Field of the Disclosure

[0001] The technology of the disclosure relates generally to power amplifiers and particularly to cascode power amplifiers.

Background

[0002] Mobile wireless communication and data transfer technologies continue to evolve from second generation (2G), third generation (3G), and 4G fourth generation (4G) long-term evolution (LTE) to, most recently, the fifth generation new radio (5G NR) standard. Each generation accesses different frequencies and has increased requirements for bandwidth and range. Since network infrastructures may not be immediately updated in all geographical areas as technologies emerge, a mobile device may encounter networks of different technology generations. Wireless devices must be designed to adapt to these different networks and their respective requirements. Consumers impose additional demands on the most common wireless mobile devices, including longer battery life, smaller device size, and lower cost. Meeting the conflicting demands of evolving technologies and consumers presents many challenges.

[0003] Aspects disclosed in the detailed description include cascode power amplification circuits, including voltage protection circuits. Related methods of protecting cascode power amplifiers are also disclosed. A power amplification circuit includes an amplifier circuit and a circuit for protecting transistors in the amplifier circuit from a destructive output voltage on an output node. The amplifier circuit includes a series of transistors, including a first cascode transistor coupled to the output node, a last cascode transistor coupled to a reference voltage node (e.g., ground), and one or more middle cascode transistors coupled between the first cascode transistor and the last cascode transistor. The output voltage is distributed across the series of transistors. The output voltage is determined by an input signal on an input terminal coupled to a control terminal of the last cascode transistor and a gain of the last cascode transistor. The circuit protecting the amplifier circuit may include one or both of a protection circuit and a stress control circuit. The protection circuit determines when the output voltage exceeds a threshold and provides a feedback signal to the bias circuit to reduce the bias voltage on at least the last cascode transistor. The stress control circuit includes a variable capacitor and a fixed capacitor coupled in series between the output node and the reference voltage node. The control terminal of the first cascode transistor is coupled to a node between the variable capacitor and the fixed capacitor. As the output voltage increases, the stress control circuit can increase a bias voltage on the control terminal of the first cascode transistor to “float up” the first cascode transistor to avoid a destructive voltage.

[0004] In one exemplary aspect, a power amplification circuit is disclosed. The power amplification circuit comprises an amplifier circuit comprising a plurality of cascode transistors coupled in a series between an output node and a reference voltage node, the series comprising a last cascode transistor coupled to the reference voltage node and determining a current in the output node. The power amplification circuit comprises a protection circuit configured to generate a feedback signal in response to an output voltage on the output node exceeding a threshold. The power amplification circuit comprises a bias circuit configured to reduce a bias voltage of the last cascode transistor based on the feedback signal.

[0005] In another exemplarity aspect, a power amplification circuit comprising an amplifier circuit and a stress control circuit is disclosed. The power amplification circuit comprises an amplifier circuit comprising a plurality of cascode transistors coupled in a series between an output node and a reference voltage node, the series comprising a first cascode transistor coupled to the output node. The power amplification circuit comprises a stress control circuit comprising a variable capacitor and a fixed capacitor coupled in series between the output node and the reference voltage node and configured to increase a bias voltage on the first cascode transistor in response to the output voltage exceeding a threshold. A control terminal of the first cascode transistor is coupled to a node between the variable capacitor and the fixed capacitor [0006] In another exemplary aspect, a power amplification circuit is disclosed. The power amplification circuit comprises an amplifier circuit comprising a plurality of cascode transistors coupled in a series between an output node and a reference voltage node, the series comprising a first cascode transistor coupled to the output node and a last cascode transistor coupled to the reference voltage node, the last cascode transistor determining a current in the output node. The power amplification circuit comprises a protection circuit generating a feedback signal in response to an output voltage on the output node exceeding a first threshold and a bias circuit reducing a bias voltage of the last cascode transistor based on the feedback signal. The power amplification circuit comprises a stress control circuit comprising a variable capacitor and a fixed capacitor coupled in series between the output node and the reference voltage node and increasing a bias voltage on the first cascode transistor in response to the output voltage exceeding a second threshold. A control terminal of the first cascode transistor is coupled to a node between the variable capacitor and the fixed capacitor.

