WO2000026680A1

WO2000026680A1 - Apparatus for power management of electrical systems

Info

Publication number: WO2000026680A1
Application number: PCT/US1999/025189
Authority: WO
Inventors: Bertram Buxton
Original assignee: Palm, Inc.
Priority date: 1998-10-30
Filing date: 1999-10-26
Publication date: 2000-05-11

Abstract

A power amplification circuit that acts to maintain the output power of the power amplifier and prevent excessive current draw from the power source by detecting and controlling the current drawn from the power source and supplied to the power amplifier is described. The circuit addresses the design problem of maintaining an output power at a certain power level or within some fixed power range by monitoring and controlling the current supply to the power amplifier. Since the output power of the power amplifier is directly proportional to this input power, the output level of the power amplifier can be controlled by reference to this input current supply.

Description

APPARATUS FORPOWER MANAGEMENT OF ELECTRICAL SYSTEMS

BACKGROUND OF THE INVENTION

Field of the Invention

The invention related to transmission electronics and in particular to a circuit and system for improved power usage for a transmission circuit.

Discussion of the Related Art

Power amplification circuitry for electrical systems have been used in many wireless communication systems. There are two major design issues that often need to be addressed in the design of power amplification circuitry. First, the power available to the power amplifier is limited by the power source. In applications where the power source has a limited lifespan (e.g., batteries), it is usually more efficient to turn off or otherwise disable the power amplifier when the amplifier is not transmitting data since the power amplifier is probably a significant power drain. In addition, the power source may have limited current or voltage capabilities, so care may need to be taken to avoid over stressing and perhaps damaging the power source by drawing too much power.

Second, the transmitter may be required to transmit at a certain power level or within some fixed power range, such as when the transmitter must comply with a formal network protocol that sets such level or range. Steps need to be taken to ensure transmission at the required power levels or within the required power range, or to address failed transmissions caused by an inadequate power supply.

One problem that can make maintenance of a fixed power level difficult is an impedance mismatch. Impedance mismatches are caused by changing loads on the power amplifier, and may increase or reduce the power demands of the transmission system. As the power demand is increased, the power amplifier may fail to produce the power required for successful transmission, or may over stress and damage the power source as it responds to the demand for increased power output. In a wireless transmission application, any touching or moving of the antenna by the operator may cause impedance mismatches.

Where the power source is a battery, such impedance mismatches may cause the power amplifier to draw too much current from the battery, perhaps exceeding recommended maximum currents and reducing the lifespan of the battery. Another problem that can make maintenance of a fixed power level difficult is another consequence of the first design issue — exhaustion of the power source. As the power supply dwindles, usually in the case of a battery, the power amplification system will have more difficulty over time achieving the required output power levels, until power supply ultimately fails.

Existing systems address the first design issue by activating and deactivating the power amplifier as necessary. The amplifier is turned on before use and turned off after use, thus minimizing unnecessary power consumption. "Failsafe" circuitry may also be included to turn off the power amplifier and shut down the system in the event the power demand threatens to damage the power supply. Existing systems address the second design issue, if at all, by use of negative feedback circuitry that stabilizes the output power of a power amplifier at some predetermined power level or within some fixed power range. This circuitry would be comprised of a power level sensor at the output of the power amplifier and a circuit to adjust the gain, and consequently the power consumption, of the power amplifier. As the measured power output falls below or above the desired power level, the gain of the power amplifier would be appropriately adjusted to stabilize the output at the desired level.

This approach, however, introduces two significant problems. First, the introduction of a power sensor introduces a parasitic resistance that reduces the efficiency of the power system. This would be of significant concern in any limited power application. Second, a feedback system that stabilizes power output by reference to some desired power output fails to take into account the limitations of the power supply. The system, as it seeks to maintain a certain power level, may draw excessive current from the power supply, thereby damaging it. If the system includes failsafe circuitry to guard against excessive current, the feedback system may, paradoxically, force the power amplifier to shut down rather than operate at the reduced, though perhaps still effective, power level it would have worked at in the absence of the feedback system.

What is desired is a power amplification circuit that supplies and maintains transmission power at a desired power level or within a fixed power range while minimizing parasitic energy loss and preventing excessive current draw that may damage the power source.

