Switch circuits having integrated overdrive protection and related transmit/receive circuits and MMIC amplifiers

Monolithic microwave integrated circuits are provided that include a substrate, a transmit/receive selection device that is formed on the substrate, a high power amplifier formed on the substrate and coupled to a first RF port of the transmit/receive selection device, a low noise amplifier formed on the substrate and coupled to a second RF port of the transmit/receive selection device and a protection circuit that is coupled to a first control port of the transmit/receive selection device.

FIELD

The inventive concepts described herein relate to microelectronic devices and, more particularly, to microelectronic devices having overdrive protection.

BACKGROUND

Most radio frequency (“RF”) electronic devices may be damaged or even destroyed if subjected to high RF signal levels. In many cases, the damage may be caused by high current levels flowing through the RF electronic device, which can result in overheating that may damage or destroy circuits within the device. The damaging RF signal levels may come from a variety of sources, some of which may be unpredictable, such as lightning strikes or RF signals transmitted by other nearby transmitters. Additionally, RF electronic devices may be damaged or destroyed when subjected to over-voltage conditions (i.e., voltage levels are applied to the device that are higher than the voltage that the device is rated to handle). This is particularly true with RF electronic devices formed from materials such as gallium arsenide (“GaAs”) or indium phosphide (“InP”) based semiconductor materials that have relatively lower critical breakdown fields as compared to, for example, devices formed from wide-bandgap materials such as gallium nitride (“GaN”) based semiconductor materials which have much higher critical breakdown fields.

One example of an RF electronic device that may be susceptible to damage when exposed to high RF signal levels is a low noise amplifier. As is known in the art, a low noise amplifier is an amplifier circuit included in many RF communications systems that is used to amplify a received RF signal before the received RF signal is passed to a receiver. Typically, the power level of RF signals that are passed to a low noise amplifier are relatively low, as the RF signals have typically been transmitted over an air interface and hence have been subjected to free space losses, which may be quite high in many cases. Low noise amplifiers may be susceptible to damage if an RF signal having a high power level is input thereto.

SUMMARY

Pursuant to some embodiments of the present invention, monolithic microwave integrated circuits are provided that include a substrate, a transmit/receive selection device that is formed on the substrate, a high power amplifier formed on the substrate and coupled to a first RF port of the transmit/receive selection device, a low noise amplifier formed on the substrate and coupled to a second RF port of the transmit/receive selection device and a protection circuit (e.g., an overdrive protection circuit) that is coupled to a first control port of the transmit/receive selection device.

In some embodiments, the transmit/receive selection device may be a transmit/receive switch such as, for example, a single-pole double-throw switch. In other embodiments, the transmit/receive selection device may be a circulator

In some embodiments, the monolithic microwave integrated circuit may further include a sampling circuit that is configured to couple a sample of an RF signal received at a third RF port of the transmit/receive selection device and to provide the sample of the RF signal to the protection circuit. The sampling circuit may be configurable to sample at least one of a first amount and a second amount of the RF signal, where the second amount is greater than the first amount. The sampling circuit may be a capacitor that is coupled to the third RF port. In some embodiments, the sampling circuit may further include a variable capacitor.

In some embodiments, the protection circuit may include an RF power detector and a control signal generator circuit that is responsive to the RF power detector, where an output of the control signal generator circuit is coupled to the first control port of the transmit/receive selection device.

The control signal generator may include a transistor that switches between an on-state and an off-state when an output of the RF power detector reaches a threshold level.

In some embodiments, the transmit/receive switch may comprise a first transistor that is coupled between a first reference voltage and a first RF transmission path that extends between the first RF port and the third RF port, a second transistor that is coupled between a second reference voltage and a second RF transmission path that extends between the second RF port and the third RF port, and a third transistor that is coupled between a third reference voltage and a first node connecting the first RF transmission path to the second RF transmission path.

In some embodiments, the protection circuit may be configured to increase reflection of RF signals received at the third RF port in response to detecting that a magnitude of the RF signal exceeds a predetermined threshold.

In some embodiments, the protection circuit may be configured to reconfigure the transmit/receive selection device in response to detecting that a level of an RF signal at the third RF port exceeds a first threshold during transmit operations and to reconfigure the transmit/receive selection device in response to detecting that the level of the RF signal exceeds a second threshold during receive operations, the second threshold being less than the first threshold.

Pursuant to further embodiments of the present invention, circuits are provided that include a substrate having a switch formed thereon that includes first through third RF ports, a first RF transmission path coupled between the third RF port and the first RF port, a second RF transmission path coupled between the third RF port and the second RF port, a first transistor that is coupled between the first RF transmission path and a first reference voltage, a second transistor that is coupled between the second RF transmission path and a second reference voltage, a third transistor that is coupled between a first node that connects the first and second RF transmission paths and a third reference voltage, and a protection circuit that is configured to turn on the third transistor in response to a level of an RF signal at the third port exceeding a preset threshold.

In some embodiments, the switch comprises a single-pole double-throw switch, the circuit further includes a high power amplifier coupled to the first RF port and a low noise amplifier coupled to the second RF port.

In some embodiments, the circuit further includes a sampling circuit that is configured to couple a sample of an RF signal received at the third RF port and to provide the sample of the RF signal to the protection circuit. The sampling circuit may comprise a capacitor voltage divider network that is coupled to the third RF port. The capacitor voltage divider network may include a variable capacitor in some embodiments.