[0007] Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.

Brief Description of the Drawings

[0008] The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure and, together with the description, serve to explain the principles of the disclosure.

[0009] Figure 1 is a schematic diagram of a cascode transistor amplifier;

[0010] Figure 2 is a schematic diagram of a power amplification circuit, including an amplifier circuit, a protection circuit, and a bias circuit coupled to the power amplification circuit; [0011] Figure 3 is a schematic diagram of the cascode transistor amplifier in Figure 1 and a first example of a stress control circuit;

[0012] Figure 4A is a graph of output voltages of signals on an output node of the cascode transistor amplifier of Figure 1 ;

[0013] Figure 4B is a graph of a capacitance of a variable capacitor in the stress control circuit of Figure 3 in response to the output voltages in Figure 4A; [0014] Figure 5 is a schematic diagram of the cascode transistor amplifier in Figure 1 with a second example of the stress control circuit;

[0015] Figure 6 is a schematic diagram of the power amplification circuit of Figure 2 and the stress control circuit as shown in Figure 3; and

[0016] Figure 7 is a schematic diagram of a two-stage power amplification circuit, including the power amplification circuit of claim 6 as a first stage.

Detailed Description

[0017] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0018] It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. [0019] It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element, or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

[0020] Relative terms such as “over” or “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” "includes," and/or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0022] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0023] Aspects disclosed in the detailed description include cascode power amplification circuits, including voltage protection circuits. Related methods of protecting cascode power amplifiers are also disclosed. A power amplification circuit includes an amplifier circuit and a circuit for protecting transistors in the amplifier circuit from a destructive output voltage on an output node. The amplifier circuit includes a series of transistors, including a first cascode transistor coupled to the output node, a last cascode transistor coupled to a reference voltage node (e.g., ground), and one or more middle cascode transistors coupled between the first cascode transistor and the last cascode transistor. The output voltage is distributed across the series of transistors. The output voltage is determined by an input signal on an input terminal coupled to a control terminal of the last cascode transistor and a gain of the last cascode transistor. The circuit protecting the amplifier circuit may include one or both of a protection circuit and a stress control circuit. The protection circuit determines when the output voltage exceeds a threshold and provides a feedback signal to the bias circuit to reduce the bias voltage on at least the last cascode transistor. The stress control circuit includes a variable capacitor and a fixed capacitor coupled in series between the output node and the reference voltage node. The control terminal of the first cascode transistor is coupled to a node between the variable capacitor and the fixed capacitor. As the output voltage increases, the stress control circuit can increase a bias voltage on the control terminal of the first cascode transistor to “float up” the first cascode transistor to avoid a destructive voltage.

[0024] Telecommunications networks are not immediately and simultaneously updated in all geographical areas as a new generation of telecommunication technology emerges. Therefore, mobile devices may need to include transmitters and receivers that operate in a variety of networks. In the evolution from second generation (2G) to third generation (3G), fourth generation longterm evolution (4G LTE), and fifth generation new radio (5G NR), the power demands of transmitters in mobile devices has changed. At the same time, the consumer market has continued to demand smaller and cheaper devices. Both the size and cost of power amplifiers in a wireless transmitter can be reduced by transitioning from amplifier circuits with larger and more expensive transistors, such as bipolar junction transistors (BJTs) made of gallium arsenide (GaAs), for example, lower-voltage transistors. As an example, the lower voltage transistors may be silicon transistors, such as field-effect transistors (FETs) (e.g., metal- oxide-semiconductor (MOS) FETs (MOSFETs)). FETs comprise a gate, a first source/drain, and a second source/drain. A bias voltage on the gate controls flow of current between the first source/drain and the second source/drain.