SUMMARY OF THE INVENTION

Accordingly, one embodiment of the invention includes a power amplification circuit that acts to maintain the output power of the power amplifier and prevent excessive current draw from the power source by detecting and controlling the current drawn from the power source and supplied to the power amplifier.

The invention addresses the design problem of maintaining an output power at a certain power level or within some fixed power range by monitoring and controlling the current supply to the power amplifier. Since the output power of the power amplifier is directly proportional to this input power, the output level of the power amplifier can be controlled by reference to this input current supply.

The input current can be determined by measuring the voltage across any resistance between the power source and the power amplifier. In an embodiment, this resistance could be the parasitic resistance of the MOSFET switch that is interjected between the power source and the power amplifier for the purpose of turning the power amplifier on or off in advance of or after use. In this case, no additional parasitic resistance would need to be added for the purpose of sensing the current, eliminating any efficiency loss in connection with the addition of the feedback loop.

The input current to the power amplifier, and hence the current demand on the power supply, is increased or decreased as the input current is determined to be below or above the reference current level. This reference current level is programmed to correlate with the desired output power level of the power amplifier under normal operating conditions. Since the negative feedback loop is designed to stabilize the input current, rather than the output power level, the system avoids the potential problem of damaging, or shutting down, the power supply, since the current demand on the power supply is always stabilized to the reference current level. An additional advantage of the various embodiments are their compact form factors. Integration of the sensing component of the feedback loop with an a power switching MOSFET reduces the relative size of these embodiments compared to the prior art. The present invention has application to a wide range of wireless communication situations, including field data transmissions and cellular communications .

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: Figure 1 shows an embodiment of the invention in a transmission circuit; and

Figure 2 is a schematic of the circuit of Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to an exemplary embodiment of the present invention, which is illustrated in the accompanying drawings. Figure 1 shows an embodiment of the invention in a hand held computer 100, which embodiment is described below. The hand held computer 100 could be a palm sized computer such is a available from Palm Computing, Inc., a division of 3COM Corporation, or a Windows CE palm sized computer. However, other embodiments of the invention do not include the hand held computer 100.

The central element of the system of Figure 1 is the power amplifier 101, which amplifies an input signal by some gain and produces an amplified output signal. The input signal is generated by the input signal circuit 114. The power amplifier 101 has a power supply input port 102 and a gain control input port

103. In the context of the preferred embodiment, the input signal can be a transmission signal intended for outbound communication, and the input signal circuit can be a digital computer microprocessor. The power amplifier 101 in the preferred embodiment is manufactured by Alpha Industries, product number AC664, and has gallium arsenide transistors for efficiency purposes.

The amplified transmission signal may be passed through one or more signal filters 104. In the preferred embodiment, the filter 104 is a ceramic low- pass filter manufactured by muRata Electronics, product number LFTB25N29E0902B. The processed transmission signal is linked to a receive/transmit switch

105. The switch 105, in the preferred embodiment, is manufactured by CP Claire, product number MRF30001. The switch 105 is also coupled to reception circuitry 106 and an antenna 107 for reception and transmission. A power supply 108 is connected, via a MOSFET switch 109, to the power supply input port 102 of the power amplifier 101. In the preferred embodiment, the power supply 108 is a NiCad battery, manufactured by Sanyo, model number 4N-50AAA-TCOM, capable of producing 1.2 amperes. The MOSFET switch 109 is capable of turning the power amplifier 101 on or off by connecting and disconnecting the power supply 108. In the preferred embodiment, the MOSFET switch 109 is manufactured by Siliconix, part number SI6415, and has an inherent resistance of approximately 30 milliohms. This finite resistance may vary from unit to unit by approximately twenty percent in the preferred embodiment.

