U.S. patent application number 16/264186 was filed with the patent office on 2019-08-08 for rf sensor in stacked transistors.
The applicant listed for this patent is Infineon Technologies AG. Invention is credited to Winfried Bakalski, Ruediger Bauder, Bernd Schleicher, Valentyn Solomko.
Application Number | 20190245533 16/264186 |
Document ID | / |
Family ID | 65324260 |
Filed Date | 2019-08-08 |
United States Patent
Application |
20190245533 |
Kind Code |
A1 |
Schleicher; Bernd ; et
al. |
August 8, 2019 |
RF SENSOR IN STACKED TRANSISTORS
Abstract
An RF switch includes series-coupled RF switch cells coupled
between an RF input and ground, a transistor including a first
current node coupled to a first load resistor, a second current
node coupled to ground, and a control node coupled to an internal
switch node, and a filter having an input coupled to the first
current node of the first transistor and an output for providing a
DC voltage corresponding to the RF power present at the internal
switch node.
Inventors: |
Schleicher; Bernd;
(Ebersberg, DE) ; Bakalski; Winfried; (Muenchen,
DE) ; Bauder; Ruediger; (FeldkirchenWesterham,
DE) ; Solomko; Valentyn; (Muenchen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Infineon Technologies AG |
Neubiberg |
|
DE |
|
|
Family ID: |
65324260 |
Appl. No.: |
16/264186 |
Filed: |
January 31, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15891025 |
Feb 7, 2018 |
10250251 |
|
|
16264186 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03K 17/6871 20130101;
H03K 17/693 20130101; H03K 17/161 20130101 |
International
Class: |
H03K 17/16 20060101
H03K017/16; H03K 17/687 20060101 H03K017/687 |
Claims
1. An RF switch comprising: a plurality of series-coupled RF switch
cells coupled between an RF input and ground; a plurality of
voltage dividers, each of the voltage dividers being coupled to a
corresponding one of the RF switch cells; and a plurality of
voltage detectors, each voltage detector having an input coupled to
a tap of a corresponding one of the voltage dividers, and an output
for providing a DC voltage, a plurality of the DC voltages
configured to provide a linearity indication of the RF switch.
2. The RF switch of claim 1, wherein each of the RF switch cells
comprises a transistor.
3. The RF switch of claim 2, wherein each of the RF switch cells
further comprises a first resistor across the transistor.
4. The RF switch of claim 3, wherein each of the RF switch cells
further comprises a second resistor coupled between an input signal
source and a control node of the transistor.
5. The RF switch of claim 1, wherein each of the plurality of
voltage dividers comprises: a first resistor coupled between the
tap and ground; and a second resistor coupled between the tap and a
corresponding RF switch cell, wherein each of the first resistors
are equal in value, and wherein each of the second resistors are
unequal in value.
6. The RF switch of claim 5, wherein the value of the second
resistor in the plurality of voltage dividers is scaled according
to a switch position of the corresponding RF switch cell associated
with the second resistor.
7. The RF switch of claim 6, wherein the value of the second
resistor comprises a linearly scaled value component that scales
with RF switch position and a constant value component that does
not scale with RF switch position.
8. The RF switch of claim 6, wherein the value of the second
resistor in a first RF switch position closest to ground comprises
a minimum value, and the value of the second resistor in a last RF
switch position closest to the RF input comprises a maximum
value.
9. The RF switch of claim 1, wherein each voltage detector of the
plurality of voltage detectors comprises a filter.
10. The RF switch of claim 1, wherein each voltage detector of the
plurality of voltage detectors comprises a rectifier circuit.
11. An RF switch comprising: a plurality of series-coupled RF
switch cells coupled between an RF input and ground; a plurality of
voltage dividers, each of the voltage dividers being coupled across
a corresponding one of the RF switch cells; and a plurality of
voltage detectors, each voltage detector having an input coupled to
a tap of a corresponding one of the voltage dividers, and an output
for providing a DC voltage, a plurality of the DC voltages
configured to monitor a plurality of internal voltage conditions of
the RF switch.
12. The RF switch of claim 11, wherein at least one of the
plurality of voltage dividers comprises a single tap voltage
divider.