[0025] Previous amplifier circuits made with GaAs BJTs require one or a few transistors that can handle large voltages. However, such large voltages would be destructive to the lower voltage transistors. An amplifier circuit can include a plurality of voltage transistors with a lower voltage limit coupled in series between an output node and a reference voltage node to overcome this problem. The output voltage of a cascode amplifier circuit is distributed across multiple transistors, such that voltage stresses to each transistor is reduced, and damage to the individual transistors may be avoided. With this type of amplifier circuit, the output voltage levels needed in each different generation of telecommunication technology may be achieved at a lower cost and in a smaller package.

[0026] Figure 1 is an illustration of an example of a cascode amplifier circuit 100 (“amplifier circuit 100”). The amplifier circuit 100 includes an input terminal 102 on which an input radio-frequency (RF) signal RFJN is received through a capacitor C102. The input RF signal RFJN is an analog signal amplified by cascode transistors 104(1 )-104(5) based on a gain of the last cascode transistor 104(5) to generate an RF output signal RF_OUT having an output voltage VOUT on output node 106. The cascode transistors 104(1 )-104(5) are coupled in series between the output node 106 and a reference voltage node (e.g., ground voltage node) GND. Although the amplifier circuit 100 in this example includes only five of the cascode transistors 104(1 )-104(5), an amplifier circuit in a power amplification circuit as disclosed herein may include fewer or more cascode transistors coupled in series. The amplifier circuit 100 may include cascode transistors 104(1 )-104(5) that are all of a first type, such as all N-channel FETs (NFETs) or all P-channel FETs (PFETs). Alternatively, an amplifier circuit of a power amplification circuit as disclosed herein may be a complementary circuit including both NFETs and PFETs.

[0027] The amplifier circuit 100 is powered by a power supply 108, such as a battery 108 providing a supply voltage VBAT. The amplifier circuit 100 includes an inductor 1 12 coupled between the power supply 108 and the output node 106. A current hoo through the cascode transistors 104(1 )-104(5) increases and decreases in response to changes in a voltage VIN on the input terminal 102. Control terminals 1 14(1 )-1 14(5) (e.g., gates) of the cascode transistors 104(1 )- 104(5) are biased to keep the cascode transistors 104(1 )-104(5) turned on (e.g., in a saturation region) to conduct the current hoo. An inductor voltage V112 is induced across the inductor 112 in response to changes in the current I100. Thus, the output voltage VOUT on the output node 106 is equal to the supply voltage VBAT plus the inductor voltage V112 (VOUT=VBAT+VH2). In this manner, the output voltage VOUT can exceed twice the supply voltage VBAT (i.e., VOUT > 2XVBAT).

Since the output voltage VOUT is distributed across source-to-drain voltages VSDI - VSDS, in the case of the cascode transistors 104(1 )-104(5) comprising FETs, the output voltage VOUT is the total of the source-to-drain voltages VSDI -VSDS. In some cases, the output voltage VOUT may not be equally divided among the source-to-drain voltages VSDI -VSDS. Thus, at a peak in magnitude of the output voltage VOUT, a destructive voltage level may be applied across one or more of the cascode transistors 104(1 )-104(5).

[0028] Figures 2, 3, and 5-7 are illustrations of examples of power amplification circuits, including circuits that protect the cascode transistors 104(1 )-104(5) by adjusting bias voltages on at least one of the control terminals 1 14(1 )-114(5) upon detecting that the output voltage VOUT exceeds a threshold. [0029] Figure 2 is an illustration of a power amplification circuit 200, including the amplifier circuit 100 of Figure 1 and a protection circuit 202. Details of the amplifier circuit 100 are shown in Figure 1 . Figure 2 also includes a bias circuit 204 coupled to the power amplification circuit 200. The bias circuit 204 is an example of a circuit that is coupled to control terminals 1 14(1 )-1 14(5) to provide bias voltages for controlling the cascode transistors 104(1 )-104(5). The protection circuit 202 provides one or more feedback signals 206 to the bias circuit 204 in response to detecting that the output voltage VOUT exceeds a threshold. The protection circuit 202 provides the one or more feedback signals 206 to control the bias circuit 204 to adjust the bias voltages provided to the control terminals 1 14(1 )-1 14(5). In particular, the one or more feedback signals 206 reduces the bias voltage on at least the control terminal 1 14(5). In some examples, the one or more feedback signals 206 cause other control terminals 1 14(1 )-1 14(4), such as the control terminal 1 14(4), to also be adjusted. Providing feedback to adjust the bias voltage(s) on one or more of the control terminals 1 14(1 )-1 14(5) reduces the current 11 oo through the amplifier circuit 100, which reduces the possibility of destructive voltages being applied to the cascode transistors 104(1 )-104(5).