In some embodiments, the protection circuit may include an RF power detector and a control signal generator circuit responsive to the RF power detector, where an output of the control signal generator circuit is coupled to a gate of the third transistor. In some embodiments, the control signal generator circuit may include a transistor that switches between an on-state and an off-state when an output of the RF power detector reaches a threshold level.

In some embodiments, the protection circuit may be configured to increase reflection of RF signals received at the third RF port in response to detecting that the level of the RF signal exceeds a preset threshold.

In some embodiments, a dissipative load may be provided between the third transistor and the third reference voltage.

Pursuant to additional embodiments of the present invention, transmit/receive switches are provided that include a first RF port, a second RF port, a third RF port that may be selectively connected to one of the first RF port and the second RF port, a first RF transmission path coupled between the third RF port and the first RF port, a second RF transmission path coupled between the third RF port and the second RF port, a first transistor that is coupled between the first RF transmission path and a first reference voltage, a second transistor that is coupled between the second RF transmission path and a second reference voltage, and a third transistor that is coupled between a first node that connects the first and second RF transmission paths and a third reference voltage.

In some embodiments, the transmit/receive switch further includes a protection circuit that is configured to turn on the third transistor in response to a level of an RF signal at the third port exceeding a preset threshold.

In some embodiments, the transmit/receive switch may be configured so that the third transistor turns on in response to a detection that the level of an RF signal at the third RF port exceeds a preset threshold.

Pursuant to still further embodiments of the present invention, methods of protecting an amplifier that is coupled to an antenna through a transmit/receive selection device from an overdrive condition are provided. Pursuant to these methods, a determination may be made that a level of an RF signal at an RF port of the transmit/receive selection device exceeds a preset threshold. The transmit/receive selection device may be configured to reflect RF energy received at an input thereof in response to determining that the level of the RF signal at the RF port exceeds the preset threshold.

In some embodiments, the method may also include taking a sample of the RF signal and passing the sample to an RF detector. A capacitor divider network may be used to extract the sample of the RF signal. In some embodiments, the capacitor divider network may include a variable capacitor. In some embodiments, the transmit/receive selection device may be a transmit/receive switch. In some embodiments, the preset threshold may be a first preset threshold when the transmit/receive switch operates in a transmit mode, and the preset threshold may be a second preset threshold when the transmit/receive switch operates in a receive mode, where the second preset threshold is different from the first preset threshold.

DETAILED DESCRIPTION

FIG. 1is a simplified block diagram of a time division duplexed RF communications system10that includes a low noise amplifier. In a time division duplexed RF communications system, the system may transmit RF signals during some time periods (called time “slots”) and receive RF signals during other time slots.

As shown inFIG. 1, the RF communications system10includes an RF transceiver20that has a transmit port22and a receive port24. The transmit port22of the RF transceiver20is coupled to a high power amplifier (HPA)30, which may be used to significantly increase the power level of an RF signal output by RF transceiver20at port22. Similarly, a low noise amplifier (LNA)40is coupled to the receive port24of the RF transceiver20. A transmit/receive switch50is provided that selectively connects the high power amplifier30and the low noise amplifier40to an antenna60.

The time division duplexed RF communications system10may operate as follows. During transmit mode time slots, the RF transceiver20may receive, for example, baseband data from baseband equipment (not shown) and may generate an RF signal that includes the data embedded therein. This RF signal is output from port22of RF transceiver20to the high power amplifier30, which increases the magnitude of the RF signal. The amplified RF signal output by the high power amplifier30passes through the transmit/receive switch50to the antenna60that radiates the amplified RF signal into free space. The RF transceiver20and the transmit/receive switch50may be controlled so that the transmit/receive switch50connects the high power amplifier30to the antenna60during the transmit time slots of the time division duplex access scheme and connects the low noise amplifier40to the antenna60during the receive time slots.

During receive mode time slots, RF signals are received by the antenna60and passed by the transmit/receive switch50to the low noise amplifier40. The low noise amplifier40is designed to amplify RF signals having relatively low power levels, as the RF signals are received over an air interface and hence subject to significant free space attenuation. Consequently, the low noise amplifier40can be subject to damage from any higher power RF signals that pass through the antenna60thereto. For example, RF signals that are transmitted by other nearby transceivers may be received at the antenna60and passed through the transmit/receive switch50to the low noise amplifier40. Due to the relatively low free space loss for close-by transmitters and/or the additive effect of multiple transmitters, the received RF signal may be large enough to damage the low noise amplifier40. As another example, if the antenna60is pointed towards a conductive surface, the RF signals transmitted by the RF transceiver20through the antenna60may be reflected back into the antenna60where they may again pass to the low noise amplifier40. If the power level of these RF signals is sufficiently large, the low noise amplifier40may be damaged or even destroyed.

Hard limiter circuits may be used to protect RF electronic devices such as the low noise amplifier40ofFIG. 1from potentially damaging RF signal levels. Hard limiter circuits may be implemented, for example, by providing one or more protection diodes that are coupled between the RF signal path and ground. These hard limiter circuits may limit the current delivered to the low noise amplifier (or other RF electronic device) to prevent damage thereto. The hard limiter circuit may be implemented on the same chip as the RF electronic device that it protects, and an additional external (off-chip) protection circuit may be provided to supplement the on-chip hard limiter circuit. The external protection circuit may be placed, for example, between an antenna and the chip that includes the RF component to be protected so that the external protection circuit will reduce the magnitude of an RF signal received at the antenna and the on-chip protection circuit may further reduce the magnitude of the received RF signal.

