* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download A protection circuit for HBT RF power amplifier under load
Immunity-aware programming wikipedia , lookup
Regenerative circuit wikipedia , lookup
Integrating ADC wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Audio power wikipedia , lookup
Index of electronics articles wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Radio transmitter design wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Negative-feedback amplifier wikipedia , lookup
Two-port network wikipedia , lookup
Schmitt trigger wikipedia , lookup
Voltage regulator wikipedia , lookup
Standing wave ratio wikipedia , lookup
Power MOSFET wikipedia , lookup
Current source wikipedia , lookup
Power electronics wikipedia , lookup
Wilson current mirror wikipedia , lookup
Valve audio amplifier technical specification wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Operational amplifier wikipedia , lookup
Surge protector wikipedia , lookup
Opto-isolator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
A protection circuit for HBT RF power amplifier under load mismatch conditions Walid Karoui Thierry Parra Freescale Semiconductors Avenue du General Eisenhower, BP 1029, 31023 Toulouse, France [email protected] LAAS-CNRS and Toulouse University, UPS 7 Avenue du Colonel Roche, 31077 Toulouse, France [email protected] Abstract—This paper investigates the radiofrequency failure mechanisms of Power Amplifier (PA) integrated in an HBT technology, and proposes a circuit solution for PA protection against impedance mismatches. It exposes the failure mechanisms that occur when a PA under extreme conditions (high battery voltage and high input power) is exposed to impedance mismatches at its output. Protection against high voltage operation is addressed by integrating a parallel base resistor which increases significantly emitter collector breakdown. A current sensor is then associated to a feedback loop on the PA biasing circuit that operates when the PA is in high dissipated power conditions by limiting the collector current. These protections are easily implemented on the PA die without any extra area. Experiments confirm the effectiveness of these principles: the protection is indexed on the collector supply voltage and acts for all output loads leading to VSWR up to 1 0:1, whereas output power and power efficiency on a 50Ω load are not affected. I. PA failures. In a second step we point out that the PA failure is mainly related to a high dissipated power at the PA’s final stage. Based on these investigations, we propose a new monolithically integrated current limiter, indexed to supply voltage, which is able to improve HBT amplifier’s ruggedness, and which avoids any isolator implementation. The effectiveness of this protection circuit is also proved as it can prevent the failure of the PA under severe mismatch conditions and high supply voltage, while maintaining RF performances on a 50Ω load. II. vswr' RF IN INTRODUCTION In a phone handset, the Power Amplifier Module (PAM) is the last active component of the transmission path (Figure1), and thus is directly exposed to impedance mismatches induced by user motion and environment variations of the handset antenna (disconnected antenna, proximity of a metallic plane, user touching the antenna…). The PA must withstand these impedance mismatches even it operates at extreme conditions such as high battery voltage and high input power. Thereby, the ruggedness of PA is a key specification for PAM in wireless applications. In order to meet the ruggedness specification, it is necessary to understand the root cause of radiofrequency failure in PA and then design an effective protection circuit. The static base collector breakdown voltage at open base (BVCEO) was used, for a long time, as a metric to qualify the robustness of Hetero-junction Bipolar Transistor (HBT). In this paper we demonstrate, in a first step, that this metric is too conservative and that the collector voltage swing of the PA can go beyond the static breakdown voltage without causing 978-1-4244-2332-3/08/$25.00 © 2008 IEEE IMPEDANCE MISMATCHES PAM vswr VC IC Isolator Antenna RFOUT R load Figure 1. Impedance mismatches in the transmission path In previous generations of handsets, an isolator was usually implemented in the transmission path between the PAM and the handset antenna, in order to isolate the amplifier from any impedance variation of the antenna terminal (Figure1). The design of modern handsets attempts the suppression of this isolator because this component makes the PAM integration incompatible with the constraints of downsizing and cost reduction. Moreover, as it inserts losses, the isolator degrades the system available output power, as well as its power added efficiency which affects directly the handset autonomy (talk time). However, the isolator 241 suppression increases the PAM sensitivity to load impedances mismatches that can translate into PAM oscillations and/or lead to its failure. This is particularly true for SiGe HBT power amplifier because the technologies remain fragile. As the reflected power at the antenna terminal can create, under certain conditions, constructive