Abstract:
A circuit for controlling a high frequency power amplifier for amplifying a signal for wireless transmission is disclosed. The circuit provides a control signal to the power amplifier at an output port thereof. The control signal has properties of rise time and fall time, where a delayed transition in this signal is based on charging and discharging of the capacitor using two current mirror circuits in order to provide the delayed transition. An advantage lies in that the time of this delayed transition is dependent primarily upon the current mirrors and substantially other than dependent upon the RC time constant of the circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates to the area of power amplifier control circuits and more specifically to the area of power amplifier integrated control circuits for use in wireless applications. 
     BACKGROUND OF THE INVENTION 
     Wireless connectivity between electronic devices is gaining popularity as wireless enabling technologies, such as Bluetooth ™ and IEEE 802.11b are integrated into an increasing amount of devices. Transceivers that comply with the Bluetooth ™ wireless specification operate in an unlicensed, 2.4 GHz radio spectrum thus ensuring communication compatibility worldwide. These transceivers use a spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz intervals and therefore provides a high degree of interference immunity while communicating using wireless technology. 
     In order for wireless devices to communicate, transmitters provide a communication signal to a receiver. Within these wireless devices, such as those using Bluetooth™, there are power amplifiers for amplifying a signal to a desired power level before they are broadcast through a transmitter antenna of the wireless transceiver. Power amplifiers are used for amplifying low intensity, low amplitude electrical signals in order to produce a higher power and higher amplitude amplified output signal. Depending on the amount of gain provided by the amplifier, the amplifiers usually dominate power consumption of the transceiver. 
     Power amplifier performance is judged under realistic operating conditions in terms of Adjacent Channel Power Ratio (ACPR) measurement. Prior to taking an ACPR measurement, a signal residing in a desired frequency channel is modulated using a digital modulation scheme, such as that set forth in adherence to the IEEE 802.11b modulation standard and output power in an adjacent channel, with respect to the desired frequency channel, is measured. The ACPR is a ratio of electrical power in a desired frequency channel compared to that in another adjacent channel, thus giving an indication of frequency spreading of the modulated input channel. If the ACPR is high then there is no energy in the adjacent channel. If spectral re-growth occurs—where electrical power is injected in frequency bands adjacent to the modulated input channel—then the ACPR is reduced. 
     When switching power amplifiers (PAs) on and off the switching causes “spectral splatter.” Spectral splatter is the generation of energy in adjacent channels. This spectral splatter is harmful especially for radio communication equipment that uses a narrow channel bandwidth. Skirts showing up in the adjacent channel as a result of modulation may affect sensitivity of radio reception. One way to reduce spectrum re-growth is to have gradual rising and falling switching edges, thereby eliminating high frequencies that are associated with sharp modulation transition edges. 
     Delay circuits, which can be used for providing gradual rising and falling switching edges, using resistor and capacitor networks and are known in the prior art. For instance: in U.S. Pat. No. 4,983,931, a resistor divider network is used to charge and discharge a capacitor. U.S. Pat. No. 5,108,133 provides a ramp circuit used for providing an EEPROM programming signal. U.S. Pat. No. 5,523,645 provides a circuit used for regulating a charge time of an output node of an amplifier on start up. U.S. Pat. No. 5,619,115 uses a current limiting circuit to charge and discharge a capacitor for purposes of charge integration. U.S. Pat. No. 5,825,218 discloses a voltage ramp generator used for discharging of a capacitor in two stages, where a discharging current source discharges the capacitor from a maximum voltage the capacitor reaches down to a low voltage above ground potential, where another discharging current source discharges the capacitor from the low voltage reached by the first source to ground. U.S. Pat. No. 6,369,626 uses a low pass filter circuit coupled to a capacitor for use in a delay lock loop circuit. Unfortunately these circuits are typically not geared towards amplifiers and more particularly feature longer rise times and fall times and would therefore not be useable for wireless applications. U.S. Pat. No. 6,169,886 a power amplifier output power level is controlled using a ramp circuit for use in amplifying an input signal of varying input power levels. However, the ramp used within this circuit is more applicable to signals having varying input power levels and not for amplitude modulating of the signal. 
     The longer the rise time or the fall time of a power amplifier the larger the reduction in spectral re-growth, however it is known to those of skill in the art that longer rise times and fall times are not conducive to fast data transmission rates. Some applications like Bluetooth ™ require rise times and fall times in the order of 2˜4μs, while other applications need shorter rise times and fall times. 