FIG. 2is a schematic block diagram of a conventional RF communications system10′ that includes a transmit/receive circuit for a time division duplex system that has a built in hard limiter circuit that provides overdrive protection for a low noise amplifier. As shown inFIG. 2, the RF communications system10′ may be identical to the RF communications system10, except that the RF communications system10′ further includes a hard limiter circuit70that is interposed between the transmit/receive switch50and the low noise amplifier40. The hard limiter circuit70is configured to attenuate any RF signals received at an input72thereto, thereby protecting the low noise amplifier40from unintended high power RF signals that may be received at the antenna60.

Unfortunately, the above-described conventional hard limiter circuit70may negatively impact the performance of the RF communications system10by decreasing output power, gain and/or system sensitivity, and/or by increasing the noise figure. For example, the above-described hard limiter circuit70may include protection diodes that may add loss and parasitic capacitance to the RF circuit path, which increases the noise figure of the low noise amplifier40and degrades system performance.

Pursuant to some embodiments of the present invention, monolithic microwave integrated circuits are provided that include at least one transistor amplifier and a protection circuit that may protect the amplifier from unpredictable and unintended high power RF signals. In some embodiments, the monolithic microwave integrated circuit may comprise a transmit/receive circuit for a time division duplex RF communications system. The transmit/receive circuit may include a high power amplifier, a low noise amplifier and a transmit/receive selection device such as a switch or a circulator that is used to selectively connect one of the high power amplifier and the low noise amplifier to an antenna. The transmit/receive selection device may be configured to protect upstream components of the RF communications system such as the high power amplifier and/or the low noise amplifier from high power RF signals that are unintentionally received by the antenna. Since the transmit/receive selection device is already a necessary part of many time division duplex RF communications systems, adding the overdrive protection function to the transmit/receive selection device may not add significant loss or otherwise degrade system performance.

In some embodiments, the monolithic microwave integrated circuit (“MMIC”) may be configured to sample an RF signal that is input thereto (e.g., from an antenna) in order to detect a power level of the RF signal (where the “detection” may merely be determining whether or not the power level of the RF signal is above or below a predefined threshold). If the power level of the received RF signal exceeds the predefined threshold, the configuration of the transmit/receive selection device may be modified so that much of the RF energy input to the MMIC device is reflected back out to the antenna or is otherwise routed away from the transmit and receive paths that connect to an external RF transceiver. In one example implementation, the MMIC includes a single-pole double-throw transmit/receive switch. A node of this switch that connects a common RF port of the switch to the other two RF ports may be selectively connected to ground when a power level of the received RF signal exceeds the predefined threshold level. When the switch is short-circuited in this fashion, the common port will reflect a large portion of the RF energy that is input thereto, thereby protecting upstream components. Since the RF communications system will typically be designed to pass higher power RF signals along the transmit path than the receive path, the RF communications system may be configured to apply different thresholds for triggering the protection function depending upon whether the system is operating in transmit or receive mode. While the power level of the RF signal may be detected in some embodiments, it will be appreciated that other levels of the RF signal (e.g., a voltage level, a current level) may be detected instead of, or in addition to, a power level.

Since the protection circuits according to embodiments of the present invention do not require the use of high power handling, low loss diodes like conventional hard limiter circuits, the protection circuit can be implemented on the same circuit substrate as the amplifier(s) and the transmit/receive switch. In some embodiments, the amplifiers, the transmit/receive switch and the protection circuits may be implemented using gallium nitride based transistors. It can be difficult to implement high power handling, low loss diodes on the same circuit substrate on which high power gallium nitride based transistor amplifiers are implemented. Pursuant to embodiments of the present invention, all of these components may be implemented on a single monolithic microwave integrated circuit, thereby reducing the size and cost of the RF communications system.

In some embodiments, the transmit/receive switch may be implemented as an RF power detector in combination with a single-pole double-throw switch that has first through third RF ports and further includes an extra “protection” transistor that is coupled between the third RF port of the transmit/receive switch and a reference voltage such as ground. During normal operation, the protection transistor may be turned off (non-conducting) and the single-pole double-throw switch may connect the third RF port to one or the other of the first RF port (which connects to a high power amplifier) and the second RF port (which connects to a low noise amplifier) based on the values of one or more control signals that are provided to the transmit/receive switch. If the RF power detector detects that the power level of the RF signal that is present at the common port of the switch is above a predefined threshold, the protection transistor may be turned on, and the short circuit to ground acts to reflect much of the RF energy that is input to the transmit/receive switch at the common port, thereby reducing the amount of RF energy that is passed to either the low-noise amplifier and/or the high power amplifier.

In some embodiments, monolithic microwave integrated circuits are provided that include a substrate, a transmit/receive selection device that is formed on the substrate, a high power amplifier formed on the substrate and coupled to a first RF port of the transmit/receive selection device, a low noise amplifier formed on the substrate and coupled to a second RF port of the transmit/receive selection device and an overdrive protection circuit that is coupled to a first control port of the transmit/receive selection device.

