Power amplifier protection circuit

Abstract

According to the present disclosure, a first diode circuit includes one diode or a plurality of diodes connected in series, and an anode-side end is connected to an input node or an output node of a power amplifier or an inter-stage node of a cascode connection circuit of a power amplifier including a plurality of cascode-connected transistors. A drain of a first transistor, which is an NMOSFET, is connected to a cathode-side end of the first diode circuit, and a power supply voltage is applied to the drain via a first resistance element. A first feedback voltage and a first reference voltage generated on the basis of the power supply voltage of the first transistor and the voltage of the cathode-side end of the first diode circuit are input to a first operational amplifier, and a first control voltage generated on the basis of the difference between the first feedback voltage and the first reference voltage is applied to a gate of the first transistor. A control circuit is configured to change the first reference voltage input to the first operational amplifier.

Description

Power amplifier protection circuit

[0001] The present invention relates to a power amplifier protection circuit.

[0002] A protection circuit including a plurality of clamp diodes connected in multiple stages is connected to an output node so that an excessive voltage exceeding the withstand voltage does not occur between the collector and emitter of a bipolar transistor in the output stage of a power amplifier (Patent Document 1). The protection circuit limits the maximum value of the voltage appearing at the output node. The maximum value of the limited voltage is referred to as the clip voltage. In the protection circuit described in Patent Document 1, a lead-out point in the middle of the plurality of clamp diodes is connected to a reference potential via a switch. By turning on and off this switch, the effective number of stages of the plurality of clamp diodes constituting the protection circuit, that is, the clip voltage can be adjusted.

[0003] Japanese Patent Application Laid-Open No. 2022-96838

[0004] In the conventional protection circuit, although the clip voltage can be adjusted, the adjusted clip voltage is limited to an integer multiple of the forward rise voltage of each clamp diode. An object of the present invention is to provide a power amplifier protection circuit capable of flexibly adjusting the clip voltage without being limited to an integer multiple of the forward rise voltage of each clamp diode.

[0005] According to one aspect of the present invention, a power amplifier protection circuit is provided comprising: a first diode circuit including one diode or a plurality of diodes connected in series, the anode end of which is connected to the input node or output node of a power amplifier, or to an interstage node of a cascode connection circuit of a power amplifier including a plurality of cascode-connected transistors; a first transistor which is an NMOSFET whose drain is connected to the cathode end of the first diode circuit and to which a power supply voltage is applied via a first resistive element; a first operational amplifier which receives a first feedback voltage and a first reference voltage generated based on the power supply voltage of the first transistor and the voltage at the cathode end of the first diode circuit, and applies a first control voltage generated based on the difference between the first feedback voltage and the first reference voltage to the gate of the first transistor; and a control circuit configured to change the first reference voltage input to the first operational amplifier.

[0006] When the control circuit changes the first reference voltage, the first control voltage applied to the gate of the first transistor changes, and the drain voltage of the first transistor changes. When the drain voltage of the first transistor changes, the voltage at the anode end of the first diode circuit 11 changes, and as a result, the clipping voltage changes. By changing the first reference voltage, the control circuit can flexibly adjust the clipping voltage.

[0007] Figure 1 is an equivalent circuit diagram of a power amplifier protection circuit according to the first embodiment. Figure 2 is a graph showing an example of the voltage waveform of the high-frequency signal RFout appearing at the output node of the power amplifier 50. Figure 3 is an equivalent circuit diagram of a power amplifier protection circuit according to a comparative example. Figure 4 is an equivalent circuit diagram of a power amplifier protection circuit according to the second embodiment. Figure 5 is a graph showing an example of the relationship between the first reference voltage Vref, the forward rise voltage Vri of the first diode circuit 11, the cathode voltage Vdk, and temperature. Figure 6 is an equivalent circuit diagram of a power amplifier protection circuit according to the third embodiment. Figure 7 is an equivalent circuit diagram of a power amplifier protection circuit according to the fourth embodiment. Figure 8 is an equivalent circuit diagram of a power amplifier protection circuit according to the fifth embodiment. Figure 9 is an equivalent circuit diagram of a power amplifier protection circuit according to the sixth embodiment. Figure 10 is a graph showing an example of the voltage waveform of the high-frequency signal RFout appearing at the output node of the power amplifier 50. Figure 11 is an equivalent circuit diagram of a power amplifier protection circuit according to the seventh embodiment. Figure 12 is an equivalent circuit diagram of a power amplifier protection circuit according to a modification of the seventh embodiment. Figure 13 is an equivalent circuit diagram of a power amplifier protection circuit according to the eighth embodiment. Figure 14 is a graph showing an example of the voltage waveform of the high-frequency signal RFin input to the input node of the power amplifier 50. Figure 15 is an equivalent circuit diagram of a power amplifier protection circuit according to the ninth embodiment.