     It is therefore an object of the invention to provide a ramping circuit for use in amplitude modulation of a power amplifier in wireless applications in order to reduce spectrum re-growth occurring in an adjacent wireless channel as a result of amplitude modulation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a circuit for controlling a high frequency power amplifier for amplifying a signal for wireless transmission comprising: an output port, a capacitor electrically coupled to the output port, a current source-sink coupled with the capacitor for providing current thereto and sinking current therefrom, the current source operable in a first state mode to charge the capacitor and operable in a second state mode to discharge the capacitor, wherein, in use, a transition between said the first modestate and the said second state mode results in a delayed transition of an output signal at the output port between first and second output signal levels having a delay transition time of the delayed transition being other than substantially related to an RC time constant of the circuit. 
     In accordance with an embodiment, the circuit is absent a resistor in series with the capacitor for forming an RC circuit. 
     In accordance with an embodiment, the circuit is for use in a BlueTooth™ wireless transmitter. 
     In accordance with another embodiment of the invention there is provided a circuit for controlling a high frequency power amplifier for amplifying a signal for wireless transmission comprising: an input port for receiving a first input signal for amplification thereof; an output port; a control port for receiving a control signal; a capacitor electrically coupled to the control port for providing a control signal thereto in dependence upon a charge on the capacitor; a source current mirror electrically coupled with the capacitor and a sink current mirror electrically coupled with the capacitor, the source and sink current mirrors for providing current to the capacitor and sinking current therefrom, the current mirrors operable in a first mode to charge the capacitor and operable in a second mode to discharge the capacitor; and, a power amplifier circuit coupled to the first input port for receiving the first input signal and for amplifying the first input signal in dependence upon the control signal, the amplified first input signal forming the output signal and provided at the output port; wherein, in use, a transition between said the first mode and the second mode results in a delayed transition of the control signal at the control port between first and second control signal levels, with a transition time the delayed transition being other than substantially related to an RC time constant of the circuit. 
     In accordance with another aspect of the invention there is provided a method of modulating a power amplifier output signal level comprising the steps of: providing a capacitor; receiving a control signal having one of a first level and a second level; initiating a charging of said capacitor at a linear rate when said control signal changes from the second level to the first level; varying the power amplifier output signal level from minimum level to a maximum level during a linear charging of said capacitor, where a time taken for said charging is a first transition time; initiating a discharge of said capacitor at the linear rate when said control signal changes from the first level to the second level; and, varying the power amplifier output signal level from the maximum level to the minimum level during a linear discharging of said capacitor, where a time taken for said charging is a second transition time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a typical prior art conventional power amplifier ramp control circuit; 
     FIG. 2 a  illustrates a novel ramp circuit in accordance with a preferred embodiment of the invention and 
     FIG. 2 b  illustrates a variation of the preferred embodiment of the invention; and,. 
     FIG. 3 illustrates output spectra of the power amplifier being operated using the conventional power amplifier control circuit and being operated using the ramp circuit in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a prior art conventional power amplifier ramp control circuit  100 , wherein based on an enable voltage applied to an enable input port  101 , a Vcc supply voltage is applied to a power amplifier circuit (PA) (not shown) through an output port  102 . Within this circuit there is a resistor  103  and a capacitor  104  forming a resistor-capacitor (RC) circuit. The RC circuit within circuit  100  causes a delayed output voltage applied to the PA in response to an enable voltage (V en ) applied to the enable input port. The amount of delay is dependent on values predetermined for RC network used to implement such functionality. This delay controls a rise time and a fall time of this circuit  100  in response to V en . The rise time is defined as an amount of time taken between a minimum predetermined supply voltage being applied to the PA circuit and a maximum predetermined supply voltage being applied to the PA, in response to a transition in V en . The fall time is defined as an amount of time taken between a maximum predetermined supply voltage being applied to the PA and a minimum predetermined supply voltage being applied to the PA, in response to the transition in V en . The transition in V en  defined as either a transition from a logic HI to a logic LO level, or a transition from a logic LO to a logic HI level. It is known to those of skill in the art that when the PA is utilized in conjunction with Bluetooth™ transmitter applications rise times or fall times within 1˜2μs are preferred. Of course, for other applications longer or shorter rise times and fall times may be preferable. 
     Unfortunately, using such a circuit is non-advantageous. There are two issues associated with the use of the RC network that render their use problematic. Firstly, it is known to those of skill in the art that RC networks have a RC time constant associated therewith, where a delay in an output signal rise and fall time of the RC network is dependent on an exponential function of the time constant. Secondly, in order to attain the preferred 2μs rise/fall time, the RC time constant needs to be around 1μs. In order to achieve this preferred time constant a 5pF capacitor is selected with a 200Ωresistor within the RC network. Of course, other combinations of resistors and capacitors will also work to attain the same time constant as would be apparent to those of skill in the art. 