In other embodiments, circuits are provided that include a substrate having a switch formed thereon. The switch includes first through third RF ports, a first RF transmission path coupled between the third RF port and the first RF port, a second RF transmission path coupled between the third RF port and the second RF port, a first transistor that is coupled between the first RF transmission path and a first reference voltage, a second transistor that is coupled between the second RF transmission path and a second reference voltage, a third transistor that is coupled between a first node that connects the first and second RF transmission paths and a third reference voltage, and an overdrive protection circuit that is configured to turn on the third transistor in response to a power level of an RF signal at the third port exceeding a preset threshold.

In still other embodiments, transmit/receive switches are provided that include a first RF port, a second RF port, a third RF port that may be selectively connected to one of the first RF port and the second RF port, a first RF transmission path coupled between the third RF port and the first RF port, a second RF transmission path coupled between the third RF port and the second RF port, a first transistor that is coupled between the first RF transmission path and a first reference voltage, a second transistor that is coupled between the second RF transmission path and a second reference voltage, and a third transistor that is coupled between a first node that connects the first and second RF transmission paths and a third reference voltage.

The RF devices according to embodiments of the present invention may help mitigate all types of overload conditions including both high current levels and overvoltage conditions. Example embodiments of the switches, transmit/receive circuits and monolithic microwave integrated circuits according to embodiments of the present invention will now be described in greater detail with referenceFIGS. 3-11.

FIG. 3is a schematic block diagram of a time division duplex RF communications system100according to embodiments of the present invention. As shown inFIG. 3, the RF communications system100includes an RF transceiver120that has a transmit port122and a receive port124. The RF transceiver120may be connected to baseband equipment (not shown). The RF transceiver120may output RF signals that are to be transmitted through the transmit port122. The transmit port122of the RF transceiver120is coupled to a high power amplifier (HPA)130, which may be used to significantly increase the power level of the RF signal output by transceiver120. Similarly, a low noise amplifier (LNA)140is coupled to the receive port124of the transceiver120. A transmit/receive switch150according to embodiments of the present invention is provided that includes first, second and third RF ports152,154,156. The high power amplifier130is coupled to the first RF port152, the low noise amplifier140is coupled to the second RF port154, and an antenna160is coupled to the third (common) RF port156. It will be appreciated that the high power amplifier130, the low noise amplifier140and the antenna160may be directly connected to the respective RF ports152,154,156or may be coupled to the RF ports152,154,156through intervening elements.

Additionally, the RF communications system100includes a protection circuit170. The protection circuit170includes a sampling circuit172, an RF detector180and a control signal generation circuit190. The RF detector180may be connected to the RF transmission path158from the antenna160to the third RF port156of the transmit/receive switch150via the sampling circuit172. The sampling circuit172may sample a small portion of the RF signal that is present on the RF transmission path158and pass the sampled RF signal to the RF detector180. The RF detector180generates an output signal based on a level of the sampled RF signal, and passes this output signal to a control signal generation circuit190. The control signal generation circuit190generates a control signal in response to the output signal from the RF detector180and uses this control signal to control operation of the transmit/receive switch150. In particular, the control signal may disable the transmit/receive switch150when the sampled RF signal exceeds a predetermined level in order to block the flow of RF signals to the high power amplifier130and the low noise amplifier140. Notably, the hard limiter used in conventional protection circuits may be omitted in the protection circuit170according to embodiments of the present invention.

FIG. 4is a schematic circuit diagram illustrating one example implementation of the RF communications system100ofFIG. 3. As shown inFIG. 4, the transmit/receive switch150may be implemented using various transistors, transmission lines, resistors and inductors. In the depicted embodiment, the transmit/receive switch150includes first, second and third transistors Q1, Q2, Q3, first and second transmission line segments T1, T2, first, second and third resistors R1, R2, R3and an inductor L1.

The inductor L1is coupled between the third RF port156and a first node N1. The inductor L1is a conventional part of a single-pole double-throw switch and may be implemented, for example, as a spiral conductive trace on a substrate. The first transmission line segment T1connects node N1to the first RF port152and the second transmission line segment T2connects node N1to the second RF port154. The first transistor Q1has a first source/drain region that is connected to the first transmission line T1and a second source/drain region that is connected to a first reference voltage such as ground. The gate of the first transistor Q1is connected to a first control input Control1through the resistor R1. The second transistor Q2has a first source/drain region that is connected to the second transmission line T2and a second source/drain region that is connected to a second reference voltage such as ground. The gate of the second transistor Q2is connected to a second control input Control2through the resistor R2. The third transistor Q3is connected between node N1and a third reference voltage such as ground. The gate of the third transistor Q3is connected to an output of the protection circuit170through the resistor R3. The first transistor Q1and the second transistor Q2may each be about a quarter wavelength (where the wavelength is the wavelength corresponding to the operating bandwidth of the device) from node N1.

The sampling circuit172includes a capacitor C1and a variable capacitor C2. The capacitor C1couples a small portion of any RF signal that is present on the transmission path158and feeds the sampled portion to the RF detector180via a transmission path174. The variable capacitor C2can be coupled between the transmission path174and a reference voltage such as ground. The variable capacitor C2may have, for example, two capacitance settings, one of which may be used when the transmit/receive switch150is set for transmission of RF signals and the other of which is used when the transmit/receive switch150is set for reception of RF signals. This allows the protection circuit170to have multiple trigger points so that the protection circuit170may be activated at a first RF power level during transmit operations and at a second, different RF power level during receive operations.

