Abstract:
A power amplifier (PA) system with PA gain correction is disclosed. The PA system includes a PA having a bias voltage input; and electrothermal feedback circuitry coupled to the bias voltage input. The electrothermal feedback circuitry is configured to receive thermal feedback generated by the PA and maintain a substantially constant PA gain by automatically changing a bias voltage level at the bias voltage input based upon the thermal feedback.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 61/878,773, filed Sep. 17, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to automatic gain control of power amplifiers (PAs). More particularly, the present disclosure provides electrothermal feedback to correct power gain of PAs during long burst and short burst operation. 
     BACKGROUND 
     Dynamic error vector magnitude (DEVM) is an important customer specification of wireless local area network (WLAN) power amplifiers (PAs). It is defined as the DEVM of the PA operated below unity duty cycles. In this operation, the PA is turned on and off and its on-off time depends on the duty cycle as well as the burst length. When a PA is pulsed-on, the temperature of the PA cells increases over the burst due to power dissipation and then decreases when the PA is pulsed-off.  FIG. 1  illustrates this situation and shows a typical corresponding temperature fluctuation profile of the PA cells. While the temperature changing during PA idle does not affect DEVM, the temperature increase during PA on causes the PA power gain, as well as its bias point during the burst, to deviate from their initial values. Both of these result in the final gain of the PA drifting gradually away from its initial value. For example,  FIG. 2  shows a typical WLAN PA gain profile over the burst. If the WLAN system is aware of this gain deviation via amplitude tracking, the gain can be corrected, but only while consuming extra system computing power. However, in order to save system power and reduce system complexity, most customers do not implement amplitude tracking. Thus, any gain correction in such a system must hold constant by way of the PA level in order to yield good DEVM. In practice however, this is a relatively challenging task since the temperature and gain profiles of a PA cell depend not only on various electrical parameters such as bias, and various thermal parameters such as the layout, but also depend on RF power level.  FIG. 1  is a graph showing a corresponding temperature fluctuation profile for related art PA cells. The temperature of a PA cell typically follows a first order heating and cooling cycle with each pulse of regulated voltage VREG and radio frequency input (RFIN).  FIG. 2  is a graph showing a typical related art wireless local area network (WLAN) power amplifier (PA) gain profile over a data burst. The PA typically deviates from the ideal PA gain with each pulse of regulated voltage VREG and RFIN. Furthermore, the PA gain profile is also dependent on the pulse length making it even more challenging to design a WLAN PA DEVM-compatible with various 802.11 standards over a wide range of burst lengths. 
       FIG. 3  is a schematic of a related art PA system  10  that includes a speed-up circuit  12  used to control the burst gain profile of a PA  14  in order to improve dynamic error vector magnitude (DEVM). The speed-up circuit  12  comprises a resistor RS 1  coupled between the regulated voltage VREG and a generic PA bias circuit  16  having a bias output  18 . A series resistor-capacitor (RC) circuit made up of a second resistor RS 2  and a capacitor CS 1  is coupled in parallel with the resistor RS 1 . The related art PA system  10  includes the generic PA bias circuit  16  that receives a collector voltage VCC and the regulated voltage VREG that is modified by the speed-up circuit  12  to become VREG′. The PA  14  includes a bias input  20  that is coupled to the bias output  18 . A plurality of bias resistors R B1  through R BN  are coupled to control inputs of transistors Q CELL-1  through +Q CELL-N . The PA  14  also includes an input IN and a coupling capacitor Cb that is coupled between the input IN and a control of the transistor Q CELL-1 . While the PA  14  can be the WLAN type, it is to be understood that the PA  14  can also be other types as well. 
       FIG. 4  shows the intended result of the operation of the speed-up circuit  12 , which is a flat gain for the PA  14 . In particular, the speed-up circuit  12  introduces a spike in a VREG′ pulse at the beginning of the on cycle, and accordingly a current spike ΔICC in a PA bias output VB. For typical class AB WLAN PAs, such a spike in the bias current will result in a proportional gain spike, ΔG, relative to its original profile (G). The resulting gain profile, G′, with the speed-up circuit, is therefore compensated to be a flat profile over a data burst. 
     By tuning the capacitor CS 1  and the resistors RS 1  and RS 2 , the amplitude and shape of AG can be controlled, and hence also the final PA gain profile. However, since there are only three circuit elements in the resistor-capacitor (RC) speed-up circuit  12 , the degree/order of pulse shaping is thus limited (in terms of the achievable pulse shape and length). Although the degree/order of pulse shaping can be improved in principle by increasing the number of RC elements in the network, in practical terms this is constrained by the complexity and cost allowed for consumer products with which the PA system  10  is integrated. For example, consumer products are limited by the number of surface mount (SMT) component spots available on a customer board and/or in the PA laminate module (not shown). Also critical is the cost of these additional components. This is especially true since the capacitors required are typically in hundreds of nF ranges. Furthermore, since the compensation is essentially open loop and does not track the root cause of the DEVM issue, which among other lesser factors is due to a temperature pulse profile of the PA cells that make up the PA  14 , it is less effective when PA operating conditions such as variations in ambient temperature and output power result in degraded DEVM over temperature and/or power. In fact, this compensation can be less effective even over PA process and SMT component tolerance, potentially resulting in low product yield. The speed-up circuit  12  is also limited in compensating an increasing burst gain profile. 