     It is also apparent to those of skill in the art that laying out of this circuit  100  on a die to form an integrated circuit typically requires a large amount of space on the die. Resistors and capacitors are known to occupy a large amount of space in integrated circuits, where capacitors are the worst offenders, occupying the most space, followed by resistors. Depending on the ramping time required, using an RC network is not optimal since a large amount of space will be utilized in order to lay these components out on the die, especially when longer delay times are required and larger RC values are utilized for larger time constants. Having a larger die also increases device cost since less devices are fit onto a same wafer. 
     FIG. 2 a  illustrates a preferred embodiment of the ramp circuit  200 . This ramp circuit  200  preferably combines a ramping function and power amplifier power supply functionality within a single integrated circuit that significantly reduces die area requirements, especially when long delay times are required. In the embodiment of FIG. 2 a , the reduced circuit size is a result of two current mirrors utilized within the circuit for providing a required delay. 
     Based on an enable voltage (Ven), or control signal, applied to an enable input port  201  a PA supply voltage (Vcc) is applied to a power amplifier circuit (PA)  213  through an output port  202 . Within the ramp circuit  200  there is a current source-sink in the form of an upper current mirror  203  and a lower current mirror  204 . The upper current mirror  203  comprises a first PMOS transistor  231  and a second PMOS transistor  232 . The lower current mirror  204  comprises a first NMOS transistor  241  and a second NMOS transistor  242 . Ven is provided to the upper current mirror first PMOS transistor  231  drain through an upper resistor  205  and to the lower current mirror first NMOS transistor  241  drain through a lower resistor  206 . The second PMOS transistor  232  drain and the second NMOS transistor  242  drain are coupled to a common node. A capacitor is provided in parallel with the second NMOS transistor  242  drain and source, with the common node coupled to a gate of a PMOS output transistor  210 . A positive supply voltage is provided to the ramp circuit  200  through a positive input port  212  and a ground potential is applied to the ramp circuit  200  through a ground input port  211 . The PMOS output transistor  210  has its source coupled to the positive input port  212  and its drain coupled to the output port  202 . FIG. 2 b  illustrates a variation of the preferred embodiment of the invention, where in this variation a small resistance  214  is disposed between the common node and the gate of a PMOS output transistor  210  and in series with the capacitor  208 . 
     In use, when V en  is LO, at a first level, the upper PMOS current mirror  203  is active, thereby operating in a first mode by charging the capacitor  208  at a linear rate until an upper limit, second output signal level, is reached. During this time the PA is enabled having its output controlled by a ramp signal caused by charging of the capacitor. This charging of the capacitor causes the PA output power to ramp from a minimum level to a maximum level in a transition time controlled by the current mirror  203  and the capacitor  208 . When V en  is HI, at a second level, the lower NMOS current mirror  204  is active, thereby operating in a second mode by discharging the capacitor at a linear rate until a lower limit, first output signal level is reached. During this time the PA is enabled, with its output power controlled by the ramp signal. This discharging of the capacitor results in the PA output power to ramp from the maximum level to a minimum level in a time controlled by the current mirror  204  and the capacitor  208 . The rise time and the fall time are determined by a charge and discharge current of the capacitor. The charge and discharge current of the capacitor is selected by choosing values for the resistors and the ratio of the current mirror currents. 
     FIG. 3 illustrates output spectra of the PA being operated using the conventional power amplifier control circuit  100  and being operated using the ramp circuit  200  in accordance with the preferred embodiment. A standard requirement is also provided on this graph to act as a guide in order to illustrate a preferred Bluetooth™ channel profile in accordance with a predetermined specification. As can be seen from this graph it is evident that operating the PA using the circuit  100  without ramp functionality results in the modulated channel profile falling outside of the requirements, such that in the adjacent channels a power level is approximately 6 dB higher for this signal without using the ramp circuit  200 . The conventional circuit also provides for a maximum amplifier gain difference of approximately 45 dB, as opposed to over 60 dB realized when using the preferred embodiment. This modulated channel causes spectral splatter in adjacent channels and for low adjacent channel modulation powers may cause erroneous information. 
     Advantageously, by using the new ramp circuit a voltage at the capacitor rises and falls linearly with a charge and discharge current that is predetermined by the current mirrors. An approximately symmetric rise and fall time is achievable without experiencing extra settling for the power amplifier when reaching the supply voltage, an important requirement for some applications. Another advantage of using the new ramp circuit is that a size of the capacitor can be greatly reduced by choosing a ratio of current mirror parameters. Advantageously, since the capacitor size is reduced the die area used by components making up the ramp circuit is also reduced, thereby offering a decreased device cost as a result of being able to manufacture more devices per wafer. Of course, the resistor is also variable to facilitate ramp circuit operation. It is also clear to those of skill in the art that the ramping circuit is not only useable in wireless application, such as those employing Bluetooth™, but also for use in cellular telephones amplifiers. 
     Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.