13. The RF switch of claim 11, wherein at least one of the
plurality of voltage dividers comprises a dual tap voltage
divider.
14. The RF switch of claim 11, wherein at least one of the
plurality of voltage dividers is associated with an RF switch cell
having an intermediate RF switch position between a first RF switch
position and a last RF switch position.
15. The RF switch of claim 11, further comprising a single resistor
coupled across a plurality of the RF switch cells not coupled to a
corresponding one of the plurality of voltage dividers.
16. The RF switch of claim 11, wherein each voltage detector of the
plurality of voltage detectors comprises a filter.
17. The RF switch of claim 11, wherein each voltage detector of the
plurality of voltage detectors comprises a rectifier circuit.
18. An RF switch comprising: a plurality of series-coupled RF
switch cells coupled between an RF input and ground; a plurality of
voltage dividers, each of the voltage dividers being coupled to a
corresponding one of the RF switch cells; and a plurality of
voltage detectors, each voltage detector having an input coupled to
a tap of a corresponding one of the voltage dividers, and an output
for providing a DC voltage.
19. The RF switch of claim 18, wherein each of the plurality of
voltage dividers is coupled across the corresponding one of the RF
switch cells.
20. The RF switch of claim 18, wherein each of the plurality of
voltage dividers is coupled between a drain node of the
corresponding one of the RF switch cells and ground.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/891,025, filed on Feb. 7, 2018, which application is
hereby incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to an RF sensor for
stacked transistors.
BACKGROUND
[0003] RF switches having stacked transistors implemented as
multiple RF cells are used in a variety of RF circuits to implement
various functions in various applications. For instance, one
application is a high voltage application such as a connection to
an antenna in a cell phone. Multiple RF cells are typically coupled
together so that any individual RF cell must only withstand a lower
voltage that is a fraction of the high voltage and is thus within
the breakdown voltage limits for the transistor manufacturing
process used. While the use of multiple RF cells ideally evenly
distributes the high voltage equally into low voltage portions
across the individual RF cells, in practice the distribution of the
high voltage can be unequal due to parasitic elements and effects.
Linearity and other performance characteristics of the high voltage
RF switch using multiple RF cells can thus be affected.
SUMMARY
[0004] An RF switch comprises a plurality of series-coupled RF
switch cells coupled between an RF input and ground; a first
transistor comprising a first current node coupled to a first load
resistor, a second current node coupled to ground, and a control
node coupled to a first internal switch node; and a first filter
having an input coupled to the first current node of the first
transistor and an output configured for providing a DC voltage
corresponding to the RF power present at the first internal switch
node.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0006] FIG. 1 is a schematic diagram of an antenna switch placed in
an example antenna arrangement;
[0007] FIG. 2 is an embodiment voltage sensor circuit embedded in
the stacked transistors of an RF antenna aperture switch;
[0008] FIG. 3(a) is a plot of a transient voltage input signal to
the antenna switch at different power levels;
[0009] FIG. 3(b) is a plot of a transient voltage associated with
an internal switch node of the RF antenna aperture switch;
[0010] FIG. 4(a) is a plot of a drain current of a transistor in
the voltage sensor circuit of FIG. 2;
[0011] FIG. 4(b) is a plot of a drain voltage of the transistor in
the voltage sensor circuit of FIG. 2;
[0012] FIG. 5(a) is a plot of the output voltage of the filter in
the voltage sensor circuit of FIG. 2;
[0013] FIG. 5(b) is a plot of the DC output voltage of the voltage
sensor circuit of FIG. 2 versus the input power into the RF antenna
aperture switch;
[0014] FIG. 6 is circuit diagram of an embodiment having a
plurality of voltage sensor circuits at different internal switch
nodes to measure different input power ranges of the RF antenna
aperture switch;
[0015] FIG. 7 is a plot of different voltage sensor output voltages
of the circuit of FIG. 6 versus input power into the RF antenna
aperture switch;
[0016] FIG. 8 is a schematic diagram of an embodiment circuit