[0008] [First Embodiment] The power amplifier protection circuit according to the first embodiment will be described with reference to Figures 1, 2, and 3.

[0009] Figure 1 is an equivalent circuit diagram of a power amplifier protection circuit according to the first embodiment. A high-frequency signal RFin is input to the input node of the power amplifier 50, and an amplified high-frequency signal RFout is output from the output node. The power amplifier 50 includes, for example, a heterojunction bipolar transistor (HBT), where the high-frequency signal RFin is input to the base of the HBT and the high-frequency signal RFout is output from the collector. A first power supply voltage Vcc is applied to the output node of the power amplifier 50, i.e., the collector of the HBT, via a choke coil 51.

[0010] The power amplifier protection circuit according to the first embodiment includes a first diode circuit 11, a first transistor 12, a first operational amplifier 13, a first resistor element 14, and a control circuit 20. The first diode circuit 11 includes one diode or multiple diodes connected in series. The anode end of the first diode circuit 11 is connected to the output node of the power amplifier 50.

[0011] The first transistor 12 is an NMOSFET. The drain of the first transistor 12 is connected to the cathode end of the first diode circuit 11. Furthermore, the second power supply voltage Vdd is applied to the drain of the first transistor 12 via the first resistor element 14. The source of the first transistor 12 is connected to the reference potential (ground potential).

[0012] The cathode end of the first diode circuit 11 is connected to a reference potential via a capacitor 15. The capacitor 15 has the function of removing high-frequency components that appear at the cathode end of the first diode circuit 11. Note that the capacitor 15 is not essential and may be omitted.

[0013] A first feedback voltage Vfb, generated based on the second power supply voltage Vdd and the cathode voltage Vdk, which is the voltage at the cathode end of the first diode circuit, is input to the non-inverting input node of the first operational amplifier 13. In the first embodiment, the drain of the first transistor 12 is connected to the non-inverting input node of the first operational amplifier 13, and the first feedback voltage Vfb is equal to the cathode voltage Vdk of the first diode circuit 11. A first reference voltage Vref is input to the inverting input node of the first operational amplifier 13.

[0014] The first operational amplifier 13 applies a first control voltage, generated based on the difference between the first feedback voltage Vfb and the first reference voltage Vref, to the gate of the first transistor 12. When the first feedback voltage Vfb exceeds the first reference voltage Vref, the first transistor 12 conducts, and the cathode voltage Vdk of the first diode circuit 11 is fixed at the first reference voltage Vref. The first transistor 12, the first operational amplifier 13, the first resistor element 14, and the control circuit 20 function as voltage sources that generate the cathode voltage Vdk.

[0015] The control circuit 20 applies a first reference voltage Vref to the inverting input terminal of the first operational amplifier 13. The control circuit 20 has the function of changing the first reference voltage Vref according to various conditions.

[0016] Next, the operation of the power amplifier protection circuit according to the first embodiment will be described with reference to Figure 2. Figure 2 is a graph showing an example of the voltage waveform of the high-frequency signal RFout that appears at the output node of the power amplifier 50. The horizontal axis represents time, and the vertical axis represents the voltage of the high-frequency signal RFout.

[0017] The high-frequency signal RFout includes a DC component corresponding to the first power supply voltage Vcc and a high-frequency component obtained by amplified high-frequency signal RFin. The forward rise voltage of the first diode circuit 11 is denoted as Vri. When the voltage of the high-frequency signal RFout reaches Vdk + Vri, current begins to flow from the output node of the power amplifier 50 to the power amplifier protection circuit. Therefore, the voltage of the high-frequency signal RFout will not exceed Vdk + Vri, as shown by the solid line in Figure 2. That is, the clipping voltage Vclp is set to Vdk + Vri. Since the cathode voltage Vdk of the first diode circuit 11 is equal to the first reference voltage Vref, the clipping voltage Vclp is set to Vref + Vri.

[0018] Next, the excellent effects of the first embodiment will be described. The control circuit 20 can change the clipping voltage Vclp by adjusting the first reference voltage Vref. The clipping voltage Vclp is not limited to integer multiples of the forward rise voltage of each diode in the first diode circuit 11, but can also be set to an intermediate value among the integer multiples of the forward rise voltage.

