Power amplifier protection circuit

[0002] The present application is directed to power amplifiers and more particularly to a circuit that protects power amplifiers from operating at excessive voltages that might cause damage to the power amplifier.

[0003] Wireless transmitters typically comprise signal processing circuitry followed by a power amplifier that elevates the power of a signal to be transmitted prior to emission by an antenna. Such power amplifiers are becoming more common as a result of the ever increasing proliferation of mobile terminals that incorporate some form of wireless transmitter.

[0004] The term “mobile terminal” encompasses pagers, cellular phones, personal digital assistants, laptops equipped with wireless modems, and the like. These mobile terminals are subject to numerous telecommunications standards and regulations which govern their behavior. Many of these standards govern the power levels with which the mobile terminals are allowed to transmit signals. This is done to prevent cross-channel interference in some cases, as well as help the mobile terminals conserve battery power. In many instances the power levels are controlled, either by a base station or by the mobile terminal at the instruction of the base station.

[0005] One side effect of this mobile environment is that the power amplifier of the transmitter has to withstand large voltage standing wave ratios (VSWR) at elevated supply voltages. GSM in particular may generate a VSWR of 15:1 at times. This can quickly lead to device burnout as voltage peaks during successive periods at collectors of the transistors forming the power amplifiers exceed tolerable voltage levels.

[0006] In addition to mobile terminals, wireless LANs of computing devices are also becoming more common. The computing devices include a wireless transmitter that likewise has a power amplifier that may be subject to extended voltage peaks and damage therefrom.

[0007] Thus, there remains a need for a way to protect power amplifiers in transmitters from voltage peaks during successive periods that exceed design parameters and may cause burnout in the power amplifier.

[0008] The present invention adds a feedback loop to a power amplifier. The power amplifier, in an exemplary embodiment, is part of a transmitter chain. The feedback loop comprises a sensing circuit and a processing circuit. When the sensing circuit senses that the output of the power amplifier is high enough that operation at that operating point for successive cycles or periods would damage the power amplifier, the sensing function causes the processing circuit to change the bias provided to the power amplifier in such a manner that the output power is lessened to an acceptable level.

[0009] In a first embodiment, the sensing circuit is formed from a transistor that acts as an avalanche diode. In a second embodiment, the sensing circuit is formed from a plurality of diodes. In a third embodiment, the sensing circuit is formed from a plurality of transistors. Common among these sensing circuits is that they remain off until the output of the power amplifier exceeds a predetermined threshold. Above the predetermined threshold, the current passing through the sensing circuit increases rapidly with small increases in applied voltage above the threshold.

[0010] The processing circuit may act on the bias circuit differently depending on different embodiments. In an exemplary embodiment, the processing circuit comprises a current mirror that sinks bias current from the input of the power amplifier, thereby reducing the output of the power amplifier. In a second exemplary embodiment, the current mirror turns off the bias current when the sensing circuit detects an output over the threshold. Alternative actions may take place depending on the nature of the bias circuit used.

[0011] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

[0012] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

[0013] FIG. 1 illustrates a top level schematic diagram according to one embodiment of the present invention;

[0014] FIG. 2A illustrates a circuit level schematic diagram according to an exemplary embodiment of the present invention with a first bias circuit;

[0015] FIG. 2B illustrates a circuit level schematic diagram according to the embodiment of FIG. 2A with a second bias circuit;

[0016] FIG. 3 illustrates a circuit level schematic diagram according to a second exemplary embodiment of the present invention;

[0017] FIG. 4 illustrates a circuit level schematic diagram according to a third exemplary embodiment of the present invention; and

[0018] FIG. 5 illustrates a circuit level schematic diagram according to a fourth exemplary embodiment of the present invention.

[0019] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention 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.

[0020] The present invention is designed to prevent burnout of power amplifiers. The most common implementation will be in wireless transmitters such as those used by mobile terminals or wireless modems; however, any power amplifier may be protected from burnout through the teachings of the present invention.

[0021] The present invention modifies a power amplifier into a power amplifier circuit 10, such as illustrated schematically in FIG. 1. The power amplifier circuit 10 comprises a power amplifier (PA) 12, a sensing circuit 14, a processing circuit 16, and a bias circuit 18. The power amplifier 12 receives an RF_INsignal, amplifies it, and produces an RF_OUTsignal. The power amplifier 12 also receives a bias signal from the bias circuit 18. In some embodiments, the bias circuit 18 may be incorporated into the semiconductor having the power amplifier 12, but need not be.

[0022] Given the current desire to minimize extra components, it is possible that the power amplifier 12, the sensing circuit 14, the processing circuit 16, and the bias circuit 18 are all printed as a single monolithic integrated circuit, but again, need not be.

[0023] The power amplifier 12 may be a transistor cell array comparable to those illustrated in U.S. patent applications Ser. No. 09/878,461, filed Jun. 11, 2001; and Ser. No. 09/952,524, filed Sep. 14, 2001; and U.S. Pat. Nos. 6,265,943; 5,608,353, and 5,629,648, which are all hereby incorporated by reference. Alternatively, the power amplifier 12 may be a single transistor, and will be represented as such in the subsequent Figures. It is to be understood, however, that despite its representation as a single transistor, the power amplifier 12 may in fact be a transistor array as described in the previously incorporated applications and patents or any other transistor amplifier configuration available to those skilled in the art.

[0024] The sensing circuit 14 is coupled to the RF_OUTsignal and detects whether the RF_OUTsignal has risen above a predetermined threshold. The predetermined threshold is, in an exemplary embodiment, set below a voltage at which extended operation would cause burnout of the power amplifier 12. The sensing circuit 14 outputs a sensed signal to the processing circuit 16 when the predetermined threshold has been exceeded.

[0025] The processing circuit 16 receives the sensed signal from the sensing circuit 14 and modifies the bias point provided by the bias circuit 18 so as to reduce the bias provided to the power amplifier 12. This causes the power amplifier 12 to have a lower output, thus reducing the chance of burnout. The precise details of the sensing circuit 14 and the processing circuit 16 depend on the embodiment and will be explored further below.

[0026] The bias circuit 18 may be of a number of different types as illustrated below. FIG. 2A illustrates a circuit level diagram of a first embodiment of the present invention with a first bias circuit 18A. The RF_INsignal is coupled to the power amplifier 12 by a capacitor 20. The RF_OUTsignal is generated by the power amplifier 12. In the embodiment shown, the collector of a transistor Q₁, provides the RF_OUTsignal. A transistor Q₃, particularly the base-emitter junction of the transistor Q₃, is used as an avalanche diode to form the sensing circuit 14. The base-emitter junction begins to pass current when the applied reverse voltage exceeds a predetermined threshold. In an exemplary embodiment, the predetermined threshold comprises approximately 8 V. The current rapidly increases from the microampere range to the milliampere range with a small increase in applied voltage above the conduction threshold.

[0027] A resistor R₃controls the magnitude of the current produced by the transistor Q₃. Resistors R₃and R₄, along with capacitor C₂, form a low pass filter for filtering the current pulses produced by the transistor Q₃. The filtered current is then applied to the current mirror 26 comprised of transistors Q₄and Q₅. The current flowing through Q₄is mirrored across to the transistor Q₅as a function of the respective sizes of the two transistors.

[0028] To create the mirrored current, the output of the current mirror 26 (transistor Q₅) sinks bias current from bias circuit 18A to ground that was destined for the power amplifier 12. In particular, as current flows through the transistor Q₅, less current is presented to diode connected transistor Q₂. Thus, less current passes through resistor R₁for use by the power amplifier 12. This reduces the bias on the power amplifier 12 when peak RF voltages start approaching the destruction limits of the power amplifier 12.

[0029] Capacitor C₂, along with resistors R₃and R₄, sets the loop response time. Resistor R₃is determined by the need to limit current spikes through transistor Q₃. The value of R₄determines the rate at which current passes into the current mirror 26. In an exemplary embodiment, the resistances of resistors R₃and R₄is 500 Ω. The capacitor C₂sets the loop time constant. In an exemplary embodiment, the capacitance of capacitor C₂is 1000 pF for loop stability. Where processing circuit 16 is integrated into the monolithic chip that contains the sensing circuit 14 and the power amplifier 12, it may be desirable to split the capacitor C₂such that a portion of the capacitance of the capacitor is off the semiconductor.

[0030] It may be preferable to have a relatively long loop time constant. For example, if the loop time constant is short, an inductive pulse may be generated at the RF_OUTport from the bias and/or matching network. This passes more current through the transistor Q₃, causing a regenerative oscillation. A longer loop time constant reduces the rate of change in the collector current of the power amplifier 12 and increases stability.

[0031] A second bias circuit 18B is illustrated in FIG. 2B. In this embodiment of the bias circuit 18, as current increases in the transistor Q₅, the transistor Q₆turns off because no current flows in R₂. As a result, the collector of the transistor Q₂has little or no current, and the power amplifier 12 has no bias input. Thus, instead of shunting the current to ground, this embodiment significantly reduces or turns off the bias current entirely.

[0032] For further information about the bias circuit 18B, reference is made to commonly owned U.S. patent application Ser. No. 09/467,415, filed Dec. 20, 1999, which is hereby incorporated by reference in its entirety. Bias circuit 18B is sometimes referred to as a buffered passive bias network.

[0033] As would be appreciated, processing circuit 16 may be modified as needed to accommodate differing bias circuits 18.

[0034] In an alternate embodiment, the sensing circuit 14 may be formed by a string of diodes 28A-28E as illustrated in FIG. 3. The number of diodes 28 is determined by the desired threshold voltage above which the feedback loop should be operative. As with the previous embodiment, in this embodiment, this value may be approximately 8 volts.

[0035] FIG. 4 illustrates yet another alternate embodiment of the sensing circuit 14. In this embodiment, the diodes 28 of FIG. 3 are replaced with diode connected transistors Q₇-Q₉. Again, the number of diode connected transistors is determined by the desired threshold voltage above which the feedback loop should be operative.

[0036] FIG. 5 illustrates still another embodiment of sensing circuit 14. In this embodiment, the transistor Q₃of FIG. 2A is replaced with a series of transistors that operate as diodes, much like in FIG. 4. However, a first transistor Q₁₀cooperates with resistors 54, 56 to form a base to emitter multiplier circuit as is well understood. The resistors 54, 56 act to increase the current that is available when the transistor Q₁₀does in fact turn on. Transistors Q₁₁-Q₁₃act to increase the threshold voltage of the sensing circuit 14 so normal operation does not actuate the feedback.

[0037] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.