     Depending on the design of PA  14 , sometimes a WLAN PA may actually behave with a decreasing gain profile and in principle a slow-down circuit  22  shown in  FIG. 5  may be used in place of the speed-up circuit  12  ( FIG. 4 ). However, the slow-down circuit  22  slows down PA turn-on undesirably and is therefore in conflict with typical PA turn-on time specifications. Thus, the slow-down circuit  22  lacks a solution for WLAN PAs with decreasing burst gain profile. What is needed is a PA system having circuitry that provides PA gain correction for PAs such as a WLAN PA that is DEVM-compatible with various 802.11 standards over a wide range of burst lengths. 
     SUMMARY 
     A power amplifier (PA) system with PA gain correction is disclosed. The PA system includes a PA having a bias voltage input; and electrothermal feedback circuitry coupled to the bias voltage input. The electrothermal feedback circuitry is configured to receive thermal feedback generated by the PA and maintain a substantially constant PA gain by automatically changing a bias voltage level at the bias voltage input based upon the thermal feedback. 
     In at least one embodiment, the PA system further includes a PA bias circuit having a bias output communicatively coupled to the bias voltage input of the PA, wherein the PA bias circuit is configured to generate a bias voltage level that is automatically modifiable by the electrothermal feedback circuitry to substantially maintain a constant PA gain based upon thermal feedback of heat generated by the PA. 
     Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  is a graph showing a corresponding temperature fluctuation profile for related art power amplifier (PA) cells. 
         FIG. 2  is a graph showing a typical related art wireless local area network (WLAN) PA gain profile over a data burst. 
         FIG. 3  is a schematic of a related art PA system that includes a speed-up circuit used to control the burst gain profile of a WLAN PA in order to improve dynamic error vector magnitude (DEVM). 
         FIG. 4  is a graph showing typical related art pulse waveforms that result from the operation of the speed-up circuit of  FIG. 3 . 
         FIG. 5  is a schematic of a related art PA system that includes a slow-down circuit used to control the burst gain profile of a WLAN PA in order to improve DEVM. 
         FIG. 6  is a schematic of a PA system that in accordance with the present disclosure includes electrothermal feedback circuitry configured to improve gain burst profile flatness over time as well as DEVM of WLAN PAs. 
         FIG. 7  is a schematic of a PA system that in accordance with the present disclosure includes modified electrothermal feedback circuitry configured to provide positive feedback to compensate for a decreasing profile over a data burst. 
         FIG. 8  is a block diagram of an exemplary layout and location of a reference device and sensor device for a PA system. 
         FIG. 9  is a schematic of a PA system that in accordance with the present disclosure includes modified electrothermal feedback circuitry configured to minimize RF coupling between the RF path and the electrothermal feedback circuitry. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure 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. 
       FIG. 6  is a schematic of a power amplifier (PA) system  24  that in accordance with the present disclosure includes electrothermal feedback circuitry  26  configured to improve gain burst profile flatness over time as well as dynamic error vector magnitude (DEVM) of wireless local area networks (WLAN) PAs. 
     A transistor Q 1  is a reference device and is thermally de-coupled from the PA cells making up the PA  14 . A reference electrical current I REF  that flows through a resistor R 1  during operation is set by the size of the transistor Q 1  as well as the value of resistance of resistor R 1 . The resistor R 1  is coupled between the regulated input VREG and a current input  28  of the transistor Q 1 . Another transistor Q 2  is a sensor device that is thermally coupled with the PA cells of PA  14 . A second resistor R 2  is coupled between the bias output  18  of the generic PA bias circuit  16  and the bias input  20  of the PA  14 . The second resistor R 2  limits the current flowing to the bias input  20  of the PA  14 . The resistor R 1  and the second resistor R 2  can be the variable resistance type to allow adjustable resistance control to compensate for variability in manufacturing processes, etc. A current input  30  of the transistor Q 2  is coupled to the bias input  20  of the PA  14 . Control input  32  of transistor Q 1  and the control input  34  of transistor Q 2  are coupled together via a third resistor R 3 , and the control input  32  of the transistor Q 1  is coupled to the current input  28  of the transistor Q 1  in a self-bias configuration. The transistor Q 2  converts, together with transistor Q 1 , the temperature profile of the PA cell into an electrical current I s  that is proportional to and substantially in the shape of an increasing temperature profile of the PA cells of PA  14 . That is,
 