including a plurality of voltage dividers and voltage sensors
circuits coupled to an RF antenna aperture switch configured to
provide a linearity indication;
[0017] FIG. 9(a) is a plot of the voltages across the RF switch
cells of the circuit of FIG. 8 without the voltage dividers being
connected;
[0018] FIG. 9(b) is a plot of the voltages across the RF switch
cells of the circuit of FIG. 8 with the voltage dividers being
connected;
[0019] FIG. 10 is a schematic diagram of an embodiment RF voltage
sensor circuit, including a reference branch and an operational
amplifier;
[0020] FIG. 11 is a schematic diagram of the circuit of FIG. 10
including an example circuit implementation of the operational
amplifier;
[0021] FIG. 12(a) is a plot of the sense and reference voltages of
the circuit of FIG. 10 versus stack input power levels;
[0022] FIG. 12(b) is a plot of the output voltage of the
operational amplifier of the circuit of FIG. 10;
[0023] FIG. 13 is a plot of the voltage across the RF switch cells
of, for example the circuit of FIG. 8, with and without the voltage
sensor circuits being connected showing a negligible difference
therebetween;
[0024] FIG. 14(a) is a plot of the voltage distribution across the
probing resistors of the circuit of FIG. 8; and
[0025] FIG. 14(b) is a plot of a similar voltage distribution
across the operational amplifier voltage outputs of the circuit of
FIG. 8.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0026] RF switches having a plurality of coupled RF switch cells
are sometimes realized using a bulk CMOS technology, which uses a
biased substrate. The transistors used in the RF switch circuit can
have a parasitic substrate capacitance and a parasitic resistance
coupled between the source of the transistor and ground, and
between the drain of the transistor and ground. The voltage at the
source and the drain of the transistor thus causes a corresponding
parasitic current to flow between the transistor source and drain,
and ground. These parasitic elements and effects cause a high
voltage impressed across a plurality of coupled RF switch cells to
be unequally distributed amongst the individual RF switch cells,
with a corresponding degradation of the linearity of the switch as
well as a degradation of other performance characteristics. For
example, a first RF cell in a plurality of coupled RF cells nearest
to the RF source may have a maximum fraction of the high voltage RF
input signal; whereas a last RF cell in a plurality of coupled RF
cells furthest from the RF source and coupled to ground may have a
minimum fraction of the high voltage RF input signal. Ideally, each
RF cell in the plurality of coupled RF cells would all have the
same fraction of the high voltage RF input voltage for maximum
linearity and circuit performance.
[0027] One such area of RF switches is antenna aperture switches,
which are used to improve the radiation performance of a mobile
phone antenna. Proximate to the switch, external inductances and/or
capacitances can be switched-in at positions far from the
feed-point of the antenna and tune the antenna to radiate better at
selected frequencies. The usage is illustrated in FIG. 1. FIG. 1
shows an antenna switch 106 placed in an example antenna
arrangement boo including an RF signal feed 102, a planar inverted
F-antenna, an inductor L1, and a capacitor C1.
[0028] Important figures of merit for such switches are its
resistance in on-mode (Ron), its capacitance in isolation mode
(Coff) and the maximal RF voltage swing it can withstand in
isolation mode. The switches are designed in a standard body-biased
bulk CMOS or Silicon-On-Insulator (SOI) technology, using single or
dual gate-oxide transistors. The nominal voltage withstood by one
dual gate-oxide transistor is typically in the range of 1.5 V to
2.5 V, the required RF voltage swing is often much higher, e.g. 45
V, 80 V, or up to 100 V. This requires a stacking of transistors as
previously explained to be able to handle the occurring voltage
swings. By stacking, the voltage swing should ideally distribute
equal between the drain and source node of the transistors, which
is supported by the gate-drain and gate-source capacitances.
However, due to small parasitic capacitances from drain, gate and
source to common ground or to substrate potential, the voltage
distribution is unequal. This unequal distribution limits the
sustainable overall RF voltage swing and causes an earlier failure
of the device, because one transistor experiences a larger voltage
swing than the others and therefore has an earlier
breakthrough.