interferences at the collector port of the PAM, the mismatches can lead to high collector voltage, if the phase of the load is around zero degree (high impedance mismatch), or high collector current, if the phase of the load is around 180 degree (low impedance mismatch) (Figure2). From current specifications, the high impedance mismatch is close to the open circuit (OC), whereas the low impedance mismatch is close to the short circuit (SC). In intermediate situations, the imaginary part of the load can resonate with parasitic capacitances and inductances of the PAM transistors and can lead to oscillations [1]. Low Impedance mismatch (High Current) High Impedance mismatch (High Voltage) B A of the PAM transistors, we can expect that the main failure source under RF operations becomes the dissipated power that runs over the limit. In order to verify this statement, we performed experiments on a three stage GaAs PA integrating a parallel base resistor on RF transistors. The PA is supplied with high battery voltage (5V). The level of the RF input signal is chosen to put the PA into saturation on a 50Ω load. Then, an impedance mismatch with a VSWR (Voltage Standing Wave Ratio) of 10:1 is set at the PA output with a VSWR tuner, and the phase of the load is varied by the tuner slide. BVCER Figure 3. Collector emitter breakdown voltage versus base paralell resistance (example of SiGe HBT) This 10:1 VSWR at the PA output corresponds to a 20:1 VSWR at the antenna terminal, assuming that the front-end between the PA and the antenna (filter and antenna switch) introduces 1 dB of losses. A spectrum analyzer is used to measure the PA output power and detect potential oscillations. Finally, the collector current of each stage of the PA is measured for each phase of the load. The test bench is depicted on Figure4. 10:1 VSWR Figure 2. Cases of impedance mismatches for a 10:1 VSWR III. FAILURE MECHANISMS IN HBT RF POWER AMPLIFIER Two issues can affect the ruggedness of HBTs: breakdown voltage and thermal issue due to high dissipated power [2]. The open base static collector emitter breakdown voltage (BVCEO) was commonly considered as the voltage limit of the RF output signal swing. However, it has been demonstrated that the collector voltage can run over BVCEO without causing PAM failure [3], [4]. This can be explained by the fact that the collector emitter breakdown voltage depends strongly on the impedance which is presented on the base-emitter junction of the transistor, for example by the bias circuit. By providing a low impedance path for the avalanche current, resulting from high voltage at the collector-base junction, an amount of this avalanche current flows through this impedance instead of flowing through the emitter-base junction, delaying on that way the bipolar positive feedback that causes HBT failure. So, for RF HBT PA applications, the collector emitter breakdown voltage with the base grounded through a resistance (BVCER) appears more relevant (Figure 3). Because the high value reached by the breakdown voltage BVCER when a 5 kΩ resistor is implemented on the HBT base Tektronics Scope HP 6626A Supply 1 2 3 4 Function Generator Yokogawa 1 2 FSIQ7 Spectrum Analyzer 3 R & S SMIQ VSWR Tuner Vreg Vbat -3dB RFin_HB RFout_HB RFin_LB RFout_LB -0.5dB Coupler Amplifier Under Test Pin VSWR Figure 4. Ruggedness test bench The measurement results are given in Figure5. When the load impedance phase varies from 0 to 360 degrees, the collector current of the final stage (IC3) varies whereas the current of drivers (IC12) remains constant. Thus, in the case of a multistage PA, only the final stage need to be protected against impedances mismatches. The PA failure occurs for 242 load phases near 180 degrees (low impedance mismatch zone) when the collector current of the final stage exceeds 2.7A. No failure or damage is observed for phases around 80 degrees where the PA collector voltage is very high and exceeds significantly the BVCEO. 1.3V (the turn on voltage of a GaAs HBT). Consequently the transistor Qfb turns on and decreases the final stage bias current. This feedback on the current mirror of the PA biasing circuit hence reduces the amplifier gain. Finally, the low pass filter R2C2 reduces the Qs collector voltage swing and prevents the voltage saturation of Qs. 4000 Failure Zone 3500 3000 IC (mA) 2500 2000 1500 Ic12 (mA) 1000 Ic3 (mA) 500 0 0 40 80 120 160 200 240 Phase (degree) 280 320 360 Current limiter Figure 7. Layout of the protected PA final Stage Figure 5. PA collector current versus load phase at VSWR of 10:1 IV. PROTECTION CIRCUIT DESIGN As we previously demonstrated, when a parallel base resistor is implemented on the HBT, the main failure reason becomes the dissipated power which runs over the limit when the RF PA is subject to load impedance mismatch. Moreover, only the final stage of a multistage PA needs to be protected. So, the protection circuit we propose is a current limiter implemented on the final stage of the PA. It is presented on figure 6 [5]. Vbat RFOUT Vreg Vbat QRF3 RFIN R2 Ibias Mirror C2 C R This protection circuit has been implemented on the final stage of a GaInP/GaAs HBT RF power amplifier. The layout of the