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Abstract: In this paper a transformer matched power amplifier for operation at millimeter-wave frequencies is presented. The SiGe single-stage push-pull ...
Proceedings of ESSCIRC, Grenoble, France, 2005

SiGe Transformer Matched Power Amplifier for Operation at Millimeter-Wave Frequencies Ullrich R. Pfeiffer(1) , David Goren(2) , Brian A. Floyd(1) and Scott K. Reynolds(1) (1) IBM T.J. Watson Research Center, PO Box 218 Rt 134, NY 10598, USA. (2) IBM Haifa Research Laboratories, Mount Carmel, Haifa 31905, Israel. [email protected]

Abstract: In this paper a transformer matched power amplifier for operation at millimeter-wave frequencies is presented. The SiGe single-stage push-pull amplifier uses a stacked transformer above a ground shield for output matching. The millimeter-wave transformer has a high coupling factor k = 0.8 and provides a very compact circuit layout. At 61.5 GHz the class-AB biased amplifier achieves a power gain of 12 dB with 8.5 dBm output power at a 1 dB compression. The saturated output power was measured up to Psat = 14 dBm with a maximum PAE of 4.2 %.

1.

(a)

Introduction

Transformer Structure

Monolithic on-chip transformers have been used for matching and power combining purposes in the past up to a few tens of GHz, where the tuned circuits used for matching have been formed by the transformer primary inductances and additional capacitors to achieve the bandwidth and efficiency required [5]. In order to operate any transformer in the mmWave frequency range, however, its primary inductance has to be reduced substantially, which in turn, requires the values of additional tuning capacitors to be extremely small. Therefore, it is crucial to have a transformer structure which allows accurate modeling and the prediction of parasitic effects. Commonly

0-7803-9205-1/05/$20.00 ©2005 IEEE

Primary Secondary

Silicon germanium (SiGe) process technologies have demonstrated operation in the millimeter-wave (mmWave) frequency range for applications like highly integrated radio circuits [1, 2]. The high level of integration possible in a SiGe process requires new passive devices with improved model accuracy to be used at mmWave frequencies [3]. Besides transmission-lines, coupled-wires are the most commonly used passive structures in mmWave circuits. Coupled-wires and like-wise transformers can be used for matching purposes or can act as a building block for various power combiners, baluns, distributed active transformers, and filters [4, 5]. In this paper a singlestage push-pull power amplifier is presented which uses a stacked transformer, fully integrated within the backend of a SiGe process technology. In this design, the transformer provides an extremely compact and optimized structure for millimeter wave operation.

2.

used on-chip transformers are made of coplanar coupled wires. Such coplanar transformers are typically used at lower frequencies where low coupling factors, substrate and skin effect losses, and inaccuracies caused by model to hardware discrepancies can be tolerated [4]. Stacked transformers provide better performance on silicon substrates if they are used in conjunction with perpendicular ground wires. Such wires can effectively shield from the lossy substrate and improve the wires coupling factor at the same time [3].

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Ground−Shield

(b)

k=0.8

Primary Conductor Secondary Conductor

Ground−Shield Side−Bars

Figure 1: Transformer cross-section is shown in (a) with its primary and secondary conductor above a ground shield. Figure (b) shows a 3D view of the transformer from which the ground shields perpendicular slots and side shields can be seen. The transformer stack-up used in this paper is shown in Figure 1 (a). The transformer is arranged in a “sandwichlike” structure where the primary inductor is stacked vertically above the secondary inductor. Both wires are located above a ground shield and achieve a coupling factor of k = 0.8. Figure 1 (b) shows a 3D view of the transformer which uses a ground shield with perpendicular slots. To improve the ability to predict the structures parasitic effects, side-bars have been added which act as a well-defined return path, and a closed environment EM condition for compact modeling at millimeter wave frequencies. Such modeling is scalable by length and insensitive to close-by metal structures that may be present dependent on the application and circuit layout. The transformer model used for circuit simulations is implemented in a filter RLC network plus dependent sources.