[0030] Before providing details of each of the features of the protection circuit 202, a brief description of the operation of the protection circuit 202 is provided. The protection circuit 202 includes a peak voltage circuit 208 coupled to the output node 106. The peak voltage circuit 208 generates a peak voltage VPEAK, which indicates a highest magnitude of the output voltage VOUT. The peak voltage VPEAK is provided to a voltage-to-current circuit 210 that generates an output current IOUT based on the peak voltage VPEAK. The protection circuit 202 also includes a threshold current circuit 212 that generates a threshold current ITH based on a threshold value. The output current IOUT is compared to the threshold current ITH to determine whether the output voltage VOUT is above a desired (i.e., threshold) level. Specifically, the protection circuit 202 includes a feedback circuit 214 and an acceleration circuit 216 that respond to the output current IOUT being higher than the threshold current ITH. The feedback circuit 214 generates the one or more feedback signals 206. The acceleration circuit 216 increases the responsiveness of the protection circuit 202, so the feedback circuit 214 will respond more quickly and/or to a stronger degree when, for example, the output current IOUT is significantly higher than the threshold current ITH. The features of the protection circuit 202 and their individual operation details are described further below.

[0031] The peak voltage circuit 208 includes diodes D1 -D4 coupled in series with the anode of the first diode D1 coupled to the output node 106 and the cathode of the last diode D4 coupled to the voltage-to-current circuit 210. The peak voltage circuit 208 also includes a capacitor C208 with one terminal coupled to the power supply 108. The other terminal of the capacitor C208 is coupled to the cathode of diode D2 and the anode of diode D3. The peak voltage VPEAK is a voltage on the cathode of the last diode D4. The peak voltage VPEAK provided to the voltage-to-current circuit 210 is lower than the actual output voltage VOUT due to voltage drops across the diodes D1 -D4 but is based on the output voltage VOUT. In some examples, the diodes D1 -D4 are MOS diodes (e.g., P-channel MOS diodes).

[0032] The voltage-to-current circuit 210 includes a first resistor R1 in series with a second resistor R2. The first resistor R1 may be much larger than the second resistor R2, the same size, or smaller than the second resistor R2. The voltage-to-current circuit 210 generates the output current IOUT on a comparison node 218 based on the peak voltage VPEAK. The output current IOUT may be proportional to the peak voltage VPEAK. In some examples, the output current IOUT is determined by a difference in voltage between the peak voltage VPEAK and a voltage VCOMP on the comparison node 218 and also on the total resistance of the resistors R1 and R2. In this way, the magnitude of the output current IOUT on the comparison node 218 corresponds to the magnitude of the peak voltage VPEAK and, therefore, corresponds to the output voltage VOUT. [0033] The threshold current circuit 212 conducts a threshold current ITH that corresponds in magnitude to the output current IOUT generated when the output voltage VOUT has reached a desired maximum. Beyond the desired maximum of the output voltage VOUT, destructive voltages may be applied to the cascode transistors 104(1 )-104(5). In other words, if the output current IOUT is greater than the threshold current ITH, the output voltage VOUT may be high enough to cause destructive voltages on at least one of the cascode transistors 104(1 )-104(5). In this context, a destructive voltage can cause permanent physical damage. In this example, the threshold current circuit 212 includes a threshold register 226 configured to store the threshold value. The threshold register 226 is coupled to a digital-to-analog converter (DAC) 228 that generates an analog control signal based on the threshold value. The analog control signal controls a current generator circuit 230 to conduct the threshold current ITH from the comparison node 218.