[0029] Embodiments are made and described below to measure the RF
voltage swing present at one or all of the transistors in the RF
stack without influencing the wanted behavior of the circuit and
report the occurring RF voltage by a DC readout. A first embodiment
circuit measures the RF voltage swing across at least one
transistor and provides RF swing information regarding internal
switch nodes and therefore in the application of the switch, and to
provide an output used to initiate measures to reduce the input
power if necessary, or to take other measures. A second circuit
allows determining the RF voltage across all or a subset of
transistors in the stack and by this to report the voltage
distribution across each transistor of the stack. This allows
visualizing insights of the circuit such as switch linearity and
therefore allows comparing semiconductor technologies and circuit
design concepts among each other. This information can also be used
by a system to change the operating condition of the switch if
desired.
[0030] The maximal RF voltage swing can also be measured by a
destructive measurement, were the RF voltage swing is increased
until the device breaks. The breakdown voltage is noted and
compared. While this method is effective in determining the voltage
limits of the RF switch in a given operating environment, it is of
course not effective for use in a system wherein the integrity of
the switch is preserved to report out internal switch information
so that a system using the switch can take measure to change the
operating condition of the switch if desired.
[0031] Embodiments therefore allow measuring internal voltages of
the circuit, without detuning the general operation of the switch
or otherwise affecting the performance of the switch. According to
embodiments, only a negligible part of the voltage swing is used to
monitor the distribution. The advantage is that voltages of
internal nodes are measured and can be read out for to implement
further measure.
[0032] FIG. 2 shows a switch arrangement 200 including an RF input
R.sub.Fin at node 202, a voltage impressed on the RF switch Vin,
and an RF output node R.sub.Fout at node 204. The RF switch and a
sensing circuit are also shown and described in detail below.
[0033] A first embodiment voltage sensing circuit is described
below with reference to FIG. 2. The sensing circuit senses the RF
voltage present in the switch branch in an isolation mode and can
be used to report the measured voltage swing to a control unit. In
FIG. 2, the stacked transistors T.sub.1 to T.sub.x of an RF antenna
aperture switch are shown on the left side. The stacked transistors
can be switched by a control signal at their gates, shown as
V.sub.G. The control signal or input signal V.sub.G is used to
energize the gates of transistors T.sub.1 through T.sub.x through
gate resistors R.sub.G1 through R.sub.Gx. The resistances parallel
to the drain and source of each transistor R.sub.DS1 to R.sub.DSx
are used for a good biasing of each transistor and are typically in
the range of 3 k to 40 k Ohm. In the off-state, all transistors are
pinched off and the series connection of the drain-source resistors
is the dominant off-state resistance.
[0034] The voltage sensor circuit of FIG. 2 described in further
detail below is placed at the lowest drain-source resistor
R.sub.DS1 or a portion of the drain-source resistor R.sub.DS1.1 to
ground node. Resistors R.sub.DS1.1 and R.sub.DS1.2 form a voltage
divider with a tap providing the V.sub.P (probe) voltage as shown,
and a total resistance value of R.sub.DS1, which is similar in
value to the other source to drain resistors shown in FIG. 2.
[0035] FIG. 2 shows an embodiment voltage sensor that can be
embedded in the stacked transistors of an RF antenna aperture
switch. The voltage sensor includes a sense portion and a filter
portion. Sense portion includes a sense transistor T.sub.sens and a
sense load resistor R.sub.sens. The node coupling the sense
transistor and the load resistor has a corresponding node voltage
V.sub.D. The current flowing into the drain of the sense transistor
is labeled I.sub.D. The sense portion is coupled between V.sub.DD
and ground as shown. The filter portion is a low pass filter
including a filter resistor R.sub.Filt and a filter capacitor
C.sub.Filt. The output of the filter is labeled V.sub.out at node
206. The filter portion is coupled between the internal sense
circuit node and ground as shown.
[0036] FIG. 3 (a) shows an example input voltage sweep 302 up to a
voltage of V.sub.in=20 V at an example frequency of 1 GHz, which is
brought to the input terminal of the RF switch. A proportional
fraction of the RF voltage swing is present across the lowest
resistor R.sub.DS1.1, as can be seen in FIG. 3 (b) on an example
stack of 18 transistors, drain-source resistors R.sub.DS2 to
R.sub.DSx of 3 kOhm and the lowest drain-source resistance
R.sub.DS1 being divided to R.sub.DS1.2=2 kOhm and R.sub.DS1.1=1
kOhm. The sensing circuit of FIG. 2 uses this voltage and converts
it to a DC readout at node 206 with a sufficient amplitude to be
used for further action.
