1834 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 9, SEPTEMBER 2007 Power Ampliﬁer Protection by Adaptive Output Power Control André van Bezooijen, Member, IEEE, Freek van Straten, Member, IEEE, Reza Mahmoudi, Member, IEEE, and Arthur H. M. van Roermund, Senior Member, IEEE Abstract—Cellular phone power ampliﬁers (PAs) operate in strongly varying environments and have to withstand extreme conditions. To avoid destructive breakdown a generic protection concept is proposed that is based on adaptive control of the output power. It provides over-voltage, over-temperature, and/or over-current protection by detection of the collector peak voltage, die temperature, and/or collector current to reduce the effective power control voltage once a threshold level is crossed. By ap- plying protections, PAs can be implemented in low-cost silicon technology competitively to GaAs HBT implementations. In ad- dition, requirements on package thermal resistance are relaxed. In this paper a theoretical analysis is given on the behavior of a class-AB ampliﬁer under mismatch conditions. Measurement results on a silicon bipolar power transistor with integrated protection circuits are presented, proving the concept of adaptive protection. For a supply voltage of 5 V and nominal output power of 2 W no breakdown is observed for a VSWR of 10 over all phases when output power is adaptively reduced by 2.7 dB at most. Index Terms—Adaptive control, avalanche breakdown, over-current, over-voltage, power ampliﬁers, protection, tem- perature. I. INTRODUCTION H ANDHELD cellular phone power ampliﬁers (PAs) op- erate in strongly varying environments and are designed to withstand extreme conditions. Destructive breakdown of the power transistor has to be prevented, even when a combination of extremes is present. For a bipolar power transistor three dif- ferent causes of break-down can be distinguished: 1) avalanche break-down of the collector–base junction; 2) run-away due to electrothermal instability; and 3) interconnect blow-out due to local dissipation. This is illustrated as a chain of causes and ef- fects in Fig. 1. Avalanche breakdown [1]–[4] of the power transistor col- lector–base junction depends on (A) output power, (B) collector load impedance, affected by antenna body-effects, [5] and [6], and (C) battery charge. To prevent avalanche breakdown, power transistors are commonly implemented in an IC technology with high breakdown voltages (typically 20 V for GSM Manuscript received December 18, 2006; revised April 15, 2007. A. van Bezooijen and F. van Straten are with NXP Semiconductors, 6534 AE Nijmegen, The Netherlands (e-mail: Andre.van.Bezooijen@nxp.com; Freek.van.Straten@nxp.com). R. Mahmoudi and A. H. M. van Roermund are with the Eindhoven Uni- versity of Technology, 5600MB, Eindhoven, The Netherlands (e-mail: R.Mah- moudi@tue.nl; A.H.M.v.Roermund@tue.nl). Digital Object Identiﬁer 10.1109/JSSC.2007.900783 PAs). This is achieved by using a relatively thick epi-layer of a silicon transistor (or thick mesa-layer of a GaAs HBT device) and by optimizing the collector doping proﬁle. However, such a ruggedness optimization inevitably results in a reduction of the , and consequently, in a lower gain-bandwidth product of the power transistor and thus in a lower PA efﬁciency [7]. This tradeoff can be shifted towards improved performance under nominal operating conditions by adopting over-voltage protec- tion circuits that prevent breakdown under extremes [8]–[10]. However, they do not provide protection against over-heating. Electrothermal instability [1], [2], [11], and [12], due to over- heating of a bipolar power transistor depends on (A) output power, (B) collector load impedance, (C) battery charge, as well as on (D) ambient temperature. In PA module design several countermeasures are taken against thermal runaway like the use of: emitter and/or base-degeneration resistors, heat spreading in- terconnect, exposed heat sinks, plated and/or copper ﬁlled lam- inate vias, and proper distribution of RF load and source im- pedances over the entire transistor array. All these measures are aiming for a reduction in the absolute die temperature, and for a reduction in temperature differences between transistors in the array. Moreover, as over-heating accelerates failure mech- anisms, these measures also increase product life-time. Major disadvantages of these countermeasures are associated pack- aging cost and size and reduced efﬁciency in case of emitter degeneration. As an alternative to these solutions, we present a more generic protection concept that is based on output power adaptation [13]–[15]. In principle, it can be used for (I) over-voltage, (II) over-temperature, and/or (III) over-current protection. More- over, it can be applied in open-loop as well as in closed-loop power controlled PAs. An integrated voltage detector, tem- perature detector and current detector are used to monitor the collector peak voltage, the die temperature and the collector current respectively. Once a detected parameter value crosses a certain threshold level the effective power control voltage is reduced to limit the ampliﬁer output power. The power control loop (PCL) is made part of the protection loops in order to avoid opposite control of the PCL and that of the protection circuits, which might occur when protection circuits, based on local feedback, limit the output power under extremes, while the power control loop tries to increase it up to the requested level. Hence, by applying over-voltage protection circuits a low cost main-stream silicon IC-technology (with lower breakdown voltages) can be used to integrate rugged power ampliﬁers, including biasing circuits, power control functions and control 0018-9200/$25.00 © 2007 IEEE