAs: Operational Transconductance Amplifiers)
104
_
1
,
104
_
2
, and
104
_
3
, capacitors
104
_
4
,
104
_
5
,
104
_
6
, and
104
_
7
, and a resistor
104
_
8
. The capacitors
104
_
4
,
104
_
5
,
104
_
6
, and
104
_
7
all have equal capacitance C, and the resistor
104
_
8
has resistance R.
The transconductor amplifier
104
_
1
includes, as shown in
FIG. 16
, NMOS transistors
104
_
11
,
104
_
12
,
104
_
13
,
104
_
14
,
104
_
15
,
104
_
16
,
104
_
17
,
104
_
18
, and
104
_
19
, constant current sources
104
_
20
,
104
_
21
,
104
_
22
,
104
_
23
, and resistors
104
_
24
and
104
_
25
. Signals IN+ and IN−, which are different in phase by 180° from each other, are applied to the NMOS transistors
104
_
11
and
104
_
12
, respectively. An external voltage signal Vf is applied to the NMOS transistor
14
_
19
. The transconductance gm of the transconductor amplifier
104
_
1
varies depending on the value of the external voltage signal Vf applied to the NMOS transistors
104
_
19
. The transconductance gm is given by the equation:
gm
=&bgr;(
Vf−Vs−Vt
)
wherein &bgr; is the feedback factor of the NMOS transistor
104
_
19
, Vs is equal to Vs
2
(when Vs
1
>Vs
2
) or Vs
1
(when Vs
1
<Vs
2
) (Vs
1
and Vs
2
are source and drain voltages, respectively, of the NMOS transistor
104
_
19
), and Vt is the threshold voltage of the NMOS transistor
104
_
19
.
Although the circuit configuration has been described above only for the transconductor amplifier
104
_
1
, the transconductor amplifiers
104
_
2
and
104
_
3
also have a similar circuit configuration.
FIG. 17
illustrates the frequency characteristic of the biquad bandpass filter shown in FIG.
15
.
The frequency characteristic of this biquad bandpass filter
15
shown in
FIG. 17
is variable. More specifically, the cutoff frequencies f
01
and f
02
can be varied by varying the external voltage signal Vf thereby varying the transconductance gm of the transconductor amplifiers
104
_
1
,
104
_
2
, and
104
_
3
. For example, when the external voltage signal Vf applied to the transconductor amplifier
104
_
2
is varied, the center frequency f
0
shown in
FIG. 17
is given by the equation:
f
0
=
gm
2
/2&pgr;
C
where gm
2
is the transconductance of the transconductor amplifier
104
_
2
.
On the other hand, the difference between the cutoff frequency f
01
and the cutoff frequency f
02
is given by the equation:
&Dgr;f=gm
2
×
R.
In the bandpass filters
102
and
103
shown in
FIGS. 11 and 13
, respectively, their cutoff frequencies are determined by the values of passive elements. This means that, to change the cutoff frequencies, the passive elements themselves must be changed. To change the values of passive elements formed on a semiconductor chip using CMOS technology or the like, it is required to change the layout of the passive elements of the semiconductor chip. The change in the layout needs a long time and high cost and thus the change results in great disadvantages in production or development. Another problem is that passive elements occupy large areas on the semiconductor chip.
Although the biquad bandpass filter
104
shown in
FIG. 15
has the advantage that the cutoff frequencies can be controlled by the external voltage signal, the biquad bandpass filter
104
has the disadvantage that the circuit configuration of the transconductor amplifiers
104
_
1
,
104
_
2
, and
104
_
3
is complicated, needs a large number of transistors, and then needs a large-scale circuit.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a mixer and a differential amplifier which have simple circuit configurations and which allow the cutoff frequency to be easily changed.
According to an aspect of the present invention, a mixer is provided for mixing an AC signal with a reference signal having a particular frequency, wherein the mixer includes a parallel resonant circuit including an active inductor and serving as an output load.
Preferably, the AC signal is an RF signal and the reference signal is an out
Choe Henry
Kawasaki Microelectronics Inc.
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