[0037] FIG. 3 (a) thus shows a transient voltage of a 1 GHz input
signal V.sub.in to the antenna switch at different power levels and
FIG. 3 (b) shows the corresponding probe voltages V.sub.P at the
lowest resistance R.sub.DS1.1 to ground.
[0038] The voltage sensing circuit in FIG. 2 on the right side as
previously explained comprises a relatively small NMOS transistor
T.sub.sens, which is DC biased at a gate source voltage of zero
volts through the resistor R.sub.DS1.1 to ground. In this condition
biasing condition a small drain current ID flows through transistor
T.sub.sens. When the RF voltage swing is present, the positive
voltage swing of V.sub.p activates the sensing transistor and an
increased, rectified drain current I.sub.D starts to flow through
the transistor. At the resistor R.sub.sens, connected between the
drain and V.sub.DD, a voltage drop V.sub.D builds up, which is
caused by the rectified current of I.sub.D. The half wave rectified
current I.sub.D and voltage V.sub.D can be seen in FIGS. 4 (a) and
4 (b).
[0039] FIG. 4 (a) thus shows the drain current 402 and FIG. 4 (b)
shows the drain voltage 404 transients, occurring at the sensing
NMOS transistor drain. The different traces of drain current 402
and drain voltage 404 represent different power levels of the 1 GHz
input signal.
[0040] The rectified voltage V.sub.D shown in FIG. 4 (b) contains a
DC voltage component, which is filtered by a subsequent low-pass
filter, e.g. formed by R.sub.Filt and C.sub.Filt in an embodiment.
In FIG. 5 (a) this filtered transient output signal V.sub.out
labeled 502 can be seen, which has a clearly visible DC component
and only a small RF ripple. Stronger filtering can even further
decrease the ripple, if desired for a specific application. In FIG.
5 (b), the DC voltage labeled 504 of V.sub.D or V.sub.out is
plotted versus the input power being present at V.sub.in. It can be
seen, that the output voltage can be attributed to an input power
in a certain power sensing range.
[0041] FIG. 5 (a) thus shows a transient voltage 502 at the output
of the RC-filter for different power levels, and FIG. 5 (b) shows
the DC voltage 504 of the voltage sensor plotted versus input power
into the RF stack.
[0042] The sensing range can be varied as desired by the use of
different circuit elements. For example, increasing the lowest
drain-source resistance R.sub.DS1.1 and decreasing the
corresponding resistance R.sub.DS1.2 increases the amount of
voltage swing V.sub.P entering transistor T.sub.sens and therefore
increases the amount of drain current I.sub.D being rectified in
the sensing transistor. Increasing the resistor might have a
drawback of detuning the voltage stack, because the parasitic
elements of transistor T.sub.sens can have a stronger influence.
Changing circuit element values to provide different sensing ranges
that have to be monitored carefully to preserve switch performance.
Another possibility to vary the sensing range could be to increase
the resistance R.sub.sens. Increasing the resistance causes the
voltage drop due to the drain current I.sub.D to increase and a
smaller portion of the probe voltage V.sub.p is needed to build up
a DC potential. But increasing the sense resistance has to be
traded-off with the quiescent current I.sub.D, which occurs in
T.sub.sens for a DC gate biasing of zero volts, as well as with the
maximal voltage swing occurring at the transistor T.sub.sens before
cutting of the transistors VDS operation. A further influence on
the obtainable output voltage is influenced by the transistor
geometry, which changes the detuning to the RF stack by its
parasitic capacitances and the current increase by inherent
influence of the transconductance. Relatively small sizes for
transistor T.sub.sens are ideally used.
