Patent Application
20110273355
The invention relates generally to antenna switches, and more particularly, to systems and methods for complementary metal-oxide-semiconductor (CMOS) differential antenna switches using multi-section impedance transformations.
In achieving fully integrated wireless communication systems, an antenna switch is utilized to change modes (e.g., transmit and receive modes) or frequency bands (e.g., high and low bands). In performing these tasks, the insertion loss of the antenna switch should be minimized to guarantee a high efficiency of the transmitter as well as a low noise figure of the receiver. The antenna switch should also isolate the receiver from the transmitter effectively during respective receive and transmit modes, and vice versa. In addition, high power signal from the transmitter should be handled without significant distortions by the antenna switch to preserve the linearity of transmitters.
The power handling capability of an antenna switch depends primarily on the voltage swing over the OFF-state receiver switches of the antenna switch. A large signal from the transmitter induces the unwanted channel formation and forward biases junction diodes of the OFF-state receiver switch devices. Also, this can cause a device breakdown, which results in linearity degradation of the transmitter. Because transmitted signals from a power amplifier can have large voltage swing (e.g., more than 1 W based upon peak-to-peak 20V at 50Ω load) in the case of cellurar applications, reducing the voltage swing over the OFF-state receiver switches is important to enhance the power handling capability of the antenna switches.
The efficiency of a power amplifier is one of the most dominant factors in determining the whole transmitter performance. Particularly, output matching network of the power amplifier takes a critical portion of it. Since the output impedance of the power amplifier is usually small enough to generate a high power signal, the output matching network of the power amplifier is forced to have a large impedance transformation ratio to match the output impedance to the antenna. As the impedance transformation ratio increases, the efficiency of the matching network is typically degraded.
According to an example embodiment of the invention, there is a CMOS differential antenna switch. The CMOS differential antenna switch may be fabricated using a standard 0.18-μm process, although other process may be utilized without departing from the embodiments of the invention. The CMOS differential antenna switch may include two (or more) identical or substantially similar single-ended antenna switches to relieve the voltage stress in half (or less) on receiver switches by providing two (or more) signal paths. Each single-ended antenna switch may include a plurality of switch devices to sustain the large voltage swing from a transmitter by distributing the stress over the multiple switch devices. The input signal of the differential antenna switch comes through the output matching network of power amplifier (e.g., transformers), and the output signal of the differential antenna switch is combined by an LC balun to transmit the signal via a single-ended antenna.
According to an example embodiment of the invention, there may be an LC balun, which may include plurality of inductors and capacitors. In an example embodiment of the invention, LC balun may combine the output signals of two single-ended antenna switches to transmit the signal through the single-ended antenna, and may provide the optimal impedance for the differential antenna switch operation by impedance transformation. A voltage stress over the receiver switches can be relaxed for a certain level of power with a reduced switch operating impedance which is obtained by implementing the LC balun as an impedance matching network between differential antenna switch and antenna. Thus, the reducing operating impedance of the antenna switch helps to enhance the power handling capability of the antenna switch.
According to an example embodiment of the invention, there may be a transformer as an output matching network of power amplifiers. To generate a high power, output powers of multiple power amplifiers are combined by an output matching network in transmitter systems. In combining the output powers, transformers are widely used due to its advantage of compact size comparing to the LC counterparts. Since the efficiency of the transformer usually depends on its impedance transformation ratio, the efficiency can be improved by reducing the impedance for the antenna switch operation minimizing the impedance transformation ratio of the transformer. Particularly, since the quality factor of the inductors used in the transformer is higher when it operates in differential mode than in single-ended mode, efficiency of the transformer is enhanced even more by implementing a differential antenna switch at the output of the transformer.
According to an example embodiment of the invention, there may be a transmitter module which consists of an antenna switch and a power amplifier. By implementing multi-section impedance transformation networks with transformer and LC balun, to match the low impedance of the output of a power amplifier to 50Ω antenna, for example, the burden of impedance transformation is distributed to those two matching networks. Since the output impedance of the power amplifier is typically low, the optimal impedance for the high power antenna switch operation can be positioned between the output impedance of the power amplifier and the antenna impedance. As a result, power handling capability of the antenna switch and the efficiency of the transmitter module can be enhanced at the same time, by employing a two-step impedance matching with a proper choice of the optimal impedance for the antenna switch operation even though an additional matching network is implemented.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
Example embodiments of the invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
Example embodiments of the invention may provide for complementary metal-oxide-semiconductor antenna switches. To increase the power handling capability of the CMOS antenna switches, differential switches can be utilized in conjunction with multi-section impedance transformations described herein. Compared to a non-differential structure, differential switches may reduce voltage stress on receiver switches by spreading voltage stress across two or more parallel signal paths. Indeed, the differential architecture may help to relieve the large voltage swing from power amplifiers by distributing the voltage stress over the receiver switch with two or more of the identical or substantially similar single-ended switches.
Likewise, multi-section impedance transformations described herein can be utilized to provide at least (i) a first impedance transformation network between amplifiers (e.g., power amplifiers) and first ports of the differential switch (e.g., from a few ohms to 35 ohms), and (ii) a second impedance transformation network/stage between second ports of the differential switch and at least one antenna (e.g., from 35 ohms to 50 ohms). The combination of the first and impedance transformation networks/stages can provide an effective impedance transformation need to match the output impedance of the amplifiers to that of the at least one antenna. However, the use of two stages relaxes the impedance transformation to be performed by the first impedance transformation network/stage between the amplifiers and the first ports of the differential switch. In particular, since only a portion of the full impedance transformation between the amplifiers and the antenna is being performed by the first impedance transformation network/stage, the operating impedance of the differential antenna switch may be reduced. This reduction in the operating impedance may help to relieve the impedance transformation ratio of the first impedance transformation network/stage, thereby resulting in an improved efficiency of the matching network for the first impedance transformation network/stage. It will be appreciated that any degraded insertion loss due to the impedance transformation technique can be compensated for by selecting an optimal impedance for the antenna switch operation. In this way, the use of the multi-section impedance transformation technique with the differential antenna switch architecture may enable to achieve a high power handling capability for the antenna switch and a reasonable efficiency for the transmitter module at the same time, according to an example embodiment of the invention.
During a transmit (TX) mode, the differential outputs of the differential PAs can be provided to respective switches of the differential antenna switch block 115 via a first matching network 110. The first matching network 110 may be operative to perform an a first impedance transformation to increase the impedance between the PA differential outputs and the differential antenna switch block 115. The amount of the first impedance transformation may be selected in order to reduce the operating impedance of the differential antenna switch block 115, thereby resulting in an improved efficiency of the first matching network 110. Likewise, degraded insertion loss due to the first impedance transformation can be compensated for by selecting an optimal impedance for the differential antenna switch block 115 operation. It will be appreciated that the first matching network 110 can comprise passive devices such as one or more of a transformer, inductor, capacitor, resistor, etc. However, the matching network 110 can likewise comprise one or more active devices as well without departing from example embodiments of the invention. The matching network 110 can also be configured to combine differential outputs from a plurality of PAs of the TX block according to an example embodiment of the invention.
The differential antenna switch block 115 may include at least two functional switches provide the equivalent of at least two single-ended logical switches for communicating differential outputs from the transmitter block. The respective switches of the differential switch block can be implemented using one or more transistors such as MOSFETS used in a CMOS technology. However, it will be appreciated that other transistors and FETs can be utilized for implementing the logical switches of the differential antenna switch block 115 without departing from example embodiments of the invention.
During the transmit (TX) mode, the differential antenna switch block 115 can operate the switches to communicate the differential outputs of the first matching network 110 to the differential inputs of a second matching network 120. The second matching network 120 may be operative to perform a second impedance transformation to increase the impedance between the outputs of the differential antenna switch block 115 and the two or more differential antennas 125. Generally, the differential nature of the outputs of the differential antenna switch block 115 may be preserved by the second matching network 120 and delivered to the respective differential antennas 125, thereby providing from a fully differential system. Likewise, the second matching network 120 can include respective impedance transformation paths for each differential signal path. It will be appreciated that the second matching network 120 can comprise passive devices such as one or more of a transformer, inductor, capacitor, resistor, etc. However, the matching network 120 can likewise comprise one or more active devices as well without departing from example embodiments of the invention.
It will be appreciated that the combination of the impedance transformations of the first matching network 110 and the second matching network 120 may be sufficient to increase the output impedance of the PAs of TX block 150 to match the impedance of antennas 125.
Still referring to
It will be appreciated that many variations of the system 100 of
As shown in the example embodiment of
The second matching network 203 can include an LC balun in order to convert balanced, differential signals to an unbalanced, single-ended signal for TX mode, and a single-ended signal to differential signals for RX mode, according to an example embodiment of the invention. As shown in
Differential antenna switch 201 includes two identical or substantially similar single-ended switches 208 and 209, which are switched from a respective first position to a respective second position, or vice versa, depending on whether a TX mode or RX mode is selected. For example, the single-ended switches 208, 209 can be operated in a respective first position to connect the differential outputs of PAs 250/first matching network 202 to the second matching network 203. On the other hand, the single-ended switches 208, 209 can be operated in a respective second position to connect the antenna 255/second matching network 203 to the receiver block, according to an example embodiment of the invention.
With continued reference to
The receiver switch 350 can include a series receive switch 303, and a shunt receive switch device 304. The series receive switch 303 may be utilized to provide the main receive signal path from the antenna 306 to the receiver block 307 during an receive (RX) mode. The shunt switch device 304 may be utilized to improve the isolation between the receiver block 307 and the transmitter block. For example, in a TX mode when the transmit block 305 is ON and the receiver block 307 is OFF, the shunt switch device 304 may be enabled to connect the main receive signal path to ground.
Still referring to
It will be appreciated that the shunt transmit switch 302 and the series receive switch 303 should be able to sustain a large voltage stress from a transmitter because the switches are in an OFF-state during the TX mode. In order to avoid channel formation of OFF-state switches and breakdown of these devices, switch devices may be stacked for the shunt transmit switch 302 and the series receive switch 303. By stacking the switch devices, the large voltage swing may be distributed to the stacked devices reducing the voltage stress on each of the switch device. However, the insertion loss and the isolation in receive mode may be degraded as the number of stacked devices increases. Thus, the number of stacked devices should be chosen by considering the trade-off between the power handling capability in transmit mode and the insertion loss in receive mode. In an example embodiment of the invention, three switch devices 308, 309, and 310 are stacked for the series receive switch 300. More specifically, the drain of device 308 can be connected to the source of device 309, and the drain of device 309 can be connected to the source of block 310. Likewise, the four switch devices 311, 312, 313, and 314 are stacked for the shunt transmit switch 302. In particular, the source of device 311 is connected to the drain of device 312. The source of device 312 is connected to the drain of device 313, and the source of device 313 is connected to the drain of device 314. These switch devices illustrated in
As shown in
Still referring to
Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