[0034] The protection circuit 200 also includes resistor R3 coupled to the comparison node 218 and a capacitor C218 coupled between the resistor R3 and the reference voltage node GND. The resistor R3 and the capacitor C218 create a zero in the response of the feedback circuit 214. Current through the comparison node 218 that is not conducted through the threshold current circuit 212 may be conducted through the resistor R3 and the capacitor C218.

[0035] The output current IOUT may be compared to the threshold current ITH to determine whether the output voltage VOUT has exceeded the desired maximum. In addition, when the output current IOUT exceeds the threshold current ITH, the voltage VCOMP on the comparison node 218 increases. In this manner, the feedback circuit 214 detects whether the output current IOUT exceeds the threshold current ITH. If an increase of the voltage VCOMP is detected by the feedback circuit 214, the feedback circuit 214 generates the one or more feedback signals 206 to the bias circuit 204 to reduce the bias voltage(s) on one or more of the control terminals 114(1 )-114(5). In more detail, the feedback circuit 214 includes at least one transistor 220(1 )-220(X) coupled to a circuit 234, where X is an integer value. In some examples, the integer X may correspond to the number of cascode transistors in the amplifier circuit 100. Thus, in the example of cascode transistors 104(1 )-104(5), X may be any integer from 1 to 5. In some examples, the circuit 234 may control a power supplied to the at least one transistor 220(1 )-220(X). Gate terminals 222(1 )-222(X) of the at least one transistor 220(1 )-220(X) may be coupled to the comparison node 218 and receive the voltage VCOMP. The voltage VCOMP of the comparison node 218 remains at a known level while the output current IOUT is less than or equal to the threshold voltage ITH. If the output current IOUT increases above the threshold voltage ITH, the voltage VCOMP will increase, and the bias voltage(s) on the gate terminals 222(1 )-222(X) will increase, which causes the current of the one or more feedback signals 206 provided to the bias circuit 204 to also increase. In response, the bias circuit 204 reduces a bias voltage on the control terminal 114(5) of the last cascode transistor 104(5) and may also reduce a bias voltage on one or more of the cascode transistors 104(1 )-104(4).

[0036] The acceleration circuit 216 includes a bypass transistor 224 that couples to terminals of the resistor R2 of the voltage-to-current circuit 210 to bypass the resistor R2 when the acceleration circuit 216 detects that the output current IOUT exceeds a predetermined stress level. In some examples, the stress level may be determined by a voltage between the resistors R1 and R2 of the voltage-to-current circuit 210. In some examples, the stress level may be determined by a current through the resistor R3. In this regard, the acceleration circuit 216 may be coupled to the resistor R3 and the capacitor C218. The stress level may be indicated by a stress value stored in a register 232. The acceleration circuit 216 may activate the bypass transistor 224 when either the voltage VCOMP increases or when the acceleration circuit 216 determines that the output current IOUT is greater than the stress current. In some examples, the acceleration circuit 216 activates the bypass transistor 224 when it is determined that the output current IOUT exceeds the threshold current ITH by a predetermined margin. Activating the bypass transistor 224 reduces a resistance in the voltage- to-current circuit 210, which increases the output current IOUT for a given voltage. A lower resistance between the peak voltage circuit 208 and the comparison node 218 increases the current to the comparison node 218, which increases the speed and/or magnitude of the response by the feedback circuit 214.

[0037] Figure 3 is an illustration of a power amplification circuit 300, including the amplifier circuit 100 of Figure 1 and a stress control circuit 302 that protects the cascode transistors 104(1 )-104(5) by adjusting bias voltages on at least one of the control terminals 114(1 )-114(5) upon detecting that the output voltage VOUT exceeds a threshold. The threshold to which the stress control circuit 302 responds may be the same threshold or a different threshold than the threshold discussed above, to which the feedback circuit 214 responds. Figure 3 also includes a bias circuit 304 that provides bias voltages for controlling the cascode transistors 104(1 )-104(5). The stress control circuit 302 adjusts the bias voltage on at least one of the control terminals 114(1 )-114(5) in response to the output voltage VOUT exceeding a threshold.