I s =αTc,  EQ. 1
 
where Tc is the PA cell temperature profile over the burst and α is some positive proportionality constant, which can be shown to be dependent on R 1 , R 3  and the size of Q 1  and Q 2 . The electrical current I s  is then converted into PA cell bias voltage VB via a small series resistor, R 2 , between the generic PA bias circuit and the PA cell as
 
 ΔVB=VB′−VB=−i   s   R 2=−α TcR 2,  EQ. 2
 
where VB is the bias voltage immediately after the generic PA bias, VB′ is the bias voltage for the PA cells and ΔVB is the change of the PA bias voltage due to I s . If we assume the small change of ΔVB induce, proportionally, a small change of PA cell gain, i.e.
 
Δ G=βΔVB   EQ. 3
 
where β is some positive proportionality constant and ΔG is the PA gain change due to PA bias change ΔVB, then we have:
 
 ΔG=β (−α TcR 2)=(−αβ r 2) Tc.   EQ. 4
 
     The term ΔG is also referred to as the electrothermal loop gain and ΔG is negative as PA cell temperature rises due to the minus sign in the equation above. As a result, ΔG evaluates as a decreasing gain for an increasing Tc. Furthermore, the magnitude of ΔG can be controlled easily by R 2  and/or the proportionality constant α. Since a can be shown to be dependent on R 1 , R 2 , R 3  and the size of Q 1  and Q 2 , the electrothermal loop gain ΔG can therefore be easily controlled by resistors R 1 , R 2 , and R 3  along with the size of Q 1  and Q 2 . Thus, by controlling these parameters, a raw PA gain deviation profile such as that shown in  FIG. 2  can be compensated by equal and opposite electrothermal loop gain ΔG to yield a flat overall gain profile over the burst. The flat overall gain profile solves the DEVM issue of the WLAN PA. 
     Unlike related art techniques, the electrothermal feedback circuitry  26  ( FIG. 6 ) directly senses a PA cell temperature deviation profile which is strongly correlated to the raw PA gain profile of the PA cell in terms of both magnitude and shape. Therefore, any conditions that will affect the deviating PA temperature profile, such as the RF power level, process change, as well as the burst length is tracked by the electrothermal feedback circuitry  26 . Since the raw PA gain profile is closely related to the PA temperature profile, the electrothermal feedback circuitry  26  is relatively more effective at maintaining flat gain over process, power, and burst length than related art gain control circuitry and techniques. Moreover, the gain control provided by the electrothermal feedback circuitry  26  is free of component tolerances associated with surface mount (SMT) devices. Further still, the electrothermal feedback circuitry  26  does not require the relatively large nF capacitors often needed by related art circuitry and, as a result, needs relatively little die area to implement. By exchanging the reference device that is transistor Q 1  and the sensor device that is transistor Q 2 , the electrothermal feedback circuitry  26  can be reconfigured to yield a positive electrothermal loop gain. 
       FIG. 7  shows a reconfigured electrothermal feedback circuitry  36  that provides a positive loop gain. That is,
 
Δ G=+γTc,   EQ. 5
 
where γ is a positive proportionality constant. Under this modified configuration, the reconfigured electrothermal feedback circuitry  36  is now capable of compensating raw PA gain profile that has a decreasing profile over the burst (i.e., a “drooping” profile).
 
