Fast settling power amplifier regulator

[0001] This invention relates to the field of power amplifiers. In particular, this invention is drawn to techniques for dynamically biasing a regulator for a power amplifier.

[0002] In some applications utilizing power amplifiers, there is a need to vary the output power delivered to a load. For example, in a cell phone environment, it is desired to vary the output power of the cell phone based on various factors. For example, a base station may dictate the power level at which each cell phone should transmit (based on factors such as the physical distance from the base station, for example). A standard method of controlling the output power of a power amplifier is to use a voltage regulator to regulate the battery or power supply voltage.

[0003] FIG. 1 is an example of an RF power amplifier 100 regulated by a voltage regulator 102. The power amplifier 100 receives an RF input signal (RF_IN) and amplifies the signal to generate an RF output signal (RF_OUT). The voltage regulator 102 is connected between the power amplifier 100 and a supply voltage (V_BATTERY) and provides a regulated voltage to the power amplifier 100. The voltage regulator 102 receives a power control signal (V_APC) which relates to a desired output power level.

[0004] FIG. 2 illustrates an example of a prior art power control circuit for regulating a supply voltage using an op-amp feedback circuit. FIG. 2 shows a voltage regulator 102 connected between a voltage supply and a power amplifier 100. The voltage regulator 102 is comprised of a switching device M1 and an op-amp 104, which form a feedback circuit. Since the power amplifier 100 requires high DC current to be drawn from the supply voltage, the switching device M1 will be large and will have a large gate capacitance. It is standard practice in the prior art to compensate the op-amp with the gate capacitance of switching device M1. FIG. 3 illustrates an example of a prior art power control circuit using a current regulator. FIG. 3 shows a current regulator 103, connected between a voltage supply and a power amplifier 100. The current regulator 103 is comprised of switching device M1, an op-amp 104, summer 105, and resister R3. The circuit develops a voltage across the resistor R3 that is proportional to the DC current supplied to the power amplifier 100, which is provided as an input to the op-amp 104.

[0005] One problem with a prior art power control circuit, such as the circuit shown in FIG. 2, is that switching device M1 saturates at high power levels, especially when the battery is low. When the switching device M1 saturates, the voltage between the source and drain of the switching device M1 and the loop gain of the feedback loop goes to zero. The unity gain frequency of the feedback loop also goes to zero. When the unity gain frequency of the feedback loop goes to zero, the waveform illustrated in FIG. 4 is observed. FIG. 4 illustrates the voltage regulator output, V_REG (at the output of voltage regulator 102), versus time. T_X represents the delay from a desired waveform V_APC (at the output of voltage regulator 102) which is introduced to V_REG by regulator 102 when M1 saturates. The switching power envelope at the output of power amplifier 100 that corresponds to waveform V_REG may violate the switching transient specification for an industry standard (e.g., a GSM TDMA standard). Therefore, there is a need to solve the problems described above.

[0006] A circuit is provided for regulating the output power of a power amplifier during a switching transient comprising: a detector circuit coupled to a regulator and to a control signal that is generated to control the output power of the power amplifier, wherein the detector circuit detects switching transients of the power amplifier; and a circuit coupled to the regulator for applying a signal to decrease the settling time of the regulator during a detected switching transient.

[0007] One embodiment includes a circuit for regulating the output power of a power amplifier during a switching transient comprising: a regulator; a detector for detecting a condition relating to the operation of the regulator; and control circuitry coupled to the regulator for decreasing the settling time of regulator in response to a detected condition.

[0008] Another embodiment provides a method of controlling a regulator comprising the steps of: detecting a condition relating to the operation of the regulator; and in response to a detected condition, applying a signal to decrease the settling time of the regulator.

[0009] Another embodiment provides a method of controlling a regulator used with a power amplifier comprising the steps of: providing a first and second regulators; using the first regulator to provide power to the power amplifier; detecting a condition relating to the operation of the regulators; in response to a detected condition, using any combination of the first and second regulators to provide power to the power amplifier.

[0010] Another embodiment provides a method of controlling a regulator for a power amplifier comprising the steps of: providing a regulator; providing current to the regulator to power the power amplifier; and selectively applying a signal to the regulator to control the settling time of the regulator.

[0011] Another embodiment provides a circuit for regulating the output power of a power amplifier during a switching transient comprising: a regulator; a detector for detecting a condition relating to a power amplifier control signal; and control circuitry coupled to the regulator for decreasing its settling time in response to a detected condition.

[0022] The present invention may be used with wireless transmission systems, such as with a power amplifier for a cellular telephone or other device. The invention may also be applied to other types of power amplifiers or circuits that require a regulated voltage supply.

[0023] As mentioned above, when using a power control circuit such as that shown in FIG. 2, the switching device M1 may saturate at high power levels, causing the problems described. One solution is to apply high bias current to the op-amp 104. This results in a high gain and thus requires less voltage across the drain and source of the switching device M1 for a given unity gain frequency. While this solution works, it results in the op-amp 104 burning too much power and decreasing battery life. Described below are additional solutions that overcome these problems without significantly increasing power consumption.

[0024] In general, the present invention provides techniques for dynamically changing the biasing conditions of a regulator to overcome the problems caused by various conditions, such as those described above. The invention is capable of decreasing the delay in the regulator performance by decreasing the settling time of the reulator. In one example, the invention includes a detector for detecting one or more conditions relating to encountered problems. A bias control circuit is coupled to the detector for applying a signal to the voltage regulator to compensate for the detected condition. A timer may be used to control the duration at which the bias current is applied. Following are examples of the present invention, although many more embodiments are possible with the spirit and scope of the invention.

[0025] FIGS. 5 and 6 are block diagrams of a power amplifier having a dynamically biased regulator. The regulators may be any desired type of regulator (e.g., voltage regulators, current regulators, etc.). FIG. 5 shows a regulator 102 coupled between a voltage supply (V_BATTERY) and an RF power amplifier 100. Note that while the figures illustrate an RF power amplifier, the present invention may be used with other types of amplifiers or circuits. Also, while the voltage supply is shown as a battery, other types of voltage supplies may be used. A detector 106 is connected to an amplifier power control signal V_APC. The detector 106 is used to detect one or more conditions relating to problems such as those described above. For example, the detector 106 may be used to detect the power amplifier switching time. In another example, the detector 106 may be used to detect the desired output power level. In another example, the detector 106 may be used to detect when the input voltage provided to the regulator is less than the supply voltage. In another example, the detector 106 may be used to detect when the power amplifier 100 is requesting maximum power. In another example, the detector 106 may be used to detect when regulator 102 is saturated. When a condition is detected, the timer 108 is triggered, which controls the duration in which bias current is applied. In another example, the detector 106 controls the duration in which bias current is applied. The timer 108 is connected to the bias control circuit 110. The bias control circuit 10 is coupled to the regulator 102 and applies bias current to the regulator 102 in response to the detected condition.

[0026] FIG. 6 is a block diagram of a power amplifier having a dynamically biased regulator. Like FIG. 5, FIG. 6 shows a regulator 102, power amplifier 100, detector 106, and a bias control circuit 110. However, FIG. 6 shows an example of a dynamically, biased regulator without the timer 108 shown in FIG. 5. In this example, the bias control circuit 110 may change the bias conditions of the regulator during the time which a condition is detected.

[0027] FIG. 7 is a schematic diagram of one example of a power amplifier having a dynamically biased voltage regulator. In the example shown in FIG. 7, the bias control circuit dynamically biases an op-amp during the switching time of the power amplifier. In this example, the bias current is increased while the power amplifier turns off.

[0028] FIG. 7 shows a power amplifier 100 coupled to a voltage regulator 102. The voltage regulator 102 is coupled between the power amplifier 100 and a voltage supply V_BATTERY and includes a switching device M1 and a feedback loop formed by the switching device M1 and an op-amp 104. A power control signal V_APC is connected to a low pass filter formed by resistor R1 and capacitor C1. The filtered power control signal is connected to one input of the op-amp 104. The unfiltered power control signal V_APC is provided to a detector formed by capacitor C2, resistor R2, and comparitor 112. The sizes of capacitor C2 and resistor R2 are chosen so that they have a slow time constant. A DC current source is also connected to capacitor C2 and resistor R2. The inputs of the comparitor 112 are connected to opposite terminals of the resistor R2, as well as to an arbitrary reference voltage V_REF. When the voltage of the power control signal V_APC drops (such as when the power amplifier turns off), the detector detects the falling edge of V_APC and trips the comparitor 112. The output of the comparitor 112 is connected to a timer 108. In the example shown in FIG. 7, the timer 108 is a 1-shot timer. When the comparitor 112 is tripped, a pulse is sent to the 1-shot timer 108, which turns the timer 108 on for a duration T_D, chosen to be approximately equal to the switching time of the power amplifier 100. The timer 108 is connected to the bias control circuit, which includes switching device M2. The output of the timer 108 causes the switching device M2 to be turned on during T_D (the power amplifier switching time). The switching device M2 is connected to a first current source 114, formed by switching devices M3 and M4 (which is shown connected to a DC current source or a source proportional to the degree of saturation of switching device M1). Another current source 116 is formed by switching devices M5 and M6, which is also connected to switching device M2. The current source 114 supplies the desired bias current to the current source 116 while switching device M2 is turned on by the timer 108. The bias current supplied to the op-amp 104 by the current source 116 then supplies bias current to the op-amp 104 during the duration T_D.

[0029] FIG. 8 is a block diagram of another example of a power amplifier having a dynamically biased voltage regulator. In FIG. 8, an op-amp 104 is shown, including the output stage of the op-amp 104. The output stage of the op-amp 104 includes switching devices M10 and M11 coupled between a voltage supply V_BATTERY and ground. A slew detector 107 is connected to the switching devices M10 and M11 to detect when the output stage of the op-amp 104 is slewing by detecting when I₁₀>k.I₂₀, where I₁₀ and I₂₀ are the currents through the switching devices M10 and M11, respectively, and k is a constant. When the slew detector 107 detects that the op-amp 104 is slewing V_G higher to recover from a saturated condition (e.g., op-amp 104 is saturated), it is desired to quickly increase V_G in order to begin lowering V_REG. For the duration that the slew detector 107 detects this condition, the control circuit 111 turns on switching device M12, which quickly raises the voltage V_G.

[0030] FIGS. 9-12 are timing diagrams illustrating the operation of the circuit shown, in FIG. 7. FIG. 9 is a diagram showing the amplifier control signal V_APC versus time. The waveform shown in FIG. 9 is representative of when the power amplifier 100 switches off at time T1. As the voltage of the power control signal V_APC begins to drop at time T1, the detector detects the falling edge of V_APC. FIG. 10 is a diagram of the voltage V₁ present at the node connecting the capacitor C2, resistor R2, and comparitor 112 (FIG. 7). When the control signal V_APC drops at T1, the voltage V₁ drops down, and slowly rises again (depending on the time constant selected). When V₁ drops at time T1, the comparitor 112 is tripped. When the comparitor 112 is tripped, the 1-shot timer 108 is activated. FIG. 11 is a diagram of the output V₂ of the timer 108. In this example, the output V2 is high from T1 to T2, which corresponds to a duration chosen to approximate T_D, the switching time of the power amplifier. From T1 to T2, the output of the timer 108 turns on switching device M2, which connects the current source 114 to the current source 116, which provides bias current to the op-amp 104. When the bias current is provided to the op-amp 104, the op-amp goes to a high gain state, which overcomes the problems described above. FIG. 12 is a diagram illustrating V_REG with the bias current applied during T1 to T2. As shown, V_REG is equal to V_APC.

[0031] FIG. 13 is a block diagram of another example of the present invention. FIG. 13 shows a power amplifier 100, a detector 106, control circuitry 109, and switching circuitry 124. Rather than a single regulator, FIG. 13 shows first and second regulators 120 and 122 coupled between a voltage source V_BATTERY and the switching circuitry 124. The output of the switching circuitry 124 is coupled to the gate of the switching device M1, which is coupled between V_BATTERY and the power amplifier 100. When the detector 106 detects one or more conditions, the control circuitry 109 is triggered. In one example, during normal operation, the regulator 120 supplies power to the power amplifier 100 via switching circuitry 124 and switching device M1. When the control circuitry 109 is triggered, the switching circuitry 124 causes the second regulator 122 to decrease the settling time of the power supplied through switching device M1. Note that any desired number of regulators may be used. The regulators 120 may be comprised of any desired combination of voltage and/or current regulators. For example, the regulators 120 and 122 may each be voltage or current regulators. In another example, one regulator may be a voltage regulator and the other a current regulator.

[0032] In the examples above where a condition is detected based on the waveform of the amplifier control signal V_APC, the bias control circuit or switching circuitry can be triggered in alternate ways. FIG. 14 is a timing diagram showing a control signal V_ACP corresponding to a power amplifier turning on at T1 and turning off at T2. If the goal is to apply bias current while a power amplifier turns off (at T2), then T2 can be detected directly (described above) or indirectly. For example, the detector can detect T2 by detecting T1 and adding a delay to T1. Similarly, the detector can detect T2 by detecting T0 and adding a delay to T0. The delay circuitry may be incorporated in the timer.

[0033] The present invention has several advantages over the prior art. First, no headroom is required on the regulating switching device M1, so the power amplifier circuit can be designed for lower supply voltages. Second, for devices operating with a battery supply, long battery life is preserved by increasing the current to the power regulator, or by using the current to turn on another op-amp, for a very low duty cycle. Another advantage is that switching transients are suppressed by the invention.

[0034] In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof. Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.