U.S. patent application number 13/732104 was filed with the patent office on 2014-07-03 for linearity improvement over temperature using temperature dependent common-mode voltages in active mixer.
This patent application is currently assigned to FUTUREWEI TECHNOLOGIES, INC.. The applicant listed for this patent is FUTUREWEI TECHNOLOGIES, INC.. Invention is credited to Yoonhyuk Ro.
Application Number | 20140184301 13/732104 |
Document ID | / |
Family ID | 50896805 |
Filed Date | 2014-07-03 |
United States Patent
Application |
20140184301 |
Kind Code |
A1 |
Ro; Yoonhyuk |
July 3, 2014 |
LINEARITY IMPROVEMENT OVER TEMPERATURE USING TEMPERATURE DEPENDENT
COMMON-MODE VOLTAGES IN ACTIVE MIXER
Abstract
A mixer comprising a Gilbert cell configured to receive a
baseband signal and a local oscillator signal, wherein the Gilbert
cell comprises a baseband transistor and a local oscillator
transistor, and a biasing circuit configured to generate a bias
voltage that is inversely related to temperature and coupled to the
local oscillator transistor.
Inventors: |
Ro; Yoonhyuk; (Gainesville,
FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUTUREWEI TECHNOLOGIES, INC. |
Plano |
TX |
US |
|
|
Assignee: |
FUTUREWEI TECHNOLOGIES,
INC.
Plano
TX
|
Family ID: |
50896805 |
Appl. No.: |
13/732104 |
Filed: |
December 31, 2012 |
Current U.S.
Class: |
327/359 |
Current CPC
Class: |
H03D 7/1441 20130101;
H03D 2200/0098 20130101; H03D 7/1458 20130101; H03D 7/1491
20130101 |
Class at
Publication: |
327/359 |
International
Class: |
H03D 7/14 20060101
H03D007/14 |
Claims
1. A mixer comprising: a Gilbert cell configured to receive a
baseband signal and a local oscillator signal, wherein the Gilbert
cell comprises a baseband transistor and a local oscillator
transistor; and a biasing circuit configured to generate a bias
voltage that is inversely related to temperature and coupled to the
local oscillator transistor.
2. The mixer of claim 1, wherein the temperature dependence of the
bias voltage is due to a current that is inversely related to
temperature.
3. The mixer of claim 1, wherein the biasing circuit is configured
to generate the current inversely related to temperature by
subtracting a first current that is directly related to temperature
from a second current that is temperature independent.
4. The mixer of claim 1, wherein the biasing circuit further
comprises: a first current source to produce a first current; and a
second current source to produce a second current, wherein the
first current source and the second current source are operatively
coupled through a plurality of transistors to generate a current
inversely related to temperature, and wherein the temperature
dependence of the bias voltage is due to the current that is
inversely related to temperature.
5. The mixer of claim 1, wherein the mixer is coupled to a ground
through a resistor and the biasing circuit is coupled to the
resistor so that a current inversely related to temperature is
passing through the resistor to form a voltage drop.
6. The mixer of claim 5, wherein the voltage drop is the bias
voltage.
7. A mixer comprising: a Gilbert cell having a baseband input and a
local oscillator (LO) input; and a biasing circuit coupled to a LO
input transistor, wherein the biasing circuit produces a
temperature dependent biasing voltage that is inversely related to
temperature.
8. The mixer of claim 7, wherein the temperature dependent biasing
voltage is produced by a temperature dependent current.
9. The mixer of claim 8, wherein the temperature dependent current
is proportional to an inversely proportional to absolute
temperature (IPTAT) current.
10. The mixer of claim 7, wherein the biasing voltage is the
voltage drop across a resistor that connects the LO input
transistor to a ground.
11. The mixer of claim 7, wherein the biasing voltage is due to an
inversely proportional to absolute temperature (IPTAT) current
running through a resistor.
12. The mixer of claim 7, wherein an inversely proportional to
absolute temperature (IPTAT) current is produced by subtracting a
proportional to absolute temperature (PTAT) current from an
independent to absolute temperature (ITAT) current.
13. The mixer of claim 7, wherein the biasing circuit further
comprises: an independent to absolute temperature (ITAT) current
source operatively coupled to a proportional to absolute
temperature (PTAT) current source through a number of current
mirrors to generate an inversely proportional to absolute
temperature (IPTAT) current.
14. The mixer of claim 13, wherein the IPTAT current is temperature
dependent.
15. The mixer of claim 13, wherein the IPTAT current is inversely
related to temperature.
16. The mixer of claim 7, wherein the Gilbert cell further
comprises a baseband input transistor, and wherein the biasing
circuit causes an operating region of the baseband input transistor
to remain in a saturation region of operation at high operating
temperatures of the Gilbert cell.
17. The mixer of claim 16, wherein the baseband input transistor
remains in the saturation region of operation due to the baseband
input transistor's source-to-drain voltage increasing at high
temperature due to the reduction of the biasing voltage.
18. A wireless communication device comprising: a mixer comprising:
a Gilbert cell; a common-mode baseband biasing circuit coupled to
the Gilbert cell; and a common-mode local oscillator (LO) biasing
circuit coupled to the Gilbert cell wherein the common-mode LO
generates a biasing voltage that is inversely related to
temperature.
19. The wireless communication device of claim 18, wherein the
biasing voltage is generated by passing a current that is inversely
related to temperature through a resistor that connects a
common-mode LO input transistor to a circuit ground.
20. The wireless communication device of claim 18, wherein the
common-mode LO biasing circuit further comprises: a current source
configured to produce a proportional to absolute temperature (PTAT)
current; and a current source configured to produce an independent
to absolute temperature (ITAT) current, wherein the PTAT current
source and the ITAT current source are operatively coupled through
a plurality of transistors to subtract PTAT current from ITAT
current to produce an inversely proportional to absolute
temperature (IPTAT) current, which equals a current that is
inversely related to temperature.
21. A method comprising: generating, by a biasing circuit, a first
current that is proportional to absolute temperature (PTAT);
generating, by the biasing circuit, a second current that is
independent to absolute temperature (ITAT); combining, by the
biasing circuit, the first current and the second current to
produce a third current that is inversely proportional to absolute
temperature (IPTAT); and producing, by the biasing circuit, a
temperature dependent biasing voltage that is inversely related to
temperature.
22. The method of claim 21, wherein the biasing circuit is coupled
to a common-mode local oscillator transistor of a Gilbert cell.
23. The method of claim 21, wherein the temperature dependent
biasing voltage is produced by passing the IPTAT current through a
resistor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
BACKGROUND
[0004] Frequency mixers, or mixers, may be used in radio frequency
(RF) communication systems to shift a center frequency of a data
signal for more convenient processing of the data signal. Mixers
may be active or passive devices. A conventional active mixer may
employ a Gilbert cell.
[0005] A gain of a conventional active mixer employing a Gilbert
cell may be sensitive to temperature variation. A conventional
technique may compensate for temperature variation by making a
common-mode voltage of baseband input transistors temperature
dependent. However, this technique has a drawback that the input
transistors may deviate from behaving as an ideal current source,
especially at higher temperatures, which may degrade linearity of
the mixer. There is thus a need to improve the linearity of active
mixers employing Gilbert cells, especially as operating temperature
increases.
SUMMARY
[0006] In one embodiment, the disclosure includes a mixer
comprising a Gilbert cell configured to receive a baseband signal
and a local oscillator signal, wherein the Gilbert cell comprises a
baseband transistor and a local oscillator transistor, and a
biasing circuit configured to generate a bias voltage that is
inversely related to temperature and coupled to the local
oscillator transistor.
[0007] In another embodiment, the disclosure includes a mixer
comprising a Gilbert cell having a baseband input and a local
oscillator (LO) input, and a biasing circuit coupled to a LO input
transistor, wherein the biasing circuit produces a temperature
dependent biasing voltage that is inversely related to
temperature.
[0008] In yet another embodiment, the disclosure includes a
wireless communication device comprising a mixer comprising a
Gilbert cell, a common-mode baseband biasing circuit coupled to the
Gilbert cell, and a common-mode LO biasing circuit coupled to the
Gilbert cell and that generates a biasing voltage that is inversely
related to temperature.
[0009] These and other features will be more clearly understood
from the following detailed description taken in conjunction with
the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of this disclosure,
reference is now made to the following brief description, taken in
connection with the accompanying drawings and detailed description,
wherein like reference numerals represent like parts.
[0011] FIG. 1 illustrates an embodiment of a wireless communication
device using an active mixer.
[0012] FIG. 2 is a circuit diagram of an embodiment of a
conventional active mixer with a bias circuit for a baseband
common-mode voltage and a DC voltage for LO common-mode
voltage.
[0013] FIG. 3 is a block diagram of an embodiment of a mixer with
biasing circuitry for a baseband common-mode voltage and LO
common-mode voltage.
[0014] FIG. 4 is a circuit diagram of an embodiment of a mixer with
biasing circuitry for a baseband common-mode voltage and LO
common-mode voltage.
[0015] FIG. 5 depicts the temperature dependence of a biasing
circuit's biasing voltage along with Gilbert cell circuitry.
[0016] FIG. 6 illustrates third-order inter modulation versus LO
bias voltage for different LO common-mode voltage biasing
schemes.
[0017] FIG. 7 illustrates fifth-order inter modulation versus LO
bias voltage for different LO common-mode voltage biasing
schemes.
[0018] FIG. 8 illustrates adjacent channel leakage ratio versus
output power for different LO common-mode voltage biasing
schemes.
[0019] FIG. 9 is a block diagram illustrating an embodiment of a
method for generating a biasing voltage that is inversely
proportional to temperature.
DETAILED DESCRIPTION
[0020] It should be understood at the outset that, although an
illustrative implementation of one or more embodiments are provided
below, the disclosed systems and/or methods may be implemented
using any number of techniques, whether currently known or in
existence. The disclosure should in no way be limited to the
illustrative implementations, drawings, and techniques illustrated
below, including the exemplary designs and implementations
illustrated and described herein, but may be modified within the
scope of the appended claims along with their full scope of
equivalents.
[0021] FIG. 1 illustrates an embodiment of a wireless communication
device 100. The wireless communication device 100 comprises an
active mixer 104, which comprises biasing circuitry 106 as shown in
FIG. 1. The active mixer 104 may have an LO input at one frequency,
a data signal input at a separate frequency and an RF output at yet
another frequency. The active mixer 104 may mix the LO input and
the data signal to generate the RF output. The RF output may be the
product of the LO input and the data signal and may be at a same
frequency as the LO input or a different frequency than either the
LO input frequency or the data signal frequency. The RF output may
be coupled to an antenna subsystem or it may couple to another
mixer so that the output signal is converted to another frequency
before transmission. As understood by a person of skill in the art,
the antenna subsystem may generate a transmitted signal. The
antenna subsystem may comprise components suitable for transmitting
a signal wirelessly. That is, the antenna subsystem may include one
or more antennas, filters, and/or amplifiers.
[0022] Active mixers, such as active mixer 104, may be affected by
temperature changes, especially when the temperature of the circuit
increases well above room temperature. The higher operating
temperatures may be the result of smaller devices containing fewer
outlets for heat dissipation. At high operating temperatures,
active mixer 104 may experience degraded linearity resulting in
poor performance of the mixer circuit and the wireless
communication device 100. The degraded linearity may be due to a
change in the operation of active mixer's 104 transistors due to
the increase in temperature.
[0023] FIG. 2 is a circuit diagram of an embodiment of a
conventional active mixer 200 with a bias circuit for a baseband
common-mode voltage and a DC voltage for LO common-mode voltage.
The active mixer 200 comprises a Gilbert cell 202, a bias circuit
204 for baseband common-mode voltage, and a DC voltage source 206
for LO common-mode voltage, baseband signal input ports or
connections, LO signal input ports or connections, and an RF output
port or connection configured as shown in FIG. 2. The active mixer
200 may be configured to receive a differential baseband signal and
a differential LO signal as shown in FIG. 2. The active mixer 200
may be configured to generate a differential RF output as shown in
FIG. 2.
[0024] The Gilbert cell 202 comprises two baseband input
transistors M0 and M3, and four local oscillator (LO) input
transistors M1, M2, M4, and M5 configured as shown. The baseband
signal inputs M0 and M3 may be used for introducing a data signal
into Gilbert cell 202 and the LO signal inputs may be used for
introducing an LO signal into Gilbert cell 202. Gilbert cells, such
as Gilbert cell 202, may both shift the frequency of signals and
amplify an output signal due their differential gain configuration.
Here, Gilbert cell 202 may produce an RF signal that is a mix of
the baseband signal and the LO signal, i.e. the RF signal may
reproduce the information in the data signal at a frequency that
may be a combination of the frequencies of the LO and baseband
signals.
[0025] The Gilbert cell 202 may be configured so that the two
baseband input transistors M0 and M3 may receive the baseband data
signal. The LO input transistors M1, M2, M4, and M5 may be
configured to receive the LO signal. The RF output of the Gilbert
cell 202 may be used to mix, or multiply, the baseband data signal
and the LO input signal. The RF output may comprise two signals.
One signal may be at a frequency equal to the frequencies of the LO
input signal and the baseband data signal added and another signal
may be at a frequency equal to subtracting the baseband data signal
frequency from the LO input signal frequency.
[0026] Some conventional mixers comprise a Gilbert cell, such as
Gilbert cell 202, but little to no biasing circuitry, such as
biasing circuit 204. Such active mixers may be sensitive to
temperature variation because a transconductance of the Gilbert
cell may change with temperature. The focus in addressing this
issue has traditionally been placed on the baseband input
transistors since it is their operation region which may be most
affected by temperature changes, which in turn affects the
linearity of the active mixer's gain over temperature.
[0027] An attempt at solving this gain variation over temperature
problem resulted in the baseband input transistors being driven
into a less optimal operation region at high temperature, where
they deviate from ideal current sources and suffer from nonlinear
characteristics.