Paper 2.F.4

Proceedings of ESSCIRC, Grenoble, France, 2005 This models the skin and proximity effects between the three transformer conductors up to the third harmonic of the fundamental frequency (180 GHz). Compared to other approaches [6], such modeling enhances its frequency range up to a point where it can be used to simulate nonlinear effects in mmWave amplifier designs. The silicon substrate induced losses and added frequency dependence is being effectively canceled by the shielding effect of the perpendicular wires of the bottom ground shield. The model is designed to describe the transformer operation in all its operation modes, namely it does not assume in advance that the transformer is being matched to a given input and output impedance. This allows for the correct tuning and matching of the transformer using the model inside a circuit level simulation. The model has been tuned and verified using 3D EM solver [7].

3.

Circuit Architecture

output match

Advanced SiGe bipolar technologies have demonstrated operation at mmWave frequencies due to continued improvement in their cutoff frequencies (fT , fmax ). The power amplifier described here was designed in IBM’s advanced bipolar technology SiGe8HP [8]. It is a 0.12 µm SiGe technology with a cutoff frequency fT ≈ 207 GHz and fmax ≈ 285 GHz. The five-layer back-end of the line has three copper layers with two thick aluminum for low loss interconnects available. The breakdown voltages are BVCEO ≈ 1.7 V and BVCBO ≈ 5.5 V.

out

transformer would act as a balun while power-combining the two amplifier outputs. Note, the circuit does not include any additional tuning capacitors at the primary side, thus an extremely compact layout of the amplifier can be achieved. The scaling of SiGe processes technologies to smaller dimension leads to lower breakdown voltages (BVCEO , BVCBO ). The BVCEO limit, caused by impact ionization, can be overcome by both grounded base stages (T1 and T2) due to their low external base resistance, which allows them to operate well above BVCEO . The final amplifier operates from a 4 V supply. The input of the amplifier is conjugately matched whereas the output is matched for maximum power delivered to a 100 Ω differential load. Figure 3 shows a chip micro-graph. The pad-limited chip has a size of 0.5 × 1.25 mm2 including bond pads. To satisfy electro-migration rules, the primary and secondary inductors use the top two thick aluminum layers while the ground shield is located on a copper layer thereunder. The thicker primary inductor carries both the DC bias current and the fundamental RF current, while the thinner secondary inductor bears only the RF portion. The transformer total length is 164 µm including the centertap VCC bias supply. Note, no quarter-wave RF-chokes are required which makes the layout compact in size. The input and output pads are ground-shielded to minimize the influence from the lossy silicon substrate. Shunt transmission line stubs are used to resonate the pad capacitance, thereby providing a matched impedance into both of the 50-Ω on-chip transmission line. Such a pad structure is close to being electrically transparent and the off-chip impedance is unaltered by the pads.

4. Measured Results

VCC = 4V T1

T2 Vgbs

in T3

T4

input match

bias

Figure 2: Simplified amplifier schematic where the bias circuit has been omitted for clarity. Two cascode amplifiers are used in push-pull configuration using a center-tap transformer for output matching purposes. A simplified amplifier schematic is shown in Figure 2. Two cascode amplifiers are used in a push-pull configuration where a center-tap transformer is used for output matching purposes. The single-stage amplifier is intended to drive a differential antenna in communication circuits. Vias connect from the transistors T1 and T2 to the primary inductor on each transformer end. The secondary inductor ends are connected to differential output pads via on-chip transmission lines. If desired, the amplifier could have been easily matched to a single-ended output where the

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All measurements in this paper have been made on-wafer into a 100-Ω differential impedance using a pure-mode network analyzer concept described in [9]. External waveguide baluns have been used to provide differential signals to the input of the amplifier as well as for power combining the signals again at its output. An additional external amplifier is however required to account for additional loss in the input balun which would limit the available input power. Figure 4 shows the measured and simulated (solid lines) large-signal characteristic of the amplifier at 61.5 GHz. Simulations where performed in Spectre using the transformer equivalent circuit model, which was based on a filter RLC network plus dependent sources as previously described. All active devices use the vbic (Vertical Bipolar Inter-Company) bipolar junction transistor model. Figure 4 (a) shows the power gain and output power versus input power. Figure 4 (b) shows the power-added-efficiency (PAE) based on the amplifiers DC current shown Figure 4 (c). Figure 4 (d) shows the total power consumption based on the 4 V supply. The single stage amplifier has high power gain of 12 dB at 61.5 GHz. The output power at 1-dB compression is 8.5 dBm. The output power has not reached its saturated level due to limitations in the available input power. The highest output power was measured at 14 dBm. Deviations form the simulated compression characteristic in Figure 4 (a) can