The RF detector180includes first and second diodes D1, D2and a capacitor C3. The control signal generation circuit190includes a pair of transistors Q4, Q5that are coupled in series between a voltage source VSS (which may output a negative voltage such as, for example, −20 volts DC) and a reference voltage (e.g., ground) and a diode D3. In the depicted embodiment, the control signal generation circuit190is implemented as an inverting DC amplifier. It will be appreciated, however, that a wide variety of different control signal generation circuits190could be used, and that in some embodiments, the control signal generation circuit190could be omitted entirely.

The circuit ofFIG. 4may operate as follows. When the RF communications system100is operating in receive mode, control signals are applied to control ports Control2and Control3that turn off transistor Q2and transistor Q3, respectively. A control signal is applied to control port Control1that turns on transistor Q1. With these control signals, the transmit/receive switch150will route RF signals received at the third RF port156through to the second RF port154to the low noise amplifier140, since the short circuit to ground through transistor Q1will prevent RF energy from passing to the first RF port152. Assuming that the antenna160is not receiving an unintended signal, then the power level of the received RF signal that is passed to the third RF port156is relatively small. Consequently, the sample of the RF signal that is coupled onto transmission path174by the sampling circuit172may have a low power level, and hence the voltage at node N2(i.e., the voltage at the gate of transistor Q4) is slightly higher than VSS and transistor Q4is forward biased (conducting). As such, the source of transistor Q5is at about VSS (specifically, VSS minus the drain-source voltage of transistor Q4, which is small due to transistor Q4being in its low resistance or “on” state). The source of transistor Q5is coupled to the gate of transistor Q3through resistor R3. When the source of transistor Q5is at about VSS (which is a negative voltage), transistor Q3will be turned off, and the transmit/receive switch150will operate like a conventional single-pole double-throw switch.

If an unintended RF signal having a high power level is received through antenna160, the sample of this RF signal that is passed by the sampling circuit172to the RF detector180will have a larger power level. The RF detector180is implemented as a diode peak detector circuit that includes the small diodes D1, D2and a capacitor C3that convert the sample of the RF signal into a direct current (“DC”) voltage. In the particular implementation of the RF detector180illustrated inFIG. 4, the circuit is configured to route DC current from voltage supply VSS away from the gate of transistor Q4and so that output of the peak detector180will become more negative as the magnitude of the sample of the RF signal increases. As the voltage level at the gate of transistor Q4drops, there is little change in the voltage applied to control port Control3so long as transistor Q4remains forward biased. Once the voltage at the gate of transistor Q4becomes sufficiently negative, transistor Q4turns off, and the source of transistor Q5goes to ground (0 volts). When this occurs, transistor Q3turns on, connecting node N1to ground. This in turn shorts out the transmit/receive switch150, and hence most of the RF energy that passes from the antenna160toward the transmit/receive switch150is reflected by the third RF port156, thereby reducing the amount of RF energy that flows to the low noise amplifier140(as well as to the high power amplifier130).

When the RF communications system100is operating in transmit mode, the system will operate in the same manner discussed above, except that under normal operations the transmit/receive switch will connect the first RF port152to the third RF port156so that the RF signals output from the high power amplifier130are passed to the antenna160. In addition, the variable capacitor C2may be set to a different level to adjust the threshold at which the overdrive protection circuit170turns on the “protection transistor” Q3in order to short circuit the transmit/receive switch150. This allows the much larger RF signal that is output from the high power amplifier130to pass through the transmit/receive switch150without turning on the protection transistor Q3. A control circuit (not shown), which may be as simple as a single transistor, may be used to adjust the setting of the variable capacitor C2as the transmit/receive switch150toggles between transmit and receive modes.

In some embodiments, the control circuit may be configured so that Control1and Control2are set to zero volts when the protection circuit170turns on the “protection transistor” Q3in order to short circuit the transmit/receive switch150. This may, in some cases, further improve the isolation between RF ports152and154with respect to RF port156.

As shown inFIG. 4, the sampling circuit172connects to the transmission path158at a node N3. According to some embodiments, a minimum electrical distance may be provided between node N3and node N4of at least 1/12 of a wavelength of the RF signal (where the wavelength of the RF signal is based on the center frequency of the RF signal). The provision of the spiral inductor L1may help ensure that this minimum distance is met. Ensuring that a minimum electrical distance is provided between nodes N3and N4may help ensure that the impedance at the sampling point is not reduced when transistor Q3changes state, which would lower the sampled voltage and possibly cause the circuit to not properly operate.

While the above description dikusses the protection circuit170being activated at various RF power levels (e.g., when a first RF power level is exceeded during transmit operation and a second, lower, RF power level during receive operations), it will be appreciated that the protection circuit may detect a parameter other than power. For example, in the embodiment ofFIG. 4, the protection circuit is triggered based on a voltage at the sampling point, but the voltage at the sampling point corresponds to a power level since the transmission line impedance is fixed. Thus, herein circuit180is referred to as an “RF detector” as it may detect any appropriate characteristic of the sampled RF signal such as a voltage level, a power level, etc. that is used to trigger the protection circuit to change states.