die is presented in figure 7, where the protection circuit is highlighted. As it can be picked up, the protection circuit doesn’t increase the die area. V. VSWR 10:1 Qs 2 Vdet Rstab R1 EXPERIMENTAL RESULTS In a first step, the detection voltage (Vdet) is measured in an open loop configuration (base of Qfb not connected). Results are given in figure 8. Performed for two values of supply voltage, these measurements show that Vdet value is higher than 1.3V when the final stage collector current reaches 2.5A. This voltage is able to turn on the bias feedback transistor Qfb. Moreover, Vdet value increases when the PA supply voltage is higher, which insures an efficient dissipated power limitation which threshold is indexed on bias. 1,8 C1 Vdet (V) 1,6 Qfb 1,4 1,2 1 Figure 6. Schematic of the protected PA final Stage 0,8 The amplifier is biased using the current mirror biasing scheme which is providing temperature compensation [6]. The voltage Vreg sets the amplifier gain. The final stage collector current (Ic) is sensed by a small transistor Qs (100 µm2) in parallel with the final stage QRF3. Then, the sensed current, which is proportional to the collector current, flows through resistor R1. The voltage across R1 is filtered by the capacitor C1 and then applied to the base of the transistor Qfb. As Ic increases, the detection voltage (Vdet) applied to the base of the transistor Qfb increases. When the collector current exceeds a predefined threshold, Vdet becomes superior to Vdet Vdet @3.2V @3.2V Vdet @4V Vdet @4V 0,6 1 1,5 2 2,5 3 Ic (A) Figure 8. Sensed voltage versus final stage collector current, indexed to supply voltage Then the robustness of the protection is tested on a PA operating in CEL/EGSM frequency bands and under GSM conditions. Measurements are carried out at the maximum supply voltage (5V) and when the final stage of the PA is in 243 saturation on a 50Ω load. A VSWR of 10:1 is then set at the PA output and the load phase is varied from 0 to 360 degrees. Figure 9 shows the final stage collector current versus load phase. Without the protection circuit, this current can increase up to 3.1A before the PA fails. With the protection, this current does not exceed the set threshold of 2.7A and no damage is observed. These experimental results confirm the effectiveness of the protection to prevent PA failure under severe mismatch conditions and high supply voltage. Moreover, because the current sensor is implemented from small components (1mA current consumption, low parasites) we verify that this protection is not affecting the output power and the power added efficiency when the PA is loaded on 50Ω. VSWR=10:1 base. In these conditions, the main cause of failure becomes the high dissipated power (high collector current) at the collector terminal of the transistor. For a multistage PA, only the final stage is sensitive to impedance mismatches and should be protected. We proposed an efficient protection circuit based on a current sensor associated to a feedback loop on the PA biasing circuit. It operates when the PA is in high dissipated power conditions by limiting the collector current of the final stage. Because it eliminates the need for band-gap reference circuit, operational amplifier and coupler, this protection circuit is easily implemented on a monolithic PA die without any extra area. Experimental results confirm the protection is indexed on the supply voltage and acts for all output loads leading to VSWR up to 10:1, whereas output power and power efficiency on a 50Ω load are not affected. IPC mode : F=915 MHz, Pin=6 dBm, Vbat=5V) 4.5 REFERENCES Failure 4 [1] 3.5 IC (A) 3 2.5 [2] 2 1.5 1 IC without protection IC_SL IC with protection IC_AL 0.5 [3] 0 0 60 120 180 Phase ( °) 240 300 360 [4] Figure 9. PA collector current comparison versus load phase VI. CONCLUSION [5] We demonstrated that the collector voltage of a power amplifier can go significantly beyond the static collector emitter breakdown voltage at open base (BVCEO), without any failure of transistors, when a parallel resistor is added on the [6] 244 J.F. Imbornone, M. Murphy, R.S. Donahue and E. Heaney "New insight into subharmonic oscillation mode of GaAs power amplifiers Under Severe Output Mismatch Condition", IEEE Journal of SolideState Circuits, Vol. 32, pp. 1319-1325, September 1997. A. Inoue, S. Nakatsuka, S. Suzuki, K. Yamamoto, and T. Shimura, “Direct measurement of the maximum operating region in GaAs HBTs for RF power amplifiers,” IEEE MTT-S Digest, pp. 1687-1690, June 2001. S. Heckmann, J.M. Nébus, R. Quéré, J.C. Jacquet, D. Floriot and P. Auxemery "Measurement and modelling of Static and Dynamic Breakdowns of Power GaInP/GaAs HBT’s", IEEE MTT-S Digest, pp1001-1004, June 2002. A.J. Joseph, J. Dunn, G. Freeman, and D.L. Harame "Product Applications and technology Directions With SiGe BiCMOS", IEEE Journal of Solid State Circuits, Vol. 38, N0 9, pp 1471-1478. September 2003. W.Karoui , P.Riondet , G.Montoriol , T.Parra : "An adaptive protection circuit for a power amplifier", Freescale worldwide patent PCT/EP2005/005210. E. Jarvinen, S. Kalajo, and M. Matilainen, “Bias circuits for GaAs HBT power amplifier,” IEEE MTT-S Digest, pp 507-510. June 2001.