[0019] Furthermore, the control circuit 20 can change the first reference voltage Vref according to various conditions, thereby setting the clipping voltage Vclp to an appropriate value according to various conditions. Also, since there is no need to switch on or off when changing the clipping voltage Vclp, noise caused by switching on and off is not generated. By continuously changing the first reference voltage Vref, discontinuous changes in the clipping voltage Vclp caused by switching on and off can be avoided, and the clipping voltage Vclp can be changed continuously.

[0020] Increasing the cathode voltage Vdk allows for a reduction in the number of diodes in the first diode circuit 11. Reducing the number of diodes in the first diode circuit 11 lowers the resistance from the output node of the power amplifier 50 to the reference potential when the power amplifier protection circuit is operating. This results in the excellent effect of increasing the protection capability of the power amplifier.

[0021] Next, the superior effects of the first embodiment will be explained in comparison with the comparative example shown in Figure 3. Figure 3 is an equivalent circuit diagram of the power amplifier protection circuit according to the comparative example.

[0022] In the first embodiment, an NMOSFET is used as the first transistor 12 (Figure 1), which is a voltage source that generates the cathode voltage Vdk of the first diode circuit 11. However, in the comparative example shown in Figure 3, a PMOSFET 60 is used. The drain of the PMOSFET 60 is connected to the cathode of the first diode circuit 11 and is also connected to a reference potential via a resistor 61. The second power supply voltage Vdd is applied to the source of the PMOSFET 60.

[0023] In the comparative example shown in Figure 3, a resistor 61 is inserted between the output node of the voltage source (the drain of the PMOSFET 60) and the reference potential. In contrast, in the first embodiment (Figure 1), a first transistor 12 is inserted between the output node of the voltage source and the reference potential. As a result, the resistance between the output node of the power amplifier 50 and the reference potential in the first embodiment is lower than the resistance in the comparative example shown in Figure 3. Therefore, the protection capability of the power amplifier is higher in the configuration of the first embodiment (Figure 1) compared to the configuration of the comparative example (Figure 3).

[0024] [Second Embodiment] Next, a power amplifier protection circuit according to the second embodiment will be described with reference to Figures 4 and 5. Hereinafter, the configuration common to the power amplifier protection circuit according to the first embodiment, described with reference to Figures 1 and 2, will be omitted from the explanation.

[0025] Figure 4 is an equivalent circuit diagram of a power amplifier protection circuit according to the second embodiment. The control circuit 20 of the power amplifier protection circuit according to the second embodiment includes a temperature sensor 21. The temperature sensor 21 is formed on the same IC chip as, for example, the first transistor 12 and the first operational amplifier 13, and measures the temperature of this IC chip. The power amplifier 50 and the first diode circuit 11 are formed on other IC chips. For example, the IC chip on which the power amplifier 50 and the first diode circuit 11 are formed and the IC chip on which the temperature sensor 21 is formed are mounted on a common module board. Because the two IC chips are thermally coupled via the module board, the temperature measurement by the temperature sensor 21 reflects the temperatures of the power amplifier 50 and the first diode circuit 11.

[0026] The control circuit 20 controls the first reference voltage Vref to increase as the temperature detected by the temperature sensor 21 increases.

[0027] Figure 5 is a graph showing an example of the relationship between the first reference voltage Vref, the forward rise voltage Vri of the first diode circuit 11, the cathode voltage Vdk, and temperature. The horizontal axis represents temperature, and the vertical axis represents voltage. The forward rise voltage Vri of the first diode circuit 11 decreases as the temperature rises, as shown by the dashed line in Figure 5. The control circuit 20 increases the first reference voltage Vref as the temperature rises, as shown by the thin solid line in Figure 5. The slope of the first reference voltage Vref is greater than the absolute value of the slope of the forward rise voltage Vri of the first diode circuit 11. The cathode voltage Vdk of the first diode circuit 11 is equal to the first reference voltage Vref. Therefore, the clipping voltage Vclp, which is equal to the sum of the cathode voltage Vdk and the rise voltage Vri, increases as the temperature rises, as shown by the thick solid line in Figure 5.

[0028] Next, the excellent effects of the second embodiment will be described. In the power amplifier 50 including the HBT, the withstand voltage between the collector and emitter decreases as the temperature decreases. In the second embodiment, the clipping voltage Vclp decreases as the temperature measured by the temperature sensor 21 decreases. That is, the clipping voltage Vclp decreases in accordance with the decrease in the withstand voltage between the collector and emitter of the power amplifier 50. In this way, an appropriate temperature characteristic can be given to the clipping voltage Vclp.