[0043] An increased sensing range could be achieved for example by
placing different sensing transistors T.sub.sens between different
sensing nodes in the R.sub.Ds stack and ground potential, as can be
seen in FIG. 6. The circuit 600 of FIG. 6 is similar to the circuit
200 of FIG. 2, previously described, but with different sensors at
different nodes of the RF stack to measure an increased input
range. Circuit 600 includes an RF input 602, RF.sub.in and an RF
output 604, RF.sub.out. In the example of FIG. 6, a first voltage
divider includes resistors R.sub.DS1.1, R.sub.DS1.2, and
R.sub.DS1.3, and includes two taps for providing voltages V.sub.p1
and V.sub.p2. A second voltage divider includes resistors
R.sub.DS2.1 and R.sub.DS2.2, and a tap for providing voltage
V.sub.p3. A first sensing circuit 606 is coupled to the V.sub.p1
tap, a second sensing circuit is coupled to the V.sub.p2 tap, and a
third sensing circuit 610 is coupled to the V.sub.p3 tap. Any
number of voltage dividers, taps, and voltage sensing circuits may
be used. The exact configuration shown in FIG. 6 is therefore only
one example for providing additional sensing ranges. In FIG. 6,
sensing circuits 606, 608, and bio are substantially the same as
those previously described in circuit 200 of FIG. 2.
[0044] Circuit 600 allows several output voltages V.sub.out1,
V.sub.out2, and V.sub.out3 to be read out, which represent
different input voltage ranges V.sub.in and can be combined to
increase the overall range. The output voltage of different voltage
sensors can be seen in FIG. 7 for three sensors similar to those
shown in FIG. 6. However, as previously described care has to be
taken on the influence to the RF stack, and not to disturb proper
wanted operation.
[0045] FIG. 7 thus shows a plot 700 of voltage versus input power
of three output voltages corresponding to output voltages
V.sub.out1, V.sub.out2, and V.sub.out3. The three different sensors
606, 608, and bio at the different nodes related to the V.sub.p1,
V.sub.p2, and V.sub.p3 voltages of the RF stack are used to measure
an increased input range as was shown in FIG. 6 and described
above.
[0046] The output voltages V.sub.out from the single voltage sensor
or the multiple voltage sensors could be provided to an
analog-to-digital converter (not shown), which samples the DC
voltage values and passes it to a subsequent control-unit for
further processing. This processing could include an overvoltage
warning, which allows performing actions to protect the antenna
switch or other components in its vicinity, or to report the amount
of voltage swing present, allowing knowledge about the antenna
radiation performance. In the second case, the occurring RF voltage
swing can be a measure for the voltage distribution across the
antennas dimension and can give information about the antenna's
radiation behavior, such as linearity. As well as in the second
instance, countermeasures to improve voltage distribution such as
altering the radiation impressed on the RF switch could be
performed.
[0047] In the above described circuit embodiments, at least one RF
voltage to ground is determined and monitored. Next, embodiment
circuits to measure the differences between the drain-source nodes
of the transistors in the RF stack are discussed. In the
subsequently described circuit embodiments, the voltage
distribution across the transistors in the stack can be quantified.
As can be seen in FIG. 8, at every drain node of the stacked RF
transistors T.sub.1 to T.sub.x, a voltage divider to ground is
placed. The voltage divider connected to the drain of the lowest
transistor T.sub.1 has an overall value of R.sub.N, which is made
large enough not to distort the performance of the RF stack.
Typical values of R.sub.N could be in the range of 50 to 120 times
R.sub.DS. The resistor at the drain of the next transistor T.sub.2
has twice the value of R.sub.N and so on, until the resistance at
the highest drain node of transistor T.sub.x has a value of
xR.sub.N. The probe resistor R.sub.P is subtracted from the
resistors nR.sub.N and are all equal in value and in the range of
the value of R.sub.DS. By this arrangement, the circuit swing
across each stack resistance is scaled to a small reference voltage
swing.
[0048] In FIG. 8, an embodiment switch arrangement 800 is shown
including the RF source, the RF switch stack, a plurality of
voltage dividers, and a plurality of sense circuits. The RF input
802 and RF output 804, and the RF stack are as previously
described. The plurality of voltage dividers are coupled to the
drains of the corresponding stack transistor and one of the
resistors in the divider is scaled with switch position as
previously discussed. Sense circuits 806, 808, 810, 812, and 814
are coupled to the taps V.sub.P1, V.sub.P2, V.sub.P3, V.sub.Px-1 of
a corresponding voltage divider, V.sub.PX, and have corresponding
output voltages V.sub.Out1, V.sub.out2, V.sub.out3, V.sub.outx-1,
and V.sub.outx.