A differential amplifier 110 measures and amplifies the voltage across the MOSFET switch 109. This amplified voltage is compared to a reference voltage by an integrator 111. The integrator 111 is connected to the gain control input port 103 of the power amplifier 101, and increases or decreases the gain of the power amplifier 101 as the voltage across the MOSFET switch 109 falls below or above the reference voltage. Since the current through the MOSFET switch 109 is proportional to the voltage across the MOSFET switch 109, the input current to the power amplifier 101, and hence the output power of the power amplifier 101, is thereby maintained. In the preferred embodiment, an invertor 113 is added between the integrator 111 and the gain control input port 103 of the power amplifier 101 to provide the negative voltage required by the OPA4340 power amplifier. The invertor 113 in the preferred embodiment is manufactured by Anadigics, product number ANC7660S9C.

Figure 2 is a schematic of the preferred embodiment of the present invention.

The first element of the present invention is a high gain differential amplifier 110 configured to measure and amplify the voltage across the MOSFET switch 109. In the preferred embodiment, the high gain differential amplifier 110 has two operational amplifiers in series which amplify the voltage across the MOSFET switch 109 by an approximate factor of fifty. The first operational amplifier 201 has a positive input port 202 connected to the source of the MOSFET 109, and a negative input port 203 connected to the drain of the MOSFET 109. Both input ports are connected to the MOSFET 109 through certain intermediate resistors, including two resistors, resistor 215 and resistor 216, at the positive input port 202 configured in a voltage divider configuration, and two resistors, resistor 217 and resistor 218, in series at the negative input port 203 configured to remove the common mode voltage. In the preferred embodiment, removal of the common mode voltage is important in light of the voltage across the MOSFET switch 109 in the 5-40 millivolt range. The negative input port 203 is also connected to the output port 204 through a resistor 219 in a negative feedback configuration. The output port 204 of the first operational amplifier 201 is connected to the positive input port 206 of the second operational amplifier 205. The negative input port 207 of the second operational amplifier 205 is connected to the output port 208 of the second operational amplifier 205 via two resistors 220 221 in a negative feedback configuration.

The output port 208 of the second operational amplifier 205 is connected to the positive input port 210 of a third operational amplifier 209, which forms the basis of an integrator 111. The negative input port 211 of the third operational amplifier 209 is connected via a resistor 222 to an 8-bit digital-to- analog convertor chip 213. In the preferred embodiment, this 8-bit DAC 213 is programmed with a table of reference voltages to correspond to certain desired input current levels, and hence power output levels, of the power amplifier 101. The negative input port 211 of the third operational amplifier 209 is also connected via a capacitor 214 to the output port 212 of the third operational amplifier 209 in a negative feedback configuration. In the preferred embodiment, the capacitor 214 is 3300 picofarads. The capacitor 214 acts to dampen the changes of the output of the operational amplifier 209. The combination of the operational amplifier 209, the capacitor 214, and the digital- to-analog convert chip 213 work as a programmable voltage comparator.

By comparing the amplified voltage across the MOSFET switch 109 and the reference voltages supplied by the 8-bit DAC 213, the integrator 111 indirectly compares the input current to the power amplifier 101 and the programmed reference currents. The output of the integrator 111 is a voltage with a relative value that is proportional to the differential voltage measured by the integrator 111. In the preferred embodiment, this output voltage is between one and three volts. The output of the integrator 111 is connected to an inverter 113 that inverts the voltage from positive to negative, and the inverter 113 is connected to the gain control input port 103 of the power amplifier 101. In the preferred embodiment, the gain control input port 103 is at ground or a voltage between -3 and -1 volts. At -3 volts, the current at the power supply input port is approximately zero to thirty milliamperes, and at -1 volt, the current at the power supply input port 102 is approximately 1.2 amperes. Once programmed to a particular DAC 213 setting corresponding to a power output level the outputs of the integrator 111 and the invertor 113 change as the battery 108 voltage decrease due to discharge and power amplifier gain changes due to external influences such as antenna mismatch, and temperature change.

Note, other embodiments of the invention can employ other configurations of circuitry. For example, the elements of the power amplifier 101, the switch 109, the differential amplifier 110, the integrator 111 and/or the other elements of Figure 1 can be integrated onto one or more integrated circuits. In such cases, and in other embodiments, the passive elements can be replaced with active elements, e.g., transistors configured to act as resistors. In other embodiments, some of the elements are combined, e.g., the resistor 217 and the resistor 218. Other configuration can be used in other embodiments.

