Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
1998-12-04
2001-02-06
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S357000, C455S333000
Reexamination Certificate
active
06184739
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to mixers and in particular to a mixer that exhibits very good linearity at low voltages.
2. Description of the Related Art
A mixer is a circuit that receives separate input signals and combines the signals to produce an output signal. Such circuit is an important building block in communications, for example, wireless radio frequency devices. Mixers may be also widely used in infrared networks and fiber optic communication systems. Generally, mixers, may be used where frequency up or down conversion may be required in a modulation scheme. Frequency conversion is the changing of one frequency to another. This may occur in instances when one signal is multiplied with a second signal to produce, a sum and/or difference of the signals. By using this feature, in one example, the mixer allows very high frequency to be downconverted to baseband or intermediate frequency (IF) so that signals may be evaluated using analog or digital signal processing techniques. In another example, the mixer upconverts a low frequency to a very high frequency.
One well-known mixer configuration is the Gilbert cell which is illustrated in FIG.
1
. The configuration of the Gilbert cell may be two differential bipolar transistor pairs P
1
,P
2
cascaded with an output of another differential bipolar transistor pair P
3
to form a four-quadrant mixer. A four-quadrant mixer allows input signals to be combined for all polarities of signals. In comparison, a two-quadrant mixer allows input signals to be combined for the same polarity of signals. For practical purposes, a mixer should at least be a two-quadrant mixer. Using bipolar transistors in the Gilbert cell may be advantageous in that bipolar transistors are exponential law devices. Stated differently, a relationship between collector to base voltage V
CB
and the collector current I
C
is exponential. Although the relationship may not be purely linear in all regions, it is approximately linear in certain regions of the relationship. The equation below expresses the relationship between the differential output current and the two input voltages V
1
and V
2
of the Gilbert cell.
Δ
I
=
I
01
-
I
02
=
I
EE
[
tanh
(
V
1
2
V
T
)
]
[
tanh
(
V
2
2
V
T
)
]
I
EE
is the bias current and V
T
is the threshold voltage. Note that the hyperbolic tangent function is an infinite series representing multiple harmonics of the input signals and the output signals as expressed by the equation below.
tanh
x
=
x
-
x
3
3
…
Typically, a bandpass filter passes the desired output signal and filters out the remaining undesirable signals. While the Gilbert cell may be successfully implemented in circuits using bipolar transistors, such may not be the case where Gilbert cell configuration is used in a complementary metal oxide semiconductor (CMOS) circuit.
FIG. 2
illustrates a Gilbert cell configuration using CMOS devices. One disadvantage of CMOS is that it is a square law device. Stated differently, drain current I
D
is a square law function of gate to source voltage V
GS
, therefore, non-linear distortion, inherently exist when the mixer is driven by input signals using gate to source or gate to drain relationships. The equation below expresses the output differential current in terms of input voltages V
1
and V
2
.
Δ
I
=
I
01
-
I
02
=
I
DSS
(
1
-
V
1
V
P
)
2
(
1
-
V
2
V
P
)
2
I
DSS
is the bias current and V
p
is the pinch-off voltage. From the equation above, it may be readily understood that a square law device is inherently nonlinear.
Another disadvantage that exists regarding Gilbert cell using CMOS devices is that the Gilbert cell design is based on stacked differential pairs, (i.e. one differential pair is stacked on top of another differential pair). Stated differently, a supply voltage may need to be sufficient to sustain the collector to emitter voltages VCE of two stacked transistors and corresponding loads. A stacked differential pair is generally not a problem in bipolar transistor circuits where it is presumed to operate at high voltage supply, for example, 6-12 volts. However, CMOS circuits are generally designed to operate at lower voltages, typically, 3 volts or lower. With the advancement of integrated circuit technology, CMOS circuits may be expected to operate at 2 volts or lower. This complicates circuits using a stacking arrangement, for example, the Gilbert cell, since it generally requires voltage to be higher than 3 volts to drive stacked transistors. Therefore, a Gilbert cell using CMOS devices may not be compatible for usage in low voltage circuits, for example, portable communication devices such as cellular phones.
Another problem with the Gilbert cell design is that the bi-polar transistor being an exponential law device or, in the case of CMOS device, a square law device, the output produces multiple harmonics requiring unwanted signals to be filtered out using a filter. This requires additional circuits and results in consumption of valuable power which is a problem in portable communication devices. In response, the present invention is directed to a mixer that does not use stacked differential pairs like the Gilbert cell. Instead, the mixer uses parallel differential pairs that may allow the mixer to operate at supply voltages of 1.2 volts or lower, for example. Further, the mixer overcomes the non-linear distortion inherent in CMOS devices.
SUMMARY OF THE INVENTION
A method and apparatus for implementing a mixer. A first transformer is provided for receiving a first signal. A second transformer is provided for receiving a second signal. A device is provided for multiplying the first and the second signal, the device coupled to the first transformer for receiving the first signal and further coupled to the second transformer for receiving the second signal.
REFERENCES:
patent: 4058771 (1977-11-01), Ohsawa et al.
patent: 4313221 (1982-01-01), Mattfeld
patent: 4461042 (1984-07-01), Tanabe et al.
Ali M. Niknejad, Robert G. Meyer; “Analysis, Design and Optimization of Spiral Inductors and Trasnfomers for Si RF ICs”—1998 IEEE International Solid-State Circuits Conference.
H. M. Wang, Bell Labs, Lucent—“A IV Multi-Gigahertz RF Mixer Cove in 0;.5um CMOS”—1998 IEEE Solid-State Circuits Conferen7ce SP23.4.
J. Burghartz, D. Edelstein, M. Soyuer, H. Ainspan, K. Jenkins; “RF Circuit Design Aspects of Spiral Inductors on Silicon”—1988 IEEE International Solid-State Circuits Conference.
Jian-jun Zhou, David J. Allstot; “A Fully Integrated CMOS 900MHz LNA Utilizing Monolithic Transformers”—1998 IEEE International Solid-State Circuits Conference.
Paul R. Gray, Robert G. Meyer—“Analysis and Design of Analog Integrated Circuits”, Third Edition; John Wiley & Sons, Inc.-13 1998 IEEE International Solid-State Circuits Conference.
Patrick Favre, Norbert Joehl, Michel Declercq, Catherine Dehollain, Phillipe Deval “:A2V, 600mA. 1GHz. BICMOS Super-Regenerated Receiver”—1998 IEEE International Solid State Circuits Conference.
S.Wu, B. Razavi, “A 900 MHz/1.8BGHz CMOS Receiver for Dual Band Applications”; 1998 IEEE International Solid-State Circuits Conference.
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Wells Kenneth B.
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