[0029] Next, a modification of the second embodiment will be described. In the second embodiment, the temperature sensor 21 is formed on the same IC chip as the first transistor 12 and the first operational amplifier 13, but it may also be formed on the same IC chip as the power amplifier 50 and the first diode circuit 11. In this case, the output signal of the temperature sensor 21 is transmitted through the wiring of the module board to the IC chip on which the first transistor 12 and the first operational amplifier 13 are formed. As an alternative configuration, the temperature sensor 21 may be mounted on the module board.

[0030] [Third Embodiment] Next, a power amplifier protection circuit according to the third embodiment will be described with reference to Figure 6. Hereinafter, the configuration common to the power amplifier protection circuit according to the first embodiment, described with reference to Figures 1 and 2, will be omitted from the explanation.

[0031] Figure 6 is an equivalent circuit diagram of the power amplifier protection circuit according to the third embodiment. In the first embodiment (Figure 1), the drain of the first transistor 12 is directly connected to the non-inverting input node of the first operational amplifier 13. In contrast, in the third embodiment, the first resistor element 14 constitutes a resistive voltage divider circuit consisting of voltage divider resistors 14a and 14b, and the first feedback voltage Vfb, obtained by dividing the drain voltage of the first transistor 12 and the second power supply voltage Vdd by the resistive voltage divider circuit, is input to the non-inverting input node of the first operational amplifier 13. That is, the first feedback voltage Vfb is generated based on the second power supply voltage Vdd of the first transistor 12 and the cathode voltage Vdk of the first diode circuit 11.

[0032] If we denote the resistance value of the voltage divider resistor 14a connected to the drain of the first transistor 12 as Ra, and the resistance value of the voltage divider resistor 14b to which the second power supply voltage Vdd is applied as Rb, then the cathode voltage Vdk is expressed by the following formula: Vdk = ((Ra + Rb) / Rb)Vref - (Ra / Rb)Vdd ... (1)

[0033] As the first reference voltage Vref increases, the cathode voltage Vdk also increases, but its slope changes depending on the voltage division ratio of the first resistive element 14.

[0034] Next, the excellent effects of the third embodiment will be described. In the third embodiment, as in the first embodiment, it is possible to set the clipping voltage Vclp to an appropriate value according to various conditions. Furthermore, in the third embodiment, the slope of the change in cathode voltage Vdk with respect to the change in the first reference voltage Vref changes according to the voltage division ratio of the first resistive element 14. Therefore, the degree of freedom in setting the correspondence between the first reference voltage Vref and the clipping voltage Vclp is increased.

[0035] [Fourth Embodiment] Next, the power amplifier protection circuit according to the fourth embodiment will be described with reference to Figure 7. The following description will omit details of components common to the power amplifier protection circuit according to the third embodiment, which was described with reference to Figure 6.

[0036] Figure 7 is an equivalent circuit diagram of the power amplifier protection circuit according to the fourth embodiment. In the third embodiment (Figure 6), the voltage division ratio of the first resistor element 14 constituting the resistive voltage divider circuit is fixed. In contrast, in the fourth embodiment, the voltage division ratio of the first resistor element 14 is variable. The voltage division ratio of the first resistor element 14 is adjusted by control from the control circuit 20. For example, the voltage division ratio can be adjusted by changing the resistance value of the voltage divider resistor 14b.

[0037] For example, a voltage divider resistor 14b can be constructed by connecting a switching transistor in series to each of several resistors with different resistance values, and then connecting multiple series circuits consisting of resistors and switching transistors in parallel. The resistance value of the voltage divider resistor 14b can be changed by controlling the switching transistors to be on or off.

[0038] The resistance value of the voltage divider resistor 14a may be made variable, or the resistance values ​​of both voltage divider resistors 14a and 14b may be made variable.

[0039] The control circuit 20 includes a temperature sensor 21, an analog circuit 24, a digital circuit 23, and an E fuse 22. The digital circuit 23 reads the conduction / disconnection state of the E fuse 22 and outputs a command signal to the analog circuit 24 based on the reading result. The analog circuit 24 changes the voltage division ratio of the first resistive element 14 in response to the command from the digital circuit 23. Furthermore, the analog circuit 24 generates a first reference voltage Vref based on the temperature measurement value from the temperature sensor 21.

[0040] As shown in equation (1), changing the voltage division ratio of the first resistive element 14 changes the relationship between the cathode voltage Vdk and the first reference voltage Vref.

[0041] Next, the excellent effects of the fourth embodiment will be described. In the fourth embodiment, the voltage division ratio of the first resistive element 14 can be changed by setting the open / closed state of the E fuse, and as a result, the relationship between the cathode voltage Vdk and the first reference voltage Vref can be adjusted.

[0042] For example, due to variations within the allowable range of the manufacturing process, the appropriate clip voltage Vclp may differ between products. In this case, by setting the cut-off conduction state of the E-fuse 22 for each product, an appropriate clip voltage Vclp suitable for each product can be realized. As an example, it is advisable to set the cut-off conduction state of the E-fuse 22 so that an appropriate clip voltage Vclp can be obtained at room temperature. When the temperature changes, the clip voltage Vclp can be changed according to the detected temperature value by the temperature sensor 21.

[0043] [Fifth Embodiment] Next, a power amplifier protection circuit according to the fifth embodiment will be described with reference to FIG. 8. Hereinafter, description of configurations common to the power amplifier protection circuit according to the fourth embodiment described with reference to FIG. 7 will be omitted.

[0044] FIG. 8 is an equivalent circuit diagram of a power amplifier protection circuit according to the fifth embodiment. In the fourth embodiment, one power amplifier 50 is the protection target. In contrast, in the fifth embodiment, a plurality of power amplifiers 50, for example, two power amplifiers 50, are the protection targets. Corresponding to each of the two power amplifiers 50, the first diode circuit 11 is provided separately.

[0045] A switch 16 is inserted between the cathode-side end of each of the two first diode circuits 11 and the drain of one first transistor 12. The switch 16 selects one of the cathode-side ends of the two first diode circuits 11 and connects it to the drain of the first transistor 12 under the control from the control circuit 20.

[0046] The plurality of power amplifiers 50 do not operate simultaneously, and only one of the plurality of power amplifiers 50 operates. The control circuit 20 controls the switch 16 to connect the cathode-side end of the first diode circuit 11 connected to the operating power amplifier 50 among the plurality of power amplifiers 50 to the drain of the first transistor 12. Thereby, the operating power amplifier 50 can be protected.

[0047] The appropriate clip voltage Vclp may not be the same for different power amplifiers 50 by the power amplifier 50. The digital circuit 23 adjusts the voltage division ratio of the first resistor element 14 so as to provide an appropriate clip voltage Vclp for the power amplifier 50 to be protected.

[0048] Next, the excellent effects of the fifth embodiment will be described. In the fifth embodiment, a plurality of power amplifiers 50 can be protected by one voltage source including the first transistor 12, the first operational amplifier 13, and the first resistor element 14. Further, by adjusting the voltage division ratio of the first resistor element 14 according to the appropriate clip voltage Vclp of the power amplifier 50 to be protected, an appropriate clip voltage Vclp can be generated according to each of the plurality of power amplifiers 50.

[0049] [Sixth Embodiment] Next, a power amplifier protection circuit according to the sixth embodiment will be described with reference to FIGS. 9 and 10. Hereinafter, description of the configuration common to the power amplifier protection circuit according to the first embodiment described with reference to FIGS. 1 and 2 will be omitted.

[0050] FIG. 9 is an equivalent circuit diagram of a power amplifier protection circuit according to the sixth embodiment. The power amplifier protection circuit according to the sixth embodiment includes, in addition to the components of the power amplifier protection circuit (FIG. 1) according to the first embodiment, a second diode circuit 31, a second transistor 32, a second operational amplifier 33, a second resistor element 34, and a capacitor 35. The second diode circuit 31 includes one diode or a plurality of diodes connected in series. The cathode-side end of the second diode circuit 31 is connected to the anode-side end of the first diode circuit 11. That is, the cathode-side end of the second diode circuit 31 is connected to the output node of the power amplifier 50.

[0051] A PMOSFET is used as the second transistor 32. The drain of the second transistor 32 is connected to the anode-side end of the second diode circuit 31 and is connected to the reference potential via the second resistor element 34. The second power supply voltage Vdd is applied to the source of the second transistor 32. The anode-side end of the second diode circuit 31 is connected to the reference potential via the capacitor 35.

[0052] A second feedback voltage Vfb2, generated based on the voltage at the anode end of the second diode circuit 31 (hereinafter referred to as the anode voltage Vda) and the reference potential, is input to the non-inverting input node of the second operational amplifier 33, and a second reference voltage Vref2 is input to the inverting input node. In the configuration shown in Figure 9, the second feedback voltage Vfb2 is equal to the anode voltage Vda.

[0053] The second operational amplifier 33 applies a second control voltage to the gate of the second transistor, based on the difference between the second reference voltage Vref2 and the second feedback voltage Vfb2. The control circuit 20 supplies the second reference voltage Vref2 to the second operational amplifier 33. When the control circuit 20 changes the second reference voltage Vref2, the anode voltage Vda of the second diode circuit 31 changes in accordance with the change in the second reference voltage Vref2.

[0054] The forward rise voltage of the second diode circuit 31 is denoted as Vri2. When the voltage at the output node of the power amplifier 50 falls below Vda - Vri2, current flows from the second power supply voltage Vdd through the second transistor 32 and the second diode circuit 31 to the output node of the power amplifier 50. Therefore, the voltage at the output node of the power amplifier 50 is limited so as not to fall below Vda - Vri2. The lower limit of the voltage at the output node of the power amplifier 50 (Vda - Vri2) is referred to as the second clipping voltage Vclp2.

[0055] Next, the operation of the power amplifier protection circuit according to the sixth embodiment will be described with reference to Figure 10. Figure 10 is a graph showing an example of the voltage waveform of the high-frequency signal RFout that appears at the output node of the power amplifier 50. The horizontal axis represents time, and the vertical axis represents voltage.

[0056] Similar to the first embodiment (Figure 2), the voltage at the output node of the power amplifier 50 is limited to below the clipping voltage Vclp. Furthermore, in the sixth embodiment, when the voltage at the output node of the power amplifier 50 drops to the second clipping voltage Vclp2, it is limited so as not to fall below the second clipping voltage Vclp2.

[0057] Next, the excellent effects of the sixth embodiment will be described. In the sixth embodiment, not only the upper limit but also the lower limit of the voltage of the high-frequency signal RFout output from the output node of the power amplifier 50 can be limited. This makes it possible to limit the amplitude of the high-frequency signal RFout to below a certain magnitude. When the high-frequency signal RFout is input to a subsequent power amplifier, the amplitude of the input signal can be limited, protecting the subsequent power amplifier. It is also possible to suppress the amplitude by limiting only the upper limit of the voltage of the high-frequency signal RFout, but by limiting both the upper and lower limits, waveform distortion can be suppressed. Furthermore, by limiting the lower limit of the high-frequency signal RFout, increases in the base-collector voltage and emitter-collector voltage can be suppressed, preventing damage to the power amplifier 50.

[0058] Next, a modified version of the sixth embodiment will be described. In the sixth embodiment, a resistive voltage divider circuit may be used as the second resistive element 34, similar to the first resistive element 14 of the power amplifier protection circuit in the third embodiment (Figure 6). In this case, the voltage difference between the anode voltage Vda and the reference potential is divided to generate the second feedback voltage Vfb2. In both the sixth embodiment and its modified versions, the second feedback voltage Vfb2 is generated based on the anode voltage Vda of the second diode circuit 31 and the reference potential.

[0059] [Seventh Embodiment] Next, the power amplifier protection circuit according to the seventh embodiment will be described with reference to Figure 11. The following description will omit details of components common to the power amplifier protection circuit according to the first embodiment, which was described with reference to Figures 1 and 2.

[0060] Figure 11 is an equivalent circuit diagram of the power amplifier protection circuit according to the seventh embodiment. In the first embodiment (Figure 1), the anode end of the first diode circuit 11 is connected to the output node of the power amplifier 50. In contrast, in the seventh embodiment, a cascode amplifier circuit is used as the power amplifier 50, and the anode end of the first diode circuit 11 is connected to the interstage node of the cascode connection circuit.

[0061] More specifically, the power amplifier 50 includes an amplification transistor 52 to which a high-frequency signal RFin is input, and two cascode transistors 53 connected in a cascode configuration to the amplification transistor 52. NMOSFETs are used for both the amplification transistor 52 and the cascode transistors 53. The amplification transistor 52 and the two cascode transistors 53 are numbered sequentially from the amplification transistor 52 to distinguish between multiple transistors.

[0062] The source of the first stage transistor, the amplification transistor 52, is connected to a reference potential, and the first power supply voltage Vdd1 is applied to the drain of the third stage cascode transistor 53 via the choke coil 51.

[0063] A high-frequency signal RFin is input to the gate of the amplification transistor 52 via the capacitor 54, and a bias voltage Bias1 is supplied via the ballast resistor element 55. Bias voltages Bias2 and Bias3 are supplied to the gates of the two cascode transistors 53, respectively. The amplified high-frequency signal RFout is output from the drain of the third-stage cascode transistor 53. In other words, the drain of the third-stage cascode transistor 53 becomes the output node of the power amplifier 50.

[0064] The anode end of the first diode circuit 11 is connected to the space between the first stage amplifier transistor 52 and the second stage cascode transistor 53 (interstage node).

[0065] Next, the excellent effects of the seventh embodiment will be described. In the seventh embodiment, the voltage at the drain of the first-stage amplification transistor 52 is limited not to exceed the clipping voltage Vclp, rather than the voltage at the output node of the power amplifier 50. When a cascode amplification circuit is used as the power amplifier 50, the voltage of the high-frequency signal RFout at the output node may not be evenly distributed among multiple transistors, but may be concentrated on the lower-stage transistors.

[0066] Therefore, even if the upper limit of the output node voltage is restricted, the voltage between the source and drain of the lower stage transistor may exceed the withstand voltage. For example, a voltage greater than the withstand voltage may be generated between the source and drain of the first stage amplifier transistor 52. In the seventh embodiment, the drain voltage of the first stage amplifier transistor 52 is controlled so as not to exceed the clipping voltage Vclp, thereby protecting the amplifier transistor 52.

[0067] Next, a modified example of the seventh embodiment will be described with reference to Figure 12. Figure 12 is an equivalent circuit diagram of a power amplifier protection circuit according to a modified example of the seventh embodiment. In the seventh embodiment (Figure 11), NMOSFETs are used for the amplification transistor 52 and cascode transistor 53 of the power amplifier 50. In contrast, in the modified example shown in Figure 12, NPN bipolar transistors are used for the amplification transistor 52 and cascode transistor 53 of the power amplifier 50. Note that there is only one cascode transistor 53. The first power supply voltage Vcc is applied to the collector of the cascode transistor 53 via the choke coil 51.

[0068] The collector of the second-stage cascode transistor 53 becomes the output node of the power amplifier 50. The anode end of the first diode circuit 11 is connected to the interstage node between the first-stage amplifying transistor 52 and the second-stage cascode transistor 53.

[0069] As shown in the modified example in Figure 12, the power amplifier 50 may be constructed using bipolar transistors. In the modified example in Figure 12, there may be two or more cascode transistors 53.

[0070] Next, another modification of the seventh embodiment will be described. In the seventh embodiment (Figure 11), the anode end of the first diode circuit 11 is connected to the interstage node between the first stage amplifier transistor 52 and the second stage cascode transistor 53, but it may be connected to any other interstage node. For example, the anode end of the first diode circuit 11 may be connected to the interstage node between the second stage cascode transistor 53 and the third stage cascode transistor 53.

[0071] [Eighth Embodiment] Next, the power amplifier protection circuit according to the eighth embodiment will be described with reference to Figures 13 and 14. Hereinafter, the configuration common to the power amplifier protection circuit according to the first embodiment, described with reference to Figures 1 and 2, will be omitted from the explanation.

[0072] Figure 13 is an equivalent circuit diagram of the power amplifier protection circuit according to the eighth embodiment. In the first embodiment (Figure 1), the anode end of the first diode circuit 11 is connected to the output node of the power amplifier 50. In contrast, in the eighth embodiment, the anode end of the first diode circuit 11 is connected to the input node of the power amplifier 50.

[0073] Figure 14 is a graph showing an example of the voltage waveform of the high-frequency signal RFin input to the input node of the power amplifier 50. The horizontal axis represents time, and the vertical axis represents voltage. In the first embodiment (Figure 2), a DC component corresponding to the first power supply voltage Vcc is superimposed on the high-frequency signal RFout. In contrast, in the eighth embodiment, no DC component is superimposed on the high-frequency signal RFin. Therefore, the voltage of the high-frequency signal RFin swings in both the positive and negative directions around 0V.

[0074] The clipping voltage Vclp is equal to the sum of the cathode voltage Vdk of the first diode circuit 11 and the forward rise voltage Vri of the first diode circuit 11, as in the first embodiment. When the voltage of the high-frequency signal RFin rises and reaches the clipping voltage Vclp, current begins to flow from the input node of the power amplifier 50 to the first diode circuit 11, thereby limiting the voltage of the high-frequency signal RFin to below the clipping voltage Vclp.

[0075] Since no DC component is superimposed on the high-frequency signal RFin, the clipping voltage Vclp is set lower than the clipping voltage Vclp in the first embodiment (Figure 2). The number of stages in the first diode circuit 11 and the first reference voltage Vref should be determined according to the clipping voltage Vclp.

[0076] Next, the excellent effects of the eighth embodiment will be described. In the eighth embodiment, the amplitude of the high-frequency signal RFin input to the power amplifier 50 is limited by the clipping voltage Vclp. By limiting the power of the high-frequency signal RFin input to the power amplifier 50, the power amplifier 50 can be protected.

[0077] [Ninth Embodiment] Next, the power amplifier protection circuit according to the ninth embodiment will be described with reference to Figure 15. Hereinafter, the configuration common to the power amplifier protection circuit according to the first embodiment, described with reference to Figures 1 and 2, will be omitted from the explanation.

[0078] Figure 15 is an equivalent circuit diagram of a power amplifier protection circuit according to the ninth embodiment. In the ninth embodiment, the power amplifier 50 has a two-stage configuration consisting of a drive stage amplifier 50a and a power stage amplifier 50b. The first power supply voltage Vcc is applied to the output node of the drive stage amplifier 50a via a choke coil 51a and to the output node of the power stage amplifier 50b via a choke coil 51b.

[0079] An impedance matching circuit 56 is inserted between the output node of the drive stage amplifier 50a and the input node of the power stage amplifier 50b. The anode end of the first diode circuit 11 is connected between the impedance matching circuit 56 and the input node of the power stage amplifier 50b. Since the first power supply voltage Vcc is cut off by the impedance matching circuit 56, no DC component is superimposed on the high-frequency signal at the anode end of the first diode circuit 11.

[0080] Next, the excellent effects of the ninth embodiment will be described. In the ninth embodiment, as in the eighth embodiment (Figures 13 and 14), the power of the high-frequency signal input to the power stage amplifier 50b is limited, thereby protecting the power stage amplifier 50b.

[0081] The embodiments described above are illustrative, and it goes without saying that partial substitution or combination of the configurations shown in different embodiments is possible. Similar effects and benefits from similar configurations in multiple embodiments will not be mentioned sequentially for each embodiment. Furthermore, the present invention is not limited to the embodiments described above. For example, it will be obvious to those skilled in the art that various modifications, improvements, and combinations are possible.

[0082] 11 First diode circuit 12 First transistor 13 First operational amplifier 14 First resistor element 14a, 14b Voltage divider resistor 15 Capacitor 16 Switch 20 Control circuit 21 Temperature sensor 22 E fuse 23 Digital circuit 24 Analog circuit 31 Second diode circuit 32 Second transistor 33 Second operational amplifier 34 Second resistor element 35 Capacitor 50 Power amplifier 50a Drive stage amplifier 50b Power stage amplifier 51 Choke coil 52 Amplifying transistor 53 Cascode transistor 54 Capacitor 55 Ballast resistor element 56 Impedance matching circuit 60 PMOSFET 61 Resistor element

Claims

1. A power amplifier protection circuit comprising: a first diode circuit including one diode or multiple diodes connected in series, the anode end of which is connected to the input node or output node of a power amplifier, or to an interstage node of a cascode connection circuit of a power amplifier including multiple cascode-connected transistors; a first transistor which is an NMOSFET, the drain of which is connected to the cathode end of the first diode circuit, and to which a power supply voltage is applied via a first resistive element; a first operational amplifier which receives a first feedback voltage and a first reference voltage generated based on the power supply voltage of the first transistor and the voltage at the cathode end of the first diode circuit, and applies a first control voltage generated based on the difference between the first feedback voltage and the first reference voltage to the gate of the first transistor; and a control circuit configured to change the first reference voltage input to the first operational amplifier.

2. The power amplifier protection circuit according to claim 1, wherein the control circuit includes a temperature sensor, and the first reference voltage is increased as the temperature detected by the temperature sensor increases.

3. The power amplifier protection circuit according to claim 1 or 2, wherein the first resistive element constitutes a resistive voltage divider circuit, divides the power supply voltage and the drain voltage of the first transistor, and the divided voltage is input to one input node of the first operational amplifier.

4. The power amplifier protection circuit according to claim 3, wherein the first resistive element has a variable voltage division ratio, and the control circuit can change the voltage division ratio of the resistive voltage divider circuit.

5. The power amplifier protection circuit according to claim 4, wherein the first diode circuits are provided in multiple quantities to correspond to multiple power amplifiers, and further comprises a switch inserted between the cathode end of each of the multiple first diode circuits and the drain of the first transistor, which selects one of the cathode ends of the multiple first diode circuits and connects it to the drain of the first transistor.

6. A power amplifier protection circuit according to any one of claims 1 to 5, further comprising: a second diode circuit including one diode or a plurality of diodes connected in series, the cathode end of which is connected to the anode end of the first diode circuit; a second transistor which is a PMOSFET, the drain of which is connected to the anode end of the second diode circuit and the drain is connected to a reference potential via a second resistive element; and a second operational amplifier which applies a second control voltage to the gate of the second transistor based on the difference between a second feedback voltage generated based on the voltage at the anode end of the second diode circuit and a reference potential and a second reference voltage, wherein the control circuit provides the second reference voltage to the second operational amplifier.

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