[0049] FIG. 8 thus shows a circuit embodiment 800 to determine the
RF voltage swing across the drain-source nodes of each transistor
in the RF switch stack. As will be explained in further detail
below, a perfectly linear switch will provide a plurality of output
voltages that are all equal. A non-linear switch will provide a
plurality of output voltages that are unequal and typically show a
curve indicating that larger drain-source voltages are associated
with transistors closer to the RF source and that relatively
smaller drain-source voltages are associated with transistors
closer to ground. Of course, other anomalous non-linear patterns
can also be detected with circuit 800 of FIG. 8.
[0050] Referring now to FIGS. 9 (a), the voltage distribution is
shown on a stack of 18 R.sub.DS resistances only. In this instance,
the unbalancing effect of the sensing transistors is not present
and the influence of the resistances R.sub.N can be checked. The
distribution of the voltages 902 can be seen in FIG. 9 (a), where
the voltage across the R.sub.DS resistances is plotted. Later, once
the transistors are added back in, this will be the voltage
difference between the drain voltages of the transistors subtracted
by the source voltage of the transistors. The broadness of curve
902 indicates that a small influence of the probing arrangement
previously discussed is still present within the RF stack, but this
is negligibly small and can further be decreased by adjusting the
resistor R.sub.N resistor component in the voltage divider to
address this error. In FIG. 9 (b), the output voltage swing V.sub.P
at the resistances R.sub.P of the different nodes can be seen. Due
to the resistive divider, it is much smaller, but still a
significant voltage swing is measured. As well as shown in FIG. 9
(a), here the minor broadening of the curve 904 indicates a small
influence of the probing arrangement. The probe amplitudes can be
seen in FIG. 9 (b). Because they are more than a factor of ten
smaller than the probe voltages in FIG. 3 (b) of the previously
described circuit 200, a more refined voltage sensor is ideally
used and will be described in further detail below.
[0051] FIG. 9 (a) thus shows the drain-source voltage across the RF
stack of 18 R.sub.DS resistors at an input voltage of 45 V. FIG.
9(b) shows the probe voltages V.sub.p at resistances R.sub.p to
ground.
[0052] The voltage sensors for circuit 800 are similar to the
voltage sensor circuits as previously described, but having
additional gain to compensate for the lower value of the sensed
voltages. The simplified schematic of an embodiment voltage sensor
1000 can be seen in FIG. 10. Circuit 1000 includes the sensing
transistor T.sub.sens with drain side connected resistor R.sub.sens
and the subsequently connected low-pass filter, here for example
using R.sub.Filt and C.sub.Filt. Additionally, to increase the
sensitivity of the readout, a reference branch is connected in a
similar arrangement to the sensing branch using reference
transistor T.sub.ref, reference resistor R.sub.ref and the low-pass
filter having the same R.sub.Filt and C.sub.Filt components. The
input (gate) of the T.sub.ref transistor is terminated with a
resistance R.sub.p, but no connection to the voltage divider of the
RF stack is used. The reference branch provides the same quiescent
current through the reference transistor and builds up a reference
voltage V.sub.ref through the resistance R.sub.ref, especially,
when the values of the elements are the same as in the sensing
branch. When an RF voltage swing is present in the RF stack, a
difference between V.sub.sens and V.sub.ref is amplified by the
operational amplifier OPA designated 1002 and increases the
operational range of the voltage sensor. The benefit of using an
amplifier at this side of the chain, is that only DC signals must
be amplified, which is much easier than amplifying RF signals and
that a voltage amplification with high-ohmic circuits can be done,
which reduces the detuning of the RF stack.
[0053] FIG. 10 thus shows an RF voltage sensor circuit, including a
voltage sensor portion, a reference branch, and an operational
amplifier for amplifying a voltage difference between the voltage
sensor portion and the reference branch.
[0054] In FIG. 1i, the RF sensing circuit 1100 with the reference
branch can be seen together with an example implementation of the
operational amplifier. The sensing circuit and reference branch in
block 1102 are as previously described. In a circuit embodiment,
operational amplifier 1002 includes a differential amplifier
comprising current source Ii, differential pair of transistors
T.sub.1 and T.sub.2, and simple current mirror load T.sub.3 and
T.sub.4. Operational amplifier 1002 also includes an output stage
including transistor T.sub.5, and associated biasing and
compensation components R.sub.1, R.sub.2, and C.sub.2.
