Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
2001-06-22
2002-05-14
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S356000, C327S408000
Reexamination Certificate
active
06388501
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of Invention
This invention relates to mixers using MOSFETs, in particular the down converter of a superheterodyne radio receiver
(2) Description of the Related Art
In a conventional radio receiver, the incoming radio frequency is mixed with a local oscillator (LO) signal to produce a beat frequency, which is the intermediate frequency (IF). The IF is then amplified and filtered to attenuate other unwanted signals,
A popular mixer circuit is the Gilbert multiplier. Since MOSFETs are widely used in circuit designs today, an MOSFET version of the Gilbert multiplier is shown in FIG.
1
. Basically, a differential amplifier with a differential pair N
2
and N
3
is fed from a current source N
1
. The differential gain of the differential amplifier is proportional to the transconductance gm of N
2
and N
3
. This transconductance varies as the square root of the dc drain current of N
2
and N
3
, which is controlled by the drain current of N
1
. The dc drain current I
D
of N
1
is controlled by the dc gate-to-source voltage V
GS1
of N
1
and has a square law relationship with the gate-to-source voltage i.e. I
D1
∝(V
GS1
−Vt)
2
, where Vt is the threshold voltage of N
1
. When a local oscillator signal V
LO
of frequency f
LO
is applied differentially to N
2
an N
3
(i.e. V
LO+
and V
LO−
respectively), and a radio frequency signal V
rf
of frequency f
rf
is applied to the gate of N
1
, the output current of the differential amplifier is equal to V
LO
*gm, and the gm is proportional to Vrf*(V
GS1
−Vt). When the V
rf
is multiplied by V
LO
, a beat frequency f
if
=f
rf
±f
LO
intermediate frequency signal V
if
is produced.
While the Gilbert multiplier is widely used in the past, it has a number of drawbacks for low voltage and low power applications. In modern CMOS technology, the tendency is to use a low supply voltage V
DD
: for instance 25 V for 0.25 &mgr;m technology and 1.8 V for 0.18 &mgr;m technology. In the Gilbert mixer, the current source is operating in the current saturation region of the V
DS
vs I
D
V-I characteristic N
1
′ in
FIG. 2
to obtain a higher transconductance and has a square law relation with V
rf
. Therefore the drain voltage V
DS1
for the current source N
1
is larger than the knee voltage V
D1′
of the N
1
′ V-I characteristic curve. That knee voltage V
D1′
is equal to V
GS1
−Vt.
Similarly, the differential pair N
2
and N
3
also must have its drain voltage higher than the knee voltage, i.e. V
D2
>2(V
GS1
−Vt) as shown by the dotted V-I characteristic of N
2
in FIG.
2
. If a resistor is used as a load, another voltage drop VL will be added to V
DS1
to be supplied by the power supply V
DD
. These three stacks of voltages, V
DS1
, V
DS2
and VL, dictate that the supply voltage cannot be made very low. For a typical threshold voltage of 0.6 V, there is hardly any “head room” for signal voltage swing.
Lee et al disclosed in U.S. Pat. No. 6,194,947 a mixer structure which is basically a Gilbert mixer having a differential pair fed from a current source with its shortcomings.
Sakusabe disclosed in U.S. Pat. No. 5,789,963, FIGS.
1
&
9
”, a mixer operating with a drain to source voltage V
DS
in the current saturation region of a MOSFET without claiming the exact V
DS
. The RF signal is injected to the drain of the mixer by AC coupling (i.e. through a coupling capacitor). The AC coupling requires many additional components such the coupling capacitor Ca and other components such as Z
1
-Z
8
and capacitors C
1
-C
5
as shown in Sakusabe's
FIG. 1
Another drawback of Sakusabe's mixer is that the gate of the mixer FET2 must be adjusted to set the quiescent drain voltage to the current saturation region for different operating currents. It is desirable to set the set the quiescent operating point (i.e. V
DS
) automatically for different operating currents.
SUMMARY OF THE INVENTION
An object of this invention is to design a MOSFET mixer which requires a lower supply voltage than the Gilbert mixer or similar structure. Another object of this invention is to reduce the power consumption of the MOSFET mixer. Still another object of this invention is to provide a high conversion gain of the mixer. A further object of this invention is to set the operating point of the mixer at its optimum conversion gain automatically.
These objects are achieved by mixing the RF signal and the local oscillator signal at the knee of the output V
DS
−I
D
characteristic of a MOSFET by dc coupling. At the knee, the characteristic has the sharpest curvature. The nonlinearity produces a maximum beat frequency signal. For implementation, a mixer MOSFET is biased at the knee of the V
DS
−I
D
characteristic. The LO (or RF) signal voltage V
LO
(or V
rf
) is applied at the gate of the mixer MOSFET, and the RF (or LO) signal voltage V
rf
(or V
LO
) is injected at the drain of the mixer MOSFET. Then a beat frequency drain current is produced. Specifically, the gate of a single-ended mixer MOSFET is fed with a local oscillator signal and the drain of the mixer is dc coupled to a single-ended source follower with the gate fed from a radio frequency signal or vise versa.
REFERENCES:
patent: 4845389 (1989-07-01), Pyndiah et al.
patent: 5083050 (1992-01-01), Vasile
patent: 5379457 (1995-01-01), Nguyen
patent: 5448197 (1995-09-01), Sagawa et al.
patent: 5495194 (1996-02-01), Sugawara
patent: 5680078 (1997-10-01), Ariie et al.
patent: 5717364 (1998-02-01), Ariie et al.
patent: 5789963 (1998-08-01), Sakusabe
patent: 6194947 (2001-02-01), Lee et al.
patent: 6239645 (2001-05-01), Tsukahara et al.
Cunningham Terry D.
H.C. Lin Patent Agent
Nguyen Hai L.
Prominenet Communications Inc.
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