1 mo t orola rf applic a tions report
% ! $ " ! " ## # # $ " ! $# prepared by: julie duclercq and olivier lembeye motorola semiconductor products sector t oulouse, france introduction the performances of rf power amplifiers for base station transceivers results in a tradeo f f between linearit y , e f ficiency and gain. this tradeo f f leads to an optimum quiescent current. but the following parameters modify this bias point: temperatur e range (commonly 40 c/+85 c), supply voltage and bias voltage variations (commonly +/5%) and manufacturin g spread. the purpose of this paper is to present a new biasing circuit which minimizes quiescent current variations suitable for ldmos rf power transistors. s t andar d biasing circuit a standard biasing circuit commonly used for class ab transistors is shown in figure 1. figure 1. standard biasing circuit v bias v supp l y i q v g v d r1 r2 for each ldmos transisto r , it is necessary to adjust the r1 and r2 values in order to set the quiescent current as specified at ambient temperature. lasertrimmable resistors can be used in mass production for that purpose. the addition of a diode in series with the two resistors allows for reduction of the quiescent current variation over temperature . w e have: r2 r 1 r2 v g = r1 r 1 r2 v d (equation 1) = r1 r 1 r2 (equation 2 ) dvd v bias + then, r2 r 1 r2 dvg = r1 r 1 r2 dv d (equation 1.bis ) dv bias + when dv bias = 0, we have: dvg considering variations due to temperature, the coe f ficient dvd dt to apply to the ldmos is related to the bias point, i.e., the quiescent current. for class ab biasing, this coe f ficient is around 2mv/ c. for full thermal tracking, more than one diode may be required because of the ratio r1 r 1 r2 in equatio n 2. howeve r , this standard biasing circuit presents two major limitations: limitation 1: the manufacturing spread of threshold voltage leads to a di f ferent r1 r 1 r2 ratio for each die. therefore, all the dice is not compensated for in the same wa y . a die with a high threshold voltage requires a ratio r1 r 1 r2 lower than for a die with a low threshold voltage. subsequentl y , the thermal compensation is lower for a die with a high threshold voltage than for a die with a low threshold voltage. this results in quiescent current over or under compensation versus temperature. limitation 2: equation 1 shows that bias voltage variations induce proportional gate voltage variations weighed by a ratio r2 r 1 r2 . thus these gate voltage variations lead to quiescent current variations via the die transconductance. as a result, we have designed a new biasing circuit to cope with those limitations. order this document by an1643/d
semiconductor applic a tio n note ? motorola, inc. 1998
2 motorola rf applications report new biasing circuit the new biasing circuit is shown in figure 2. figure 2. new biasing circuit v supply i q v g v d r1 r2 v bias voltage regulator v ref the bias bridge is adjusted to set the quiescent current as specified at ambient temperature. laser trimmable resistors can be used in mass production for that purpose. thermal tracking is introduced to compensate the quiescent current variations versus temperature. a diode in the bias bridge compensates the ldmos temperature coefficient. we have now: r2 r1 r2 v g = v ref +v d (equation 3) assuming no reference voltage variation versus bias voltage and temperature (dv ref = 0), we have: (equation 4) dvg = dvd thermal tracking: the thermal compensation is now independent of r1 and r2 values and thus independent of the manufacturing spread of transistor's threshold voltage. bias voltage variations: as the bias voltage is now regulated, we can see from equation 3 that the bias voltage variations are no longer applied on the gate. this results in a constant quiescent current at a given temperature. note that in this case the bias voltage can be directly connected to the power supply voltage. implementation this new biasing circuit is already implemented in three motorola rf ldmos power modules designed for gsm base station applications. the mhw910 (10 w gsm900), MHW1810 (10 w gsm1800) and mhw1910 (10 w gsm1900) modules take advantage of this solution. this new biasing circuit allows keeping this optimum bias point in every temperature and bias voltage combination. one is now able to get the best performances, especially within the whole temperature range. another advantage from a customer standpoint is that these new power modules no longer require external bias regulation circuitry for optimum performances. the voltage regulator used in the mhw910 and MHW1810 is the lp2951. this ic provides the required performances of a very low output voltage temperature coefficient and a very good input voltage variations rejection. another feature of the lp2951 is the availability of the shutdown input which allows a logic level signal to turnoff or turnon the regulator output. this biasing circuit can be used in the same way for discrete ldmos transistors as well. special attention must be taken regarding the dc and rf decoupling. this circuit guarantees best possible performance by maintaining a constant quiescent current regardless of temperature, bias voltage and gate threshold voltage variations. motorola reserves the right to make changes without further notice to any products herein. motorola makes no warranty, represe ntation or guarantee regarding the suitability of its products for any particular purpose, nor does motorola assume any liability arising out of the applicati on or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. atypicalo para meters which may be provided in motorola data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all ope rating parameters, including atypicalso must be validated for each customer application by customer's technical experts. motorola does not convey any license under it s patent rights nor the rights of others. motorola products are not designed, intended, or authorized for use as components in systems intended for surgical imp lant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the motorola product cou ld create a situation where personal injury or death may occur. should buyer purchase or use motorola products for any such unintended or unauthorized application, buyer shall indemnify and hold motorola and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expens es, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized u se, even if such claim alleges that motorola was negligent regarding the design or manufacture of the part. motorola and are registered trademarks of motoro la, inc. motorola, inc. is an equal opportunity/affirmative action employer. mfax is a trademark of motorola, inc. how to reach us: usa / europe / locations not listed : motorola literature distribution; japan : motorola japan ltd.; spd, strategic planning office, 141, p.o. box 5405, denver, colorado 80217. 13036752140 or 18004412447 4321 nishigotanda, shinagawaku, tokyo, japan. 813 54878488 customer focus center: 18005216274 mfax ? 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