Proceedings of ESSCIRC, Grenoble, France, 2005

G

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VCC T2

500 µm

T1

MIM−Caps

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MIM−Caps Transformer

Vgbs G

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Figure 3: Chip micro-graph: The pad-limited chip has a size of 0.5 × 1.25 mm 2 including bond pads. The transformer total length is 164 µm including its center-tap connection. (a)

(b) 9

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Figure 4: Measured and simulated (solid lines) large-signal characteristic at 61.5 GHz. Power gain and output power versus input power is shown in (a), Power-added-efficiency (PAE) in (b), supply current in (c), and total power consumption in (d). be linked to the active device models being used. Highperformance SiGe HBT devices inherently have a steep fT roll-off at high injection due to the high germanium content and larger germanium gradient in the neutral base, employed to achieve their high beta and fT [10, 11]. Rolloff at high-injection depends also on the device size and is known to be a weakness of the vbic model. A discrepancy in the compression characteristic is therefore not surprising and is related to the device models and not to the transformer. Such an earlier compression limits the available PAE shown in Figure 4 (b), although, the DC current in Figure 4 (c) correlates quite well. For a comparison at 59 and 64 GHz see the summary in Table 1.

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5. Conclusions A transformer matched power amplifier for operation at mmWave frequencies has been implemented in a silicon germanium process technology. The amplifier uses a stacked transformer above a ground shield for output matching purposes. The mmWave transformer has a high coupling factor of k = 0.8 and provides an efficient impedance matching and compact circuit layout. At 61.5 GHz the single-stage amplifier achieves a power gain of 12 dB with 8.5 dBm output power at a 1 dB compression. The saturated output power was measured up to Psat = 14 dBm with a maximum PAE of 4.2 %. Compared to previously published SiGe power amplifiers [12, 13], this one has improved in efficiency and power gain, with an only slightly lower output referred compression point. The circuit layout however, requires only

Proceedings of ESSCIRC, Grenoble, France, 2005 Table 1: Summary of measured and simulated PA performance.

DC Specification (TA = +25◦ C,unless otherwise noted)

PARAMETER

Circuit 60 GHz Transformer PA Measured Simulated

DC POWER REQUIREMENTS Voltage Supply, VCC +4 +4 Cuiesent Current, IDC 66 66 AC TEST CONDITION Frequency [59,61.5,64] [59,61.5,64] DIFFERENTIAL LARGE SIGNAL Compression Point, CP1dB [9.3,8.5,7.3] [11.2,11.3,10.8] Transducer Power Gain, GT [14,12,11] [11.7,12.0,11.2] Saturated Output Power1 , Psat [14.5,14,13.5] [20,20,20] Max Power Added Efficiency, PAE [4.6,4.2,3.1] [7.7,8.5,7.6] 1 Limited by available input power of the measurement equipment about 1/3 of the silicon area and includes the option for single-ended operation at no additional expense. It has been demonstrated that fully integrated on-chip transformers can operate efficiently in SiGe mmWave circuits.

[7] [8]

6.

Acknowledgments

The authors would like to thank all who contributed to the fabrication of the chip; especially the IBM SiGe technology group, DARPA (N66001-02-C08014) and NASA (NAS3-03070) for partial funding.

References:

[9]

[10]

[1] S. Reynolds, B. Floyd, U. R. Pfeiffer, and T. Zwick. 60 GHz transceiver circuits in SiGe bipolar technology. IEEE International Solid-State Circuits Conference, pages 442–443, February 2004. [2] B. Floyd, S. Reynolds, U. R. Pfeiffer, T. Zwick, Troy Beukema, and Brian Gaucher. SiGe bipolar transceiver circuits operating at 60 GHz. IEEE Journal of Solid-State Circuits, