BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a representative plot of the output power of a prior art power amplifier in a time-division multiple-access communications system, such as in GSM cellular telephony;
FIG. 2 is a diagram of a prior art power amplifier and power control system using output voltage detection;
FIG. 3 is a diagram of a representative prior art power versus time plot in a situation where this clipping occurs;
FIG. 4 is a representative prior art power versus time plot in a situation where this wind-up occurs;
FIG. 5 is a diagram of power amplifier and power control system in accordance with an exemplary embodiment of the present invention;
FIG. 6 is a diagram of power control system with increased reliability and efficiency, in accordance with an exemplary embodiment of the present invention;
FIG. 7 is a diagram of power control system with transistors, in accordance with an exemplary embodiment of the present invention;
FIG. 8 is a diagram of a power control system with detector rf attenuators, in accordance with an exemplary embodiment of the present invention;
FIG. 9 is a diagram of power control system with offset cancellation, in accordance with an exemplary embodiment of the present invention;
FIG. 10 is a diagram of power control system with an alternative current detector configuration, in accordance with an exemplary embodiment of the present invention;
FIG. 11 is a diagram of power control system with a distributed active transformer, in accordance with an exemplary embodiment of the present invention; and
FIG. 12 is a flowchart method for power amplifier level control in accordance with an exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the description which follows, like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures may not be to scale and certain components may be shown in generalized or schematic form and identified by commercial designations in the interest of clarity and conciseness.
FIG. 5 is a diagram of power amplifier 212 and power control system 500 in accordance with an exemplary embodiment of the present invention. Power amplifier 212 and power control system 500 can be implemented in silicon, silicon germanium, gallium arsenide, or other suitable materials. Likewise, power amplifier 212 and power control system 500 can be implemented on a single integrated circuit, from discrete components, from a combination of integrated circuits and discrete components, or in other suitable manners.
Power amplifier 212 and power control system 500 can generate a signal 520 that is proportional to the rf voltage envelope at the output and another signal 523 that is proportional to the rf current envelope at the output. The rf voltage envelop of signal 520 can be generated a number of ways, such as through the use of envelope voltage detector 215. Likewise, there are a number of ways to generate a current envelope signal, such as through the use of sense transformer 521 in combination with detector 522. The feedback signal 216 of power control system 500 is generated from the greater of the voltage and current envelope signals, such as by using maximum detector circuit 524 which outputs the greater of its two inputs. Error amplifier 217, which may be an integrating amplifier, a differencing amplifier, or other suitable amplifier, compares the feedback signal 216 to a power control input signal 218 so as to adjust control signal 219 in a way which tends to reduce the difference between the feedback and input signals. In this way, control signal 219 can control the output power of power amplifier 212.
By adjusting the constants of proportionality between the rf voltage and current envelopes and their respective signals 520 and 523, the levels of signals 520 and 523 can be controlled so as to be similar when power amplifier 212 is presented with the nominal/design load, such as 50 Ohms. In this configuration, power control system 500 avoids clipping and wind-up of power amplifier 212, since the mismatch conditions which cause these events typically cause either the voltage or the current envelope to increase from their values when there is no load mismatch. As a result, feedback signal 216 can increase in these conditions, causing the system to behave as if the output power were greater than it actually is, reducing the actual output power so that clipping and wind-up can be reduced or eliminated.
Power control system 500 can also have the benefit of avoiding high voltage stress in power amplifier 212 under load mismatch. Since the output voltage or current envelope typically increases under the conditions that high voltage stress occurs on power amplifier 212, the resulting increase in feedback signal 216 can result in power amplifier 212 reducing its output power, which can reduce the stress. Similarly, power control system 500 can avoid high current stress in power amplifier 212.
FIG. 6 is a diagram of power control system 625 with increased reliability and efficiency, in accordance with an exemplary embodiment of the present invention. Power control system 625 includes detecting devices 626 and 627, which can be diodes or other suitable devices, and holding capacitor 628. The value of sense transformer 521 and holding capacitor 628 are selected so that power control system 600 generates an output signal which is proportional to the greater of the envelopes of the two inputs received from detecting devices 626 and 627. In this manner, maximum detector circuit 524 can be eliminated, which reduces the complexity and increases the reliability and efficiency of power control system 600.
FIG. 7 is a diagram of power control system 700 with transistors, in accordance with an exemplary embodiment of the present invention. Power control system 700 includes transistors 726 and 727 in place of detecting devices 626 and 627. Transistors 726 and 727 are connected so that their gates (if MOSFET), bases (if bipolar), or other suitable control terminals (if other devices) are connected to the rf signal and their sources (if MOSFET), emitters (if bipolar), or other suitable current transmitting terminal (if other devices) are connected to holding capacitor 628, which is coupled to voltage common/ground. The drains (if MOSFET), collector (if bipolar), or other suitable current receiving terminal (if other devices) can be connected to a power supply or other suitable connection. Transistors 726 and 727 allow the rf signal to be connected to a high impedance input, and the current supplied to feedback signal 216 and holding capacitor 628 can be taken from the transistor drain or collector rather than from the rf signal driver, such as power amplifier 212. This can reduce the loading on the rf signal driver.
FIG. 8 is a diagram of a power control system 800 with detector rf attenuators, in accordance with an exemplary embodiment of the present invention. Power control system 800 includes rf attenuators 829 and 830 in series with transistors 726 and 727. Rf attenuators 829 and 830 can be implemented using capacitive dividers as depicted, or in other suitable manners, and provide a predetermined signal level to the control inputs of transistors 726 and 727. For instance, the capacitors or other components of rf attenuators 829 and 830 can be selected so that the signal levels at the control inputs of transistors 726 and 727 are approximately equal to each other when there is no mismatch. Furthermore, rf attenuators 829 and 830 can also reduce the rf voltage level presented to transistors 726 and 727 if un-attenuated signals have voltage levels that are too high to be presented to the control terminals of transistors 726 and 727 directly, such as if there is a potential reliability or dynamic range issue if such signals are not attenuated.
FIG. 9 is a diagram of power control system 900 with offset cancellation, in accordance with an exemplary embodiment of the present invention. Power control system includes an offset cancellation circuit, such as transistor 931 and subtracting amplifier 936. As feedback signal 216 can have a dc offset caused by transistors 726 and 727, a dc value may need to be added to the detected output envelope. However, the dc value may vary as a function of part tolerance variations to part, temperature, or other variables. To reduce this offset variation, transistor 931, which can be more readily matched to transistors 726 and 727, can be used. By placing similar dc biasing conditions on transistors 931, 726 and 727, but no rf input signal on transistor 931, offset signal 933 can be generated that is close to the detected signal 935 that would be generated had the rf envelopes been zero. Subtracting amplifier 936 can subtract this offset signal 933 from the detected signal 935, generating feedback signal 216 with the offset reduced or removed. To ensure that detecting devices are biased properly, current sources 932 and 934 can be used as the pull down elements in the detectors. By appropriately scaling the currents provided by current sources 932 and 934, such as by sizing them proportional to the device areas of the detecting devices each is connected to, the offset may be further reduced.
FIG. 10 is a diagram of power control system 1000 with an alternative current detector configuration, in accordance with an exemplary embodiment of the present invention. Power control system 1000 includes inductor 1038, which is used to detect the current envelope. In many cases, such as when the power amplifier includes an output transformer 1037 to couple signal into the load, there are inductances present in the current path of the output transformer that have currents through them which are substantially the same as the load current, for example, where inductor 1038 is a bond wire that connects the transformer ground to the package ground or voltage common, such that the power amplifier load current effectively flows through the bond wire. In this case, power control system 1000 can sense the power amplifier current envelope by coupling the voltage drop across this bond wire to transistor 726, such as through optional rf attenuator 830. In this way, the current detection path can be made without use of additional components, such as might be used to create sense transformer 521 as shown in FIGS. 5 through 9.
FIG. 11 is a diagram of power control system 1100 with a distributed active transformer, in accordance with an exemplary embodiment of the present invention. Power control system 1100 includes distributed active transformer 1102, such as that disclosed in U.S. Pat. Nos. 6,737,948 and 6,856,199, each of which is hereby incorporated by reference for all purposes. Distributed active transformer 1102 includes a ground wire that operates as inductor 1038, which is used to detect the current envelope. Power control system 1100 senses the current envelope by coupling the voltage drop across this bond wire to transistor 726, such as through optional rf attenuator 830.
FIG. 12 is a flowchart method 1200 for power amplifier level control in accordance with an exemplary embodiment of the present invention. Method 1200 allows a power amplifier level controller to receive a single input that transitions from a voltage envelope control state to a current envelope control state without requiring multiple inputs and other circuitry for selecting between the voltage envelope and the current envelope.
Method 1200 begins at 1202 where a voltage envelope signal is received. In one exemplary embodiment, the voltage envelope signal is derived from the load voltage seen at an output, such as through a capacitive voltage dividing network or other suitable attenuator. The method then proceeds to 1204.
At 1204, a current envelope signal is received. In one exemplary embodiment, the current envelope signal can be generated by a capacitive voltage dividing network or other suitable attenuator, such as one that is in parallel with a bond wire for a secondary winding of a circular geometry power amplifier or other suitable current envelope signals. The method then proceeds to 1206.
At 1206, it is determined whether the voltage envelope signal is not equal to the current envelope signal. If the power amplifier is providing power at a level that is within the rating of the power amplifier, then the voltage envelope signal will equal the current envelope signal, and no change in state will occur until the voltage envelope signal is greater than or lesser than the current envelope signal. If the voltage envelope signal is greater than the current envelope signal, the method proceeds to 1208.
The 1208, the power control is derived from the voltage envelope signal. In one exemplary embodiment, an increase in the voltage envelope signal may occur if the load being driven by the power amplifier increases in impedance, such as due to a VSWR event, capacitive coupling, inductive coupling, or some other effect. The method then proceeds to 1208 where it is determined whether the voltage envelope signal has changed and is now less than or equal to the current envelope signal. If the voltage envelope signal has not changed or has become equal to the current envelope signal, then the method proceeds to 1216 where it is determined whether there has been any loss of signal. If no loss of signal has been detected the method returns to 1206, where it is determined whether the voltage envelope signal and current envelope signals match.
Likewise, if it is determined that 1210 that the voltage envelope signal has dropped to a level below that of the current envelope signal, the method proceeds to 1212 where power control is derived from the current envelope signal, such as to limit the power amplifier output if the load impedance has decreased below a predetermined allowable level. The method then proceeds to 1214 where it is determined whether the voltage envelope signal is now greater than or equal to the current envelope signal. If the voltage envelope signal is greater than or equal to the current envelope signal, the method returns to 1206, otherwise the method proceeds to 1218 where it is determined whether a loss of signal has occurred. If no loss of signal has occurred the method returns to 1206.
Likewise, if it is determined at either 1216 or 1218 that a loss of signal has occurred, then the method proceeds to 1220 where the power amplifier output is shut down.
In operation, method 1200 allows the control of a power amplifier to be maintained based on the greater of a voltage envelope signal or a current envelope signal without requiring separate control circuitry for each. In one exemplary embodiment, method 1200 can be used in power amplifiers having bond wire connections where a voltage is generated that is proportional to a current being provided to a load, and also where the load voltage can be measured, such as by using a capacitive voltage divider.
In view of the above detailed description of the present invention and associated drawings, other modifications and variations are apparent to those skilled in the art. It is also apparent that such other modifications and variations may be effected without departing from the spirit and scope of the present invention.
Patent Application
20080061874