     Unlike related art techniques such as the slow-down circuit  22  of  FIG. 5 , the reconfigured electrothermal feedback circuitry  36  does not compromise PA turn on response time, which is more desirable and useful for PAs requiring relatively strict turn-on time specifications. Structurally, the current input of the transistor Q 1  is coupled between the series coupling of the resistor R 2  and the bias input  20 . Moreover, the current input of transistor Q 1  and the control input of second transistor Q 2  are coupled together in a self-biasing configuration in place of the self-biasing configuration of the transistor Q 1  shown in  FIG. 6 . In this reconfiguration, the transistor Q 1  and PA  14  are thermally decoupled, and the second transistor Q 2  and PA  14  are thermally coupled. 
       FIG. 8  is a block diagram of an exemplary layout and location of a reference device and sensor device for a PA system such as the PA system  24  shown in  FIG. 6 . The layout and location of the reference and sensor devices is a critical and integral part of the electrothermal feedback circuitry  26  disclosure. Moreover, the layout and location of the reference and sensor devices include careful considerations to ensure both effective thermal coupling and de-couplings of the relevant devices and RF decoupling of the electrothermal feedback circuitry  26  circuit from RF path on the die. 
     It is relatively important to thermally de-couple transistor Q 1  from any hot spot on the die, especially from the PA cells of PA  14 . A layout procedure of a PA system such as PA  14  ( FIG. 6 ) carefully locates the transistor Q 1  on a die where the temperature fluctuation, while the PA is pulsed, is minimal. For example, transistor Q 1  is usually placed at a location that is around about at least 50 μm away from any PA cell of PA  14  and is typically placed immediately next to a metallized VIA such as a ground VIA. However, it is to be understood that distance between any PA cell of PA  14  and a thermally coupled or de-coupled device is a variable that is a function of material characteristics and other design criteria that is evaluated on a case by case basis. Thus, the given distance of &gt;50 μm in  FIG. 8  is exemplary and not to be interpreted as limiting. Typically, the reference device in this case transistor Q 1  is physically coupled to a metallization directly coupled to the reference device. As shown in the exemplary embodiment of  FIG. 8 , a chunk area of metal  38  can be attached to the emitter of transistor Q 1  to further reduce the temperature fluctuations of transistor Q 1 . The sensor device, transistor Q 2 , is relatively tightly thermally coupled to the PA cells of PA  14  in order to sense the temperature fluctuation of PA  14 . Typically, transistor Q 2  is located, but is not limited to, a closest one of the PA cells while complying with layout design rules checking (DRC) rules. 
       FIG. 8  also depicts a reconfiguration shown in short dashed line wherein the first transistor Q 1  is the sensor device and the second transistor is the reference device. By exchanging the reference device that is Q 1  and the sensor device that is Q 2 , the electrothermal feedback circuitry  26  ( FIG. 6 ) can be reconfigured to yield a positive electrothermal loop gain as shown in  FIG. 7 . In this reconfiguration, the first transistor Q 1  and the PA  14  are thermally decoupled, and the second transistor Q 2  and the PA  14  are thermally coupled. 
     It is relatively important to minimize any RF coupling from any RF paths associated with the PA system  24  ( FIG. 6 ). During layout, any opportunity for the interconnection of metals of the electrothermal feedback circuitry  26  to cross over an RF path should be minimized. Jumper wires (not shown) can be used to avoid RF coupling when crossover is otherwise inevitable. In addition, an RF decoupling capacitor should be added to various nodes of the electrothermal feedback circuitry  26  to decouple any unwanted RF energy. If RF coupling is inevitable, RC networks around the electrothermal feedback circuitry  26  can be used appropriately to attenuate coupling and minimize the impact.  FIG. 9  schematically depicts RF decoupling capacitors C 1 , C 2 , C 3 , C 4 , and C 5  as well as RC networks at various nodes of the electrothermal feedback circuitry  26  for this decoupling purpose. Depending on the actual layout, all of the capacitors and RC networks shown are not necessary, but those on the hottest node are recommended. To identify the hottest node, electro-magnetic (EM) modeling tools can be used to simulate EM coupling between the electrothermal feedback circuitry  26  and the RF path. 
     Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.