[0028] The conventional baseband bias circuit 204 may be
implemented in active mixers, such as active mixer 200, and may
include a current source I1, resistor R0, and transistors M6 and
M7. The current source I1 may generate a current that is
proportional to absolute temperature (PTAT). That is, when the
temperature increases, the current it generates may increase and
the current may decrease when the temperature decreases. The PTAT
current may run through resistor R0 due to a current mirror that is
created by transistors M6 and M7. The voltage drop generated across
R0 may then be subtracted from the power supply voltage V.sub.cc. A
resulting voltage at the gates of the baseband input transistors M0
and M3 may be their biasing voltage.
[0029] Biasing the baseband input transistors M0 and M3 in such a
fashion may generate a biasing voltage that changes with
temperature. Namely, when the temperature increases, PTAT current
may increase, which may cause the voltage drop across R0 to
increase. Then, since the biasing voltage is the voltage drop
across R0 subtracted from V.sub.cc, the biasing voltage may
decrease. And, since the bias voltage is applied to the gates of
the transistors, the gate voltage of the baseband input transistors
may also decrease. Thus, as the temperature increases, the bias
voltage and the gate voltage for transistors M0 and M3 may
decrease. Let V.sub.gs for a transistor denote the absolute voltage
difference between the transistor's gate and source, and let
V.sub.ds for the transistor denote the absolute voltage difference
between the transistor's drain and source. For M0 and M3, the
source voltage may be V.sub.cc, and the gate voltage may be the
bias voltage generated by baseband bias circuit 204. Thus, if the
gate voltage, or biasing voltage, is decreasing with temperature
and V.sub.cc is constant, then V.sub.gs may increase with
temperature.
[0030] At around room temperature or lower, V.sub.gs for M0 and M3
may cause the baseband input transistors to remain in the
saturation region of operation. However, at high operating
temperatures, the operating region may still tend to move toward
the linear region due to V.sub.ds of the baseband input transistors
M0 and M3 being fixed. Due to the relationship of a transistor's
drain current with V.sub.ds, an operating region of a transistor
operating at a fixed V.sub.ds with increasing V.sub.gs may tend to
move toward the linear region when V.sub.gs increases, which may
affect the performance of the transistors and ultimately the active
mixer 200.
[0031] Referring to FIG. 2, V.sub.ds for baseband input transistor
M0 may be the voltage difference between V.sub.cc and the source
voltages of the LO input transistors M1 and M2. Similarly, V.sub.ds
for baseband input transistor M3 may be the voltage difference
between V.sub.cc and the source voltages of the LO input
transistors M4 and M5. V.sub.ds of the baseband input transistors
being dependent upon the LO input transistors may be due to the
drain of the baseband input transistors being tied to the sources
of the LO input transistors. Source voltages for the LO input
transistors M1, M2, M4, and M5 may depend on their gate voltages.
In FIG. 2, the LO input transistors' gates may be biased by DC
voltage source 206. Consequently, the voltage at the LO input
transistors' sources is fixed to their gate voltages, which is the
DC voltage source 206, plus Vgs of the two LO input transistors'
that have their sources coupled because the LO input transistors'
sources may not be directly biased by an external source. Thus, the
voltage at M1's source may be DC voltage source 206 plus V.sub.gs
of M1 and M2, which would also be the source voltage for M2. Thus,
the voltage at M4's source may be DC voltage source 206 plus
V.sub.gs of M4 and M5, which would also be the source voltage for
M5.
[0032] Hence, the drain voltage of the baseband input transistor
may be equal to the DC voltage source 206 plus V.sub.gs of the two
LO input transistors, so that the drain voltage of M0 may be DC
voltage source 206 plus V.sub.gs of M1 and M2. The drain voltage of
M3 may then be DC voltage source 206 plus V.sub.gs of M4 and M5.
The drain voltage of the baseband input transistors may increase
with temperature because the current from M0 and M3 may increase,
which flows into M1, M2, M4, and M5 causing V.sub.gs of M1 and M2
to increase. As a result, V.sub.ds of the baseband input
transistors M0 and M3 may be the difference between V.sub.cc and
the DC voltage source 206 plus V.sub.gs of M1 and M2 (or M4 and
M5), which may decrease as temperature increases.
[0033] Since V.sub.gs of the baseband transistors M0 and M3 may
increase when the temperature increases and V.sub.ds of M0 and M3
may decrease, the operating region of transistors M0 and M3 may
shift toward the linear region at high temperatures. This potential
shift in the operating region of the baseband input transistors may
affect the performance of the entire mixer at elevated operating
temperatures. The compromised performance of the active mixer, such
as active mixer 200, manifests itself in a reduction in the
linearity of the gain over temperature. Improvements to this
technology may allow ever-shrinking hand held wireless components
to retain maximum operating characteristics at higher
temperatures.
[0034] Disclosed herein are systems and an apparatus to improve the
linearity of the active mixer while maintaining overall gain over
temperature. One embodiment to improve the linearity may involve
forcing the biasing voltage of the LO input transistors to be
temperature dependent. Another embodiment may involve the LO input
transistors being biased using a voltage source that decreases with
increasing temperature. In the later embodiment, due to both the
baseband bias voltage and the LO bias voltage being temperature
dependent, the linearity of the active mixer may be improved along
with maintaining overall gain of the cell over temperature. This
biasing technique may cause the V.sub.ds voltage of the baseband
input transistors to change with temperature. The solution may
involve the generation of a current that is inversely related to
temperature that may then be used to produce the biasing voltage
for the LO input transistors of a Gilbert cell.
[0035] One embodiment of active mixer 104 is depicted in FIG. 3.
FIG. 3 is a block diagram of mixer 300. Mixer 300 may include an LO
bias circuit 302, a Gilbert cell 304, and a baseband bias circuit
306 configured as shown in FIG. 3. The Gilbert cell 304 may
comprise a differential amplifier including modulation inputs for
the baseband frequency and LO inputs for the data modulation signal
along with an RF output. The Gilbert cell 304 may be configured the
same as the Gilbert cell 202 in FIG. 2. For example, the Gilbert
cell 304 may be connected to voltage source V.sub.cc through
baseband input transistors and connected to ground through
inductors LO and L3. The baseband bias circuit 306 may bias the
baseband input transistors by either a constant voltage source or a
temperature dependent voltage source and may be connected to the
gates of the baseband input transistors. The baseband bias circuit
306 may be configured the same as the baseband bias circuit 204 in
FIG. 2.
[0036] In accordance with various embodiments, LO bias circuit 302
may be designed to produce a temperature dependent biasing voltage
that is inversely related to temperature. By making the LO input
transistors gate voltage inversely proportional to temperature, the
change of V.sub.gs of the baseband transistors does not affect
their operation region as severely, therefore their operating
region may remain in saturation even at high temperature. The LO
bias circuit 302 may be designed to generate a voltage that is
inversely proportional to temperature by utilizing a current that
is inversely related to temperature. This current may then be
passed through a resistor that connects the LO input transistors to
ground. The voltage drop generated across the resistor may be used
as the biasing voltage of the LO bias circuit 302. Since the
current decreases with increasing temperature, the voltage drop,
and therefore the biasing voltage, may decrease with increasing
temperature.
[0037] One exemplary embodiment of LO bias circuit 302 that may be
inversely related to temperature is depicted by active mixer 400 in
FIG. 4. FIG. 4 is a circuit diagram of an embodiment of a mixer 400
with biasing circuitry 406 for a LO common-mode voltage. The mixer
400 comprises a Gilbert cell 402, a baseband bias circuit 404, and
a LO bias circuit 406 configured as shown in FIG. 4. Gilbert cell
402 comprises six transistors coupled in a differential amplifier
configuration with two baseband transistors and four LO transistors
to effectuate mixing of the baseband and LO signals. The
transistors in FIG. 4 represent PFETs, but NFETs and BJTs could
also be utilized to implement the circuit. Baseband bias circuit
404 may generate a biasing voltage on the gates of baseband input
transistors that decreases when the temperature increases and may
be substantially configured as described above.
[0038] As described above, active mixers may experience a reduction
in their linearity of their gain at high temperatures. One way to
maintain active mixer's 400 linearity at high temperature is to
make V.sub.ds of the baseband input transistors temperature
dependent. Specifically, making the LO input transistor's gate
voltage inversely related to temperature, e.g., to decrease with
increasing temperature, may cause active mixer's 400 gain to remain
in a more optimal operating region over temperature.
[0039] Referring to FIG. 4, coupling the baseband and LO
transistors as shown in Gilbert cell 402 may cause the baseband
transistors' V.sub.ds to be dependent on the LO input transistors'
V.sub.gs. The LO input transistors' V.sub.gs may be dependent on
the biasing voltage of the LO input transistors since the LO input
transistors may be biased by connecting their gates to LO bias
circuit 406. When LO bias circuit 406 as in FIG. 4 is used to bias
the LO input transistors of Gilbert cell 402, their biasing voltage
may become inversely related to temperature, meaning gate voltage
will decrease with increasing temperatures. Biasing the LO input
transistors in this fashion may cause the baseband input
transistors' V.sub.ds to become temperature dependent, which in
turn may solve the linearity problem.
[0040] The LO bias circuit 406 comprises a proportional to absolute
temperature (PTAT) DC current source I3, an independent to absolute
temperature (ITAT) DC current source I10, transistors M8, M9, M10,
M11, M12, M13, M14, and M15, and a resister R2 configured as shown
in FIG. 4. The transistors of LO bias circuit 406 are configured
into a succession of current mirrors. A current mirror may comprise
a pair of transistors connected in a manner so that one of the
transistors may be forced to carry the same current as the other
transistor, which may be acting as a reference. The two transistors
may be connected to each other's gate with one of the transistors,
the reference transistor, connected as a diode. Connecting a
transistor as a diode may require connecting its gate and drain
together. For example, transistors M8 and M9 of LO bias circuit 406
may be connected as a current mirror since their gates are
connected to each other and M9's gate and drain are connected. A
current mirror may mirror current because the non-diode connected
transistor may be forced to carry the same current as the reference
transistor, or diode-connected transistor. For example, again
referring to transistors M8 and M9 of LO bias circuit 406, M9 may
have PTAT current running through it since M9 is connected in
series with the PTAT current source, I3. Since M8's gate is
connected to M9's gate and M9 is diode-connected, M8 may be forced
to also carry PTAT current generated by I3. In other words, M8 is
current mirrored with M9, or M8 and M9 are current mirrored.
[0041] Referring back to LO bias circuit 406 of FIG. 4, it may be
evident that there are four sets of current mirrors and they are:
M8 and M9, M10 and M11, M12 and M13, and M14 and M15. Applying the
current mirror analysis to LO bias circuit 406, the current flowing
in M9 may also be flowing in M8; M10's current may be flowing in
M11; M12's current may be flowing in M13; and M14's current may be
flowing in M15. Lastly, the current flowing through M15 may also be
flowing through R2 since they are in series with each other. Thus,
the current running through R2 may generate a voltage drop using
the current being carried by M15
[0042] Referring again to LO bias circuit 406 of FIG. 4, M9 may
carry PTAT current, which may force M8 to also carry PTAT current.
M10 may carry ITAT current from current source I10 since they are
in series, which then may force M11 to carry ITAT current generated
by I10. M8 and M11 are in series with each other, but they may not
be carrying the same current. They may not be carrying the same
current since they are current mirrored to different references.
Nevertheless, M12 is connected between M8 and M11 and may be forced
to carry the difference in current between M8 and M11.
[0043] Since M8 may be forced to carry PTAT current and M11 may be
forced to carry ITAT current, M12 may then carry the current
difference of ITAT and PTAT. ITAT current minus PTAT current may be
designated as inversely proportional to absolute temperature
(IPTAT) current. Continuing on with the current tracing of the
circuit, since M12 may be carrying IPTAT current, M13 may be forced
to carry IPTAT current also. M14, in series with M13, may also be
carrying IPTAT current, which in turn may force M15 to carry IPTAT
current. Lastly, since M15 may be in series with R2, R2 may have
IPTAT current running through it, which may result in generating a
voltage drop from IPTAT current across R2.
[0044] Since IPTAT current may be inversely related to temperature,
the voltage drop generated by IPTAT passing through R2 may also be
inversely related to temperature. Since the bias voltage generated
by LO bias circuit 406 is taken from the voltage drop across R2 and
is connected to the gates of the LO input transistors, the LO input
transistors' gate voltage and their gate voltage may also be
inversely related to temperature.
[0045] Since the baseband input transistors' V.sub.ds is the
difference between V.sub.cc and the LO input transistors' source
voltage, the temperature dependence of the LO input transistors'
V.sub.gs will cause the baseband input transistors' V.sub.ds to
also be temperature dependent. In accordance with various
embodiments, utilizing the LO bias circuit 406, which is
temperature dependent, as the temperature of the Gilbert cell 402
increases, the LO input transistors' V.sub.ds may decrease and both
the baseband input transistors' V.sub.gs and V.sub.ds may increase.
Because the baseband input transistors' V.sub.gs and V.sub.ds may
be both changing together, their operation region may not be
affected. The operation region may not be affected if the
relationship between the baseband input transistors' V.sub.gs and
V.sub.ds persists at high temperature as it does at room
temperature to retain operation in the saturation region.
Therefore, if the operating region of the baseband input
transistor's is unaffected when the temperature increases, then the
linearity of the gain over temperature of the Gilbert cell 402 may
not be affected.
[0046] FIG. 5 shows the voltage dependency on temperature for both
the LO bias circuit 406 and the baseband bias circuit 404. Graph
502 illustrates the reduction in voltage of the baseband biasing
voltage due to increasing temperatures. LO bias circuit's 406
temperature dependence is shown in graph 504. Graph 504 shows three
separate plots notated with LO_CM=0, LO_CM=1, and LO_CM=2. The
different values of LO_CM represent different R2 values used in LO
bias circuit 406. By changing the ratio of PTAT to ITAT, the
temperature dependence of the biasing circuit may be altered.
Meaning, the slope of the line on graph 504 may change, thereby
changing how much IPTAT changes with temperature. Graph 506
illustrates three separate plots notated with LO_CM=3, LO_CM=4, and
LO_CM=5. The different values of LO_CM in graph 506 represent PTAT
at different percentages of ITAT used in LO bias circuit 406.
LO_CM=3 corresponds to PTAT being 15% of ITAT, LO_CM=4 corresponds
to 25%, and LO_CM=5 corresponds to 35%.
[0047] For the LO bias circuit 406, the base values of ITAT I10 and
PTAT I3 may be selected so that the current from I10 is always
greater than the current from I3 regardless of temperature. Due to
subtracting PTAT from ITAT, if PTAT current and ITAT current are
close in value, then there may be a potential for PTAT to be
greater than ITAT at high temperatures. This should be avoided
because it may cause the IPTAT current flowing through R2 to
reverse, which would potentially switch the polarity of the voltage
drop across R2 and the biasing voltage of the LO input transistors.
Also, the base values of I3 and I10 should not be selected to equal
each other. If ITAT and PTAT become equal, then LO bias circuit 406
may not have any current flow through R2, which may result in the
LO input transistors not being properly biased. The values of ITAT
and PTAT may be chosen based on the amount of gain that is desired
for the Gilbert cell 402. Typical values for ITAT may be 10 to 20
.mu.A. PTAT may be chosen to be anywhere from 15% to 35% of ITAT,
depending on the rate of change with respect to temperature desired
for IPTAT.
[0048] FIG. 6 illustrates third order inter-modulation (IM3) versus
LO bias voltage for different LO common-mode voltage biasing
circuit schemes at 85.degree. C. IM3 is a measure of the difference
between the carrier frequency and the modulated third harmonic
frequency, and a large value of IM3 is desired. The x-axis
corresponds to the three different LO_CMs shown in FIG. 5 with
LO_CM=0 corresponding to a small R2 value, LO_CM=1 corresponding to
a medium R2 value, and LO_CM=2 corresponding to a large R2 value.
The y-axis shows the IM3 value in units of dBc. The solid line
represents the use of a constant DC biasing voltage (one that does
not change with temperature) biasing the LO input transistors, such
as DC voltage source 206 in FIG. 2. The dotted line represents the
use of the LO bias circuit 406 having different R2 values, as noted
above. Changing R2 values results in creating different biasing
voltages. As can be seen, the use of the LO bias circuit 406
improves the IM3 modulation at high temperature at different R2
values. It should be noted that the baseband is being biased with
the temperature dependent biasing voltage from, e.g, baseband bias
circuit 404. A single chip containing all the circuitry shown in
FIG. 4 was designed and processed in 90 nm CMOS and was used in
producing the test results shown herein.
[0049] FIG. 7 illustrates the fifth order inter-modulation (IM5)
versus LO bias voltage for different LO common-mode voltage biasing
circuit schemes at 85.degree. C. As in FIG. 6, the use of a DC
voltage source is represented by a solid line and the use of the LO
bias circuit 406 having different R2 values is represented by a
dashed line. As can be seen, the IM5 is improved at high
temperature when using the temperature dependent LO bias circuit
406 as opposed to a DC voltage source 206.
[0050] FIG. 8 illustrates adjacent channel leakage ratio (ACLR)
versus output power for different LO common-mode voltage biasing
circuit schemes at 85.degree. C. As in FIGS. 6 and 7, the use of a
DC voltage source is represented by a solid line and the use of the
LO bias circuit 406 having different R2 values is represented by a
dashed line. ACLR is defined as the ratio of the power in the
adjacent radio channels or shoulder bands to the transmitted power.
ACLR is a measure of how well tuned the circuit is for transmitting
most of the power in the carrier frequency band instead of shoulder
bands. The y-axis corresponds to the amount of ACLR in units of dBc
and the x-axis corresponds to the output power in the transmitted
channels of the carrier frequency in units of dBm. The circuits
used for the ACLR measurements were the same as used for measuring
IM3 and IM5, discussed above. As can be seen from FIG. 8, the ACLR
is lower when using LO bias circuit 406 as opposed to the constant
DC voltage source 206.
[0051] To obtain the performance curves shown in FIGS. 5 through 8,
circuits corresponding to active mixer 400 were both simulated and
produced using a 90 nm CMOS process. For both the simulations and
the device designs and testing, the following values were used
various components of the circuit. The power supply voltage
V.sub.cc was chosen to be 2.2 V. For LO bias circuit 406 a value of
45 k.OMEGA. was chosen for R2, I10 (ITAT) was 25 .mu.A, I3 (PTAT)
was 5 .mu.A, the width over length values for transistors M10, M11,
M14, and M15 was 20/4 with both values in .mu.m. The width over
length values for transistors M8, M9, M12 and M13 was 10/4 with
both values in .mu.m. For baseband bias circuit 404 I1 (PTAT) was
20 .mu.A, R0 was 40 k.OMEGA., and the width over length values for
M6 and M7 was 10/4 with both values in .mu.m. The values for the
various components are example values for the purposes of
illustration, and the values are not limited to these values as one
of skill in the art would understand.
[0052] As shown in FIGS. 6-8, changing the biasing circuitry for a
Gilbert cell's LO input transistors does not adversely affect the
performance of the mixer and actually improves IM3, IM5, and ACLR
at high temperatures. Thus, utilizing a biasing circuit that is
inversely related to temperature for the LO input transistors of a
Gilbert cell improves performance, namely the linearity of the
gain, over temperature and especially at high temperatures.
[0053] FIG. 9 is a block diagram illustrating an embodiment of a
method for generating a biasing voltage that is inversely
proportional to temperature, for example that is generated by the
biasing circuit 406 shown in FIG. 4. Method 900 begins in block 902
with generating a first current that is proportional to temperature
(PTAT). Method 900 continues at block 904 with generating a second
current that is independent of absolute temperature (ITAT). Method
900 continues at block 906 with combining the first and second
currents to produce a third current that is inversely proportional
to absolute temperature (IPTAT). The combination of the first and
second currents to produce the IPTAT current is completed similar
to how the PTAT and ITAT currents are combined through the use of
current mirrors shown in the biasing circuit 406 of FIG. 4. Method
900 then continues at block 908 with producing a temperature
dependent biasing voltage that is inversely related to temperature.
The biasing voltage that is inversely related to temperature is
produced by passing the IPTAT current through a resistor similar to
how the biasing circuit 406 produces a biasing voltage that is
inversely related to temperature.
[0054] At least one embodiment is disclosed and variations,
combinations, and/or modifications of the embodiment(s) and/or
features of the embodiment(s) made by a person having ordinary
skill in the art are within the scope of the disclosure.
Alternative embodiments that result from combining, integrating,
and/or omitting features of the embodiment(s) are also within the
scope of the disclosure. Where numerical ranges or limitations are
expressly stated, such express ranges or limitations should be
understood to include iterative ranges or limitations of like
magnitude falling within the expressly stated ranges or limitations
(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater
than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a
numerical range with a lower limit, R.sub.l, and an upper limit,
R.sub.u, is disclosed, any number falling within the range is
specifically disclosed. In particular, the following numbers within
the range are specifically disclosed:
R=R.sub.l+k*(R.sub.u-R.sub.l), wherein k is a variable ranging from
1 percent to 100 percent with a 1 percent increment, i.e., k is 1
percent, 2 percent, 3 percent, 4 percent, 7 percent, . . . , 70
percent, 71 percent, 72 percent, . . . , 97 percent, 96 percent, 97
percent, 98 percent, 99 percent, or 100 percent. Moreover, any
numerical range defined by two R numbers as defined in the above is
also specifically disclosed. The use of the term "about" means
.+-.10% of the subsequent number, unless otherwise stated. Use of
the term "optionally" with respect to any element of a claim means
that the element is required, or alternatively, the element is not
required, both alternatives being within the scope of the claim.
Use of broader terms such as comprises, includes, and having should
be understood to provide support for narrower terms such as
consisting of, consisting essentially of, and comprised
substantially of. Accordingly, the scope of protection is not
limited by the description set out above but is defined by the
claims that follow, that scope including all equivalents of the
subject matter of the claims. Each and every claim is incorporated
as further disclosure into the specification and the claims are
embodiment(s) of the present disclosure. The discussion of a
reference in the disclosure is not an admission that it is prior
art, especially any reference that has a publication date after the
priority date of this application. The disclosure of all patents,
patent applications, and publications cited in the disclosure are
hereby incorporated by reference, to the extent that they provide
exemplary, procedural, or other details supplementary to the
disclosure.
[0055] While several embodiments have been provided in the present
disclosure, it may be understood that the disclosed systems and
methods might be embodied in many other specific forms without
departing from the spirit or scope of the present disclosure. The
present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details
given herein. For example, the various elements or components may
be combined or integrated in another system or certain features may
be omitted, or not implemented.
[0056] In addition, techniques, systems, subsystems, and methods
described and illustrated in the various embodiments as discrete or
separate may be combined or integrated with other systems, modules,
techniques, or methods without departing from the scope of the
present disclosure. Other items shown or discussed as coupled or
directly coupled or communicating with each other may be indirectly
coupled or communicating through some interface, device, or
intermediate component whether electrically, mechanically, or
otherwise. Other examples of changes, substitutions, and
alterations are ascertainable by one skilled in the art and may be
made without departing from the spirit and scope disclosed
herein.
* * * * *