[0038] The stress control circuit 302 includes a variable capacitor 308 and a fixed capacitor 310 coupled in series between the output node 106 and the reference voltage node GND. The variable capacitor 308 may also be a varactor 308. The control terminal 114(1 ) of the first cascode transistor 104(1 ) is coupled to a bias node 312 between the variable capacitor 308 and the fixed capacitor 310. For example, in Figure 3, the variable capacitor 308 is coupled between the output node 106 and the bias node 312. In addition, in the example shown in Figure 3, the variable capacitor 308 may be a MOS varactor. The fixed capacitor 310 is coupled between the bias node 312 and the reference voltage node GND. As the output voltage VOUT on the output node 106 increases, the variable capacitor 308 coupled to the output node 106 increases in capacitance. As the variable capacitor 308 increases in capacitance, a ratio of capacitance between the variable capacitor 308 and the fixed capacitor 310 increases, causing a bias voltage at the bias node 312 to increase. The bias node 312 is coupled to the control terminal 114(1 ) of the first cascode transistor 104(1 ). In the example in which the first cascode transistor 104(1 ) is an N-channel MOSFET (NFET), the control terminal 114(1 ) is a gate 114(1 ), and the output node 106 is a drain terminal 106. Raising the bias voltage on the gate 114(1 ) raises the voltage on a source terminal SRC of the NFET cascode transistor 104(1 ) based on a gate-to- source voltage. Raising the voltages on the gate 1 14(1 ) and the source terminal SRC as the drain terminal 106 of the first cascode transistor 104(1 ) increases is referred to herein as “floating up” the first cascode transistor. By increasing the voltage on the source terminal SRC, the voltage drop from the drain terminal 106 to the source terminal SRC is reduced, which avoids a destructive voltage being applied between the drain terminal 106 and the source terminal SRC of the first cascode transistor 104(1 ). It should be noted that coupling the cascode transistors 104(1 )-104(5) in series comprises, as an example, coupling a source terminal SRC of a first one of the cascode transistors 104(1 )-104(5) (e.g., 104(x)) to a drain terminal of a next one of the cascode transistors 104(1 )-104(5) (e.g., 104(x-1 )) in the series in a first direction and coupling a drain terminal of the first one (104(x)) to the source terminal SRC of a next one of the cascode transistors 104(1 )-104(5) (e.g., 104(X+1 )) in the series in the opposite direction.

[0039] In some examples, the stress control circuit 302 may include a second varactor 308 and a second fixed capacitor 310 coupled in series between the output node 106 and the reference voltage node GND with the control terminal 114(2) coupled to a node between the second variable capacitor 308 and the second fixed capacitor 310 to adjust the bias voltage of the cascode transistor 104(2). Although not shown here, the stress control circuit 302 may also include a signal or signals provided to the bias circuit 304 to prevent the bias circuit 304 from reducing the bias voltage on the control terminal 114(1 ), and possibly one or more other control terminals 114(2)-114(5), in conflict with the stress control circuit 302.

[0040] An illustration of the effect of the stress control circuit 302 is provided in Figures 4A and 4B. Figure 4A is a graph 400 illustrating the output voltage VOUT for signals 402, 404, and 406, whose voltages vary in time with respect to a direct current (DC) voltage VDC. A maximum voltage VMAX is also shown in Figure 4A. If the output voltage VOUT is above the maximum voltage VMAX, the output voltage VOUT will cause a destructive voltage across one or more of the cascode transistors 104(1 )-104(5) in the amplifier circuit 100 in Figure 1. Voltages below the DC voltage VDC do not cause destructive voltages in the amplifier circuit 100. Therefore, when the signals 402, 404, and 406 have low values that cannot cause destructive voltages to the cascode transistors 104(1 )- 104(5), there is no need to change the capacitance C308 with the output voltage VOUT below the DC voltage VDC. AS shown, the signals 402 and 404 do not reach a high enough voltage to cause a destructive voltage. However, the signal 406 exceeds the maximum voltage VMAX. The stress control circuit 302 of Figure 3 responds to increases in the output voltage VOUT by increasing a capacitance of the variable capacitor 308 to raise a bias voltage on the control terminal 114(1 ) of the first cascode transistor 104(1 ).

[0041] Figure 4B is an illustration of the capacitance C308 of the variable capacitor 308 as it changes relative to an unbiased capacitance CUNB in response to the rise and fall of the signal 406 due to the stress control circuit 302 (not shown here). As shown, the capacitance of the variable capacitor 308 is nonlinear, meaning that the response to the output voltage VOUT above the DC voltage VDC differs from the response to the output voltage VOUT below the DC voltage VDC. Specifically, the decrease to the capacitance C308 caused by the stress control circuit 302 due to a drop in the signal 406 below the DC voltage VDC is much less than a corresponding increase of the capacitance C308 due to a rise in the signal 406 above the DC voltage VDC. In this manner, the stress control circuit 302 changes the bias voltage on the first cascode transistor 104(1 ) as needed and does not change the bias voltage on the first cascode transistor 104(1 ) when none is needed.

[0042] Figure 5 illustrates an alternative example of a power amplification circuit 500, including the amplifier circuit 100 and a stress control circuit 502. Figure 5 also includes a bias circuit 504 that provides bias voltages for controlling the cascode transistors 104(1 )-104(5). The stress control circuit 502 adjusts the bias voltage on at least one of the control terminals 114(1 )-114(5) in response to the output voltage VOUT exceeding a threshold. The stress control circuit 502 includes a fixed capacitor 506 and a variable capacitor 508 coupled in series between the output node 106 and the reference voltage node GND. The variable capacitor 508 may also be a varactor 508. The control terminal 114(1 ) of the first cascode transistor 104(1 ) is coupled to a bias node 510 between the variable capacitor 508 and the fixed capacitor 506. In the example in Figure 5, the fixed capacitor 506 is coupled between the output node 106 and the bias node 510. The variable capacitor 508 is coupled between the bias node 510 and the reference voltage node GND. In addition, in the example shown in Figure 5, the variable capacitor 508 may be a varactor diode. As the output voltage VOUT on the output node 106 increases, raising the bias voltage on the control terminal 114(1 ), the variable capacitor 508 decreases in capacitance. As the capacitance of the variable capacitor 508 decreases, the ratio of the fixed capacitor 506 and the variable capacitor 508 decreases, and the bias voltage is further increased. [0043] Figure 6 is an example of the power amplification circuit 600, including the amplifier circuit 100, a protection circuit 602 corresponding to the protection circuit 202 in Figure 2, and a stress control circuit 604 corresponding to one of the stress control circuits 302 and 502 in Figures 3 and 5, respectively. Figure 6 also includes a bias circuit 606. In response to the output voltage VOUT rising above a voltage at which destructive voltages may be supplied to the cascode transistors 104(1 )-104(5), the protection circuit 602 provides a feedback signal 206 that reduces a bias voltage of at least the last cascode transistor 104(5) and the stress control circuit 604 raises a bias voltage of the first cascode transistor 104(1 ). The feedback circuit 602 reduces the bias voltage of at least the last cascode transistor 104(5) to a bias voltage closer to the reference voltage of the reference voltage node GND (e.g., ground). In contrast, the stress control circuit 604 increases the bias voltage of at least the first cascode transistor 104(1 ) to a level closer to a supply voltage (VBAT).

[0044] Figure 7 is an example of a power amplification circuit 700 to show that the power amplification circuit 600 of Figure 6, including the amplifier circuit 100, the protection circuit 602, and the stress control circuit 604, coupled to the bias circuit 606, may be a first stage 702 of a multi-stage amplifier. Figure 7 also shows that the power amplification circuit 700 can include a second stage 704. In this example, the second stage 704 includes an amplification stage 706 and the protection circuit 602 coupled to a bias circuit 606. The second stage 704 may also or alternatively include the stress control circuit 604 (corresponding to the stress control circuit 302 in Figure 3 or the stress control circuit 502 in Figure 5). Thus, as disclosed herein, cascode amplifiers and amplifier stages may be protected from destructive voltages.

[0045] Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.