FIG. 5is a graph illustrating the simulated output power of the transmit/receive switch150included in the RF communications system ofFIG. 4as a function of input power. In particular, curve200illustrates the RF power level at the first RF port152(which connects to high power amplifier130) as a function of the RF power level at the third RF port156when the system is operating in transmit mode. As shown by curve200, the power level of the RF signal at the first RF port152generally tracks the power of the RF signal at the third RF port156(the power is about 1 dB less due to losses within the transmit/receive switch150) until the RF power at the third RF port reaches about 45.5 dBm. At this power level at the third RF port156, the power at the first RF port152drops rapidly by nearly 10 dB, as the protection transistor Q3turns on and much of the RF energy (nearly 90% in this example) is reflected back towards the antenna160.

Curve210illustrates the RF power level at the second RF port154(which connects to the low noise amplifier140) as a function of the RF power level at the third RF port156when the system is operating in transmit mode. As shown, curve210has the exact same shape as curve200, but the power level is reduced by about 18 dB due to the isolation in the transmit/receive switch150. As with curve200, for power levels above 45.5 dBm, nearly 10 dB of additional isolation is achieved since the protection transistor Q3turns on at this threshold and much of the RF energy (nearly 90% in this example) is reflected back towards the antenna160. Curves220and230show the characteristics when the transmit/receive switch150is operating in receive mode. It can be seen that the exact same performance is achieved in receive mode. InFIG. 5, the points labelled250represent the threshold level where the protection circuit170starts operating when the system is operating in receive mode, while the points labelled260represent the threshold level where the protection circuit170starts operating when the system is operating in transmit mode. It should be noted that the points250are at a power level (at port156) of 37.5 dBm. This trigger point is set to protect the more sensitive low noise amplifier140from damage. Thus,FIG. 5illustrates that the protection circuit170can provide almost 10 dB reduction in the power level of a large RF signal that is input at port156of the transmit/receive switch, thereby protecting both the low noise amplifier140and the high power amplifier130from damage.

Referring again toFIG. 4, the transistor Q3replaces a matching capacitor that would be included in the transmit/receive switch150if transistor Q3had not been added, and hence the addition of transistor Q3has very little impact on the performance of the transmit/receive switch150. This is in contrast to the hard limiter protection circuit included in the RF communications10′ ofFIG. 2, which introduces losses and other performance degradations such as, for example, decreased output power, lower gain, reduced efficiency, increased system sensitivity and/or an increased noise figure. Additionally, the transmit receive circuits according to embodiments of the present invention provide protection to both the low noise amplifier140and the high power amplifier130, both of which are susceptible to damage. For instance, the high power amplifier130may be subject to damage if a conductive surface (e.g., a stabilizer on an airplane) is moved in front of the RF communications system, as such an object can reflect much of the transmitted RF energy back through the antenna160toward the transmit/receive switch150. Moreover, the protection circuit170may be readily implemented on the same circuit substrate as one or more of the transmit/receive switch150, the high power amplifier130and the low noise amplifier140. Thus, in some embodiments of the present invention, a monolithic microwave integrated transmit/receive circuit may be provided that includes all four of the protection circuit170, the transmit/receive switch150, the high power amplifier130and the low noise amplifier140implemented in a single integrated circuit chip.

FIG. 6is a graph of the measured output power at RF port154of the transmit/receive switch ofFIG. 4as a function of both the frequency and the power level of the RF signal at RF port156when the transmit/receive switch150is operating in receive mode. As shown inFIG. 6, at an input power level of 36 dBm, the transmit/receive switch150operates similar to a conventional transmit/receive switch, and the RF power level at RF port154is about 34.5 dBm, representing a loss of about 1.5 dB in the transmit/receive switch150. At input RF power levels of 38 dBm and 39 dBm, the protection circuit170of transmit/receive switch150operates normally and the RF power level at RF port154is reduced by about 9 dBm from the power level at RF port156in each case, although the response is not constant with frequency. The variation as a function of frequency may be caused by measurement limitations rather than a circuit performance issue. When the switch150enters protection mode, the reflected energy returns to the test system power amplifier which may cause the output power of the amplifier to change (due to this non-optimal load condition on the PA), and also reduces measurement accuracy of the incident power. At an input RF power level of 37 dBm at RF port156, the protection circuit170starts to operate and transient performance is seen as protection is provided at some frequencies, but not at others.

FIG. 7is a schematic block diagram of an RF communications system300according to further embodiments of the present invention. As shown inFIG. 7, the RF communications system300is very similar to the RF communications system100discussed above, except that the transmit/receive switch150of RF communications system100is replaced in RF communications system300with a circulator350. As known to those of skill in the art, a circulator is an RF circuit that typically has three or four ports that is designed so that RF energy entering the device at one port is transmitted only to the next port in a direction of rotation of the circulator. The circulator350that is included in the RF communications system300may be a non-conventional circulator that is designed to reflect RF energy received at one of the RF ports thereof in response to an RF power level present at the port exceeding a predetermined threshold.

As shown inFIG. 7, the circulator350includes four RF ports352,354,356and358. RF energy that is input to RF port352is output through RF port356. RF energy that is input to RF port356is output through RF port354. RF energy that is input to RF port354is output through RF port358. In normal operations, no RF energy will be input to RF port354, but in the event that this occurs, the RF circulator350will pass such RF energy to port358where it will be terminated in a dissipative load351.