What is important is that a switch is used to turn the power off to the power amplifier 101 and that the resistance of the switch is used to determine the transmission power from the power amplifier 101.

Accordingly, in the above described embodiments, the current supplied to the power amplifier is controlled, even for significant variations in the load on the power amplifier. These embodiments act to maintain the output power of the power amplifier while preventing excessive current draw and avoiding unnecessary parasitic energy loss.

Claims

THE CLAIMSWhat is claimed is:

1. A power amplification system capable of maintaining a fixed output power level, comprising: a power amplifier, the power amplifier having a power input port and a gain control input port; a switch having a resistance, the switch being coupled to the power input port; a voltage comparator coupled to the gain control input port; a voltage sensor coupled to the voltage comparator and the switch, the voltage sensor configured to detect the voltage across the switch; and a reference voltage generator coupled to the voltage comparator, wherein the voltage comparator controls the gain of the power amplifier using a reference voltage generated by the reference voltage generator.

2. The power amplification system of claim 1, wherein the switch is a metal oxide semiconductor field effect transistor.

3. The power amplification system of claim 1, wherein the reference voltage generator comprises digital-to-analog circuitry for generating the reference voltage.

4. The power amplification system of claim 1 , wherein the voltage comparator comprises of an operational amplifier configured as a voltage comparator.

5. The power amplification system of claim 4, wherein the voltage comparator further includes a capacitor for damping changes in the output of the voltage comparator.

6. The power amplification system of claim 1 , wherein the voltage sensor comprises a differential amplifier configured as a voltage sensor.

7. The power amplification system of claim 1, further comprising a differential amplifier coupled to a resistor and voltage sensor, the differential amplifier configured to amplify the voltage across the resistor.

8. The power amplification system of claim 1, wherein the voltage sensor includes means for removing the common mode voltage.

9. The power amplification system of claim 6, wherein the differential amplifier is comprised of an operational amplifier configured as a differential amplifier.

10. The power amplification system of claim 1 , further comprising a voltage inverter coupled between the voltage comparator and the gain control input port of the power amplifier.

11. A hand held computer system with wireless signal transmission capability, comprising: a handheld computer processing system including a transmission signal circuit for generating a transmission signal; a transmission circuit coupled to receive the transmission signal from the transmission signal circuit and to amplify the transmission signal, the transmission circuit having a power supply current input, a power switch, and a power usage detection circuit, and a power control circuit, the power switch coupled to the power supply current input, the power usage detection circuit coupled to the power switch to determine power usage of the transmission circuit for amplifying the transmission signal, the power control circuit coupled to the power usuage detection circuit to control the current received at the power supply current input; and an antenna coupled to receive and transmit an amplified transmission signal from the transmission circuit.

12. The handheld computer system of claim 11, wherein the handheld computer processing system comprises a microprocessor and software configured to produce the transmission signal.

13. The handheld computer system of claim 11 , wherein the transmission circuit comprises a power amplifier configured to amplify the transmission signal.

14. The handheld computer system of claim 11, further comprising a signal switch coupled to the transmission circuit and configured to select between the transmission signal and a reception signal, and a reception circuit coupled to the signal switch configured to communicate the reception signal to the handheld computer processing system.

15. The handheld computer system of claim 11 , further comprising a power supply wherein the power supply comprises a battery configured to supply the input current.

16. A method for generating an output power level from a power amplifier having an input power port, a resistive switching element coupled to the input power port, an input power current flowing through the resistive switching element and to the input power port, and a reference current, comprising: measuring a signal corresponding to the input power current, the signal being measured using the resistive switching element; comparing the signal to a second signal, the second signal being proportional to the reference current; and increasing or decreasing the input power current in relation to the difference between the signal and the second signal.

17. An apparatus for generating an output power level from a power amplifier, the power amplifier having an input power port, apparatus comprising: a switching means coupled to the input power port for switching an input power current on and off, the switching means having a resistance; a measuring means for measuring a signal corresponding to the input power current, the signal being measured using the resistance of the switching means; means for comparing the signal to a second signal, the second signal being proportional to a reference current; and means for increasing or decreasing the input power current in relation to the difference between the signal and the second signal.