[0055] FIG. 11 thus shows an RF voltage sensor circuit 1100,
including a reference branch and an example implementation of the
operational amplifier 1002.
[0056] In FIGS. 12 (a) and (b) a sweep of the RF input power into
the stack at one voltage sensor can be seen. In FIG. 12 (a) the
internal voltages V.sub.sens 1204 and V.sub.ref 1202 can be
compared to each other. In FIG. 12 (b), the output voltage 1208 at
one operational amplifier can be seen. For reference, the voltages
of V.sub.sens and V.sub.ref (seen as a composite trace 1206 due to
the different voltage scale in FIG. 12 (b)). It can be seen that
the output voltage V.sub.out 1208 is dependent on the input power
into the RF stack.
[0057] FIG. 12 (a) thus shows the DC voltage V.sub.sens and
V.sub.ref occurring in one voltage sensor, swept across stack input
power levels and FIG. 12 (b) shows the output amplitude V.sub.out
of one operational amplifier output swept versus stack input
power.
[0058] The RF stack of FIG. 8 is built using the 18 resistances
R.sub.DS, including the RF transistors, connecting the R.sub.N
resistance network and the voltage sensors at each node V.sub.P in
an embodiment. In FIG. 13, a comparison of the RF voltage
distribution at a low RF input voltage of 30 dBm (=10 V) into the
RF port can be seen in trace 1302. The plot of FIG. 13 shows the
magnitudes of the drain-source voltage difference versus transistor
number T1 to T18. Due to convergence issues, the simulated power
could not be increased above this value, but the voltages scale in
the same way at higher input power levels. An ideal equal
distribution is calculated by 10 V/18=0.55 V across the
drain-source voltage. The hanging voltage distribution 1302 seen in
FIG. 13 is a result of parasitic substrate capacitances and is well
explained in the literature and referenced above. As can be seen
here, the curves are substantially identical when comparing the
curves with and without resistive probing network, and so are
referred to with a single identifying numeral 1302. This indicates
that the probing network does not detune the proper circuit
operation.
[0059] FIG. 13 thus shows the RF voltage distribution across the
drain-source terminals of the 18 RF transistors in the switch
stack. The comparison shows a negligible influence for the
resistive probing network being connected or not.
[0060] In FIG. 14 (a), the voltage distribution V.sub.px 1402 of
the 18 nodes at the probe resistances R.sub.p can be seen. The same
hanging curve was shown appearing across the drain-source nodes at
the transistors directly. The similarity of the curves demonstrates
that the distribution of voltages can be probed by the arrangement
of FIG. 8. In FIG. 14 (b) the voltage distribution 1404 at the
output of the 18 operational amplifiers is shown. The shape of the
distribution 1404 is the same as was shown in FIG. 13 and is
equivalent to the drain-source voltage distribution of the RF
transistor stack.
[0061] FIGS. 14 (a) and (b) thus shows the voltage distribution at
the probing resistors Rp (FIG. 14(a)) and the output of the
operational amplifiers Vout (FIG. 14(b)), showing an equivalent
shape compared to the drain-source voltage swing of the RF
transistors.
[0062] By reading the output voltages of the voltage sensors, the
voltage distribution at each drain node of the transistor stack can
be visualized. Similar to the sensor circuit embodiments in the
first part of the description, the readout of the output voltages
can be done using an analog to digital converter. The information
about the voltage distribution can be used in a subsequent control
unit for further processing and measures such as adjusting the RF
input power or in making other adjustments in the operating
condition of the RF switch.
[0063] Circuit embodiments could be used to influence both internal
systems and customer systems. In internal systems they could be
used for a deeper circuit analysis in the design phase or during
product testing. The customer system could benefit from sensed
internal node voltages by being able to monitor these during the
customer design phase, for design-in at the customer, for debugging
or for use during normal operation.
[0064] Sensing circuits have been described in various embodiments,
which make internal circuit nodes of an RF switch available to the
outside world but not disturbing the desired operation of the RF
switch.
[0065] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. It is therefore
intended that the appended claims encompass any such modifications
or embodiments.
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