The RF communications system300may operate essentially identically to the RF communications system100discussed above, as the circulator350will functionally perform equivalent operations to the transmit/receive switch150of RF communications system100. The circulator350may include a reflection circuit (not shown) that is configured to substantially prevent RF energy input at RF port356from flowing out through RF port354toward the low noise amplifier140. As a result, most of the RF energy input at RF port356will not flow through RF port354, but instead will continue passing through the circulator350and exit the circulator350at RF port358, where it is terminated in a dissipative load351. In an alternative embodiment, a three port circulator may be used that does not include RF port358(or the dissipative load351). In this embodiment, a reflection circuit (not shown) may be provided at RF port356that, when activated, is configured to substantially prevent RF energy input at antenna160from passing into the circulator350. This reflection circuit may be implemented, for example, as a circuit that debiases the magnetic ferrite of the circulator350to make the circulator350lossy in the bandwidth of the offending (undesired) RF signal. In each of the above example embodiments, a control port that is coupled to the protection circuit170may control the reflection circuit to activate the reflection circuit when the protection circuit170determines that the RF power level at RF port356exceeds a predefined threshold.

FIG. 8is a schematic block diagram of an RF communications system400according to further embodiments of the present invention that includes a different transmit/receive switch implementation that includes a single-pole double-throw transmit/receive switch450. The single-pole double-throw switch450illustrated inFIG. 8is similar to the single-pole double-throw transmit/receive switch shown in FIG. 1 of U.S. Pat. No. 8,421,122, the entire content of which is incorporated herein by reference. However, as compared to the switch disclosed in U.S. Pat. No. 8,421,122, the single-pole double-throw switch450inFIG. 8includes an additional transistor Q14which is coupled between node N1and a reference voltage (e.g., ground).

Referring toFIG. 8, the transmit/receive switch450includes a first control input Control1, a complementary control input Control2, and first through third RF ports452,454,456. The transmit/receive switch450further includes five gallium nitride based HEMT transistors Q10to Q14. The second control input Control2is connected through resistors R2and R3to gates of transistors Q11and Q12. The first control input Control1is connected through resistors R1and R4to gates of transistors Q10and Q13. The resistors R1to R4at the gates of transistors Q10to Q13may have relatively large resistances in some embodiments, such as, for example, resistances of about 10,000 ohms RF port452is connected through an inductance L2to the sixth node N6. RF port454is connected through an inductance L3to the fifth node N5. RF port456is connected through inductance L1to the first node N1. The inductors L1to L3may include spiral inductor networks that are designed to match the switch on the monolithic die to 50 ohms impedance.

The RF communications system further includes the above-described protection circuit170. As shown inFIG. 8, the output of the protection circuit is coupled to the gate of transistor Q14. The protection circuit may be configured to turn transistor Q14on in response to detecting that the RF power level at port456exceeds a predefined threshold. As the RF communications system400may operate in essentially the same manner as the RF communications system100with the exception that a different single-pole double-throw switch design is used, hence further description of the RF communications system400ofFIG. 8will be omitted.

It should be noted that in the embodiment ofFIG. 8the added capacitance of the protection transistor Q14may force reduction in sizes of the other transistors Q10-Q13, which may increase loss, and decrease power handling and isolation. Accordingly, as shown inFIG. 12, in yet another embodiment, the protection transistor Q14may be omitted and the control circuit configured to re-bias transistors Q10and Q11with a large negative gate voltage while biasing transistors Q12and Q13to zero volts. In doing so the protection will be added without significantly impacting the RF loss, isolation, or power handling.

FIG. 9is a schematic block diagram of an RF communications system500according to further embodiments of the present invention. The RF communications system500is very similar to the RF communications system100discussed above, except that the transmit/receive switch150of RF communications system100is replaced in RF communications system500with a transmit/receive switch550that is designed to couple excessive RF energy that is present at RF port156to a load551as opposed to using a short circuit to reflect the RF energy back towards the antenna160. While not shown inFIG. 9, control circuits are coupled between Control3and Control1and Control2that switch Control1and Control2to zero volts when the protection circuit170is activated. Given the similarities between RF communications system500and RF communications system100, further description of RF communications system500will be omitted.

As discussed above, the transmit/receive circuits according to embodiments of the present invention may be implemented as monolithic microwave integrated circuits that may include, for example, a high power amplifier, a low noise amplifier, a transmit/receive switch and a protection circuit that are all implemented on a single substrate as a monolithic integrated circuit. Such an implementation may have advantages in terms of size, cost and performance.

FIG. 10is a schematic layout view of a monolithic microwave integrated circuit implementation of a transmit/receive circuit600according to embodiments of the present invention. The monolithic microwave integrated circuit shown inFIG. 10may comprise, for example, an implementation of the transmit/receive circuit portion of the RF communications system100ofFIGS. 3-4.

As shown inFIG. 10, the transmit/receive circuit600includes a plurality of circuit elements that are formed on a monolithic substrate610. The substrate610may comprise, for example, a silicon carbide or sapphire substrate that has gallium nitride based epitaxial layers formed thereon, or other suitable epitaxial layers such as, for example, silicon carbide or gallium arsenide based epitaxial layers. Metallization patterns and dielectric layers may also be formed on the substrate610to implement the various circuit elements.

The circuit elements may include a high power amplifier630, a low noise amplifier640and a transmit/receive switch650and a protection circuit670. The high power amplifier630and the low noise amplifier640may each comprise a gallium nitride based transistor amplifier implemented using a plurality of HEMT transistors. In the depicted embodiment, the high power amplifier630is illustrated as being a multi-stage amplifier that has a first plurality of unit cell transistors that form a driver amplification stage632and a second plurality of unit cell transistors634that form an output amplification stage. Each amplifier may also include one or more of an input impedance matching circuit, an output impedance matching circuit and/or inter-stage impedance matching circuit(s).

The transmit/receive switch650may comprise any of the transmit/receive switches according to embodiments of the present invention. For example, the transmit/receive switch650may comprise the single-pole double-throw switch150ofFIG. 4. The protection circuit670may comprise any circuit that may control the transmit/receive switch650to redirect RF energy present at a common port thereof when a power level of the RF energy exceeds one or more predefined thresholds. For example, the protection circuit670may comprise the protection circuit170shown inFIG. 4. The transmit/receive switch650further includes input/output pads612and additional power and ground pads614.

FIG. 11is a flow chart illustrating a method of protecting an amplifier that is coupled to an antenna through a transmit/receive selection device from an overdrive condition according to certain embodiments of the present invention. As shown inFIG. 11, operations may begin with an RF signal that is at an input port of the transmit/receive selection device being sampled (block700). In some embodiments, the transmit/receive selection device may be a single-pole double-throw switch. The sample of the RF signal may be taken using, for example, a capacitor divider network. The sample of the RF signal may be passed to an RF detector (block710). Next, a determination may be made as to whether or not a power level of an RF signal that is present at an RF port of the transmit/receive selection device exceeds a preset threshold (block720). In response to this determination, the transmit/receive selection device may be configured to reflect RF energy received at the input port thereof (block730).

It will be appreciated that the above embodiments are exemplary in nature and are not intended to be limiting to the scope of the present invention. It will also be appreciated that numerous modifications may be made to the above-described embodiments without departing from the scope of the present invention. As one simple example, the antenna160illustrated in the above embodiments is schematically shown as comprising a linear array of radiating elements such as are used in phased array antennas. it will be appreciated that the antenna160may comprise any appropriate antenna, including dipole, patch, reflector, horn, loop and numerous other antennas, and may include a single radiating element or surface or multiple radiating elements/surfaces, which may or may not be phase controlled. As another example, while the above described embodiments are implemented as monolithic microwave integrated circuits, it will be appreciated that the various elements of the communications system may be implemented as separate integrated circuits or other elements.

As another example, it will be appreciated that the present invention can be implemented in a wide variety of different switches, including switches other than transmit/receive switches and including switches other than single-pole double-throw switches. For example, in other embodiments, the systems could include a single-pole single-throw switch, a triple pole switch, etc. Thus, it will be appreciated that the scope of the present invention is defined by the attached claims rather than by the example embodiments that are described in detail herein. It will also be appreciated that the above-described embodiments may be combined in any fashion. For example, the technique discussed with reference toFIG. 9where the excess RF energy is passed to a load could be used in any of the other embodiments. It will likewise be appreciated that in some embodiments, the transmit/receive circuits may have different numbers of control inputs than shown in the above example embodiments.

The transmit/receive switches according to embodiments of the present invention may exhibit low signal loss, low distortion, high power handling, low DC power dissipation, and good impedance match characteristics. The transmit/receive switches disclosed herein may also provide high levels of isolation between a non-selected port (i.e., the RF port connected to the high power amplifier when the switch operates in receive mode, and the RF port connected to the low noise amplifier when the switch operates in transmit mode) over a wide frequency band.

Embodiments of the inventive concepts may be particularly well suited for use in connection with electronic circuits that are formed in Group III-nitride based semiconductor materials. As used herein, the term “Group III nitride” refers to those semiconducting compounds formed between nitrogen and the elements in Group III of the periodic table, usually aluminum (Al), gallium (Ga), and/or indium (In). The term also refers to ternary and quaternary compounds such as AlGaN and AlInGaN. These compounds all have empirical formulas in which one mole of nitrogen is combined with a total of one mole of the Group III elements.

For example, many high power transistor amplifiers are formed using GaN-based high electron mobility transistor (HEMT) devices. Suitable structures for GaN-based HEMTs that may be used to implement the amplifiers included in embodiments of the present invention are described, for example, in commonly assigned U.S. Publication No. 2017/0271497, published Sep. 21, 2017, the disclosure of which is incorporated herein by reference in its entirety

The transistors included in the devices according to embodiments of the inventive concepts may include a semiconductor structure that is a multiple layer structure. For example, each transistor may be formed on a substrate which may be, for example, a silicon carbide (e.g., 4H-SiC or 6H-silicon carbide, sapphire, aluminum nitride, aluminum gallium nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. A semiconductor structure may be formed on an upper surface of the substrate. The semiconductor structure may include at least a channel layer and a barrier layer, which may be sequentially stacked on the substrate and which may each comprise Group III nitride layers. Optional buffer, nucleation, strain balancing and/or transition layers may be provided between the substrate and the channel layer. One or more capping layers, such as silicon nitride layers, may be provided on an upper surface of the barrier layer.

The channel layer may comprise, for example, a Group III-nitride, such as AlxGa1-xN where 0≤x<1, provided that the energy of the conduction band edge of the channel layer is less than the energy of the conduction band edge of the barrier layer at the interface between the channel and barrier layers. The channel layer may be undoped or unintentionally doped and may have a bandgap that is less than the bandgap of the barrier layer. The channel layer may have a larger electron affinity than the barrier layer. In certain embodiments, the barrier layer is AlN, AlInN, AlGaN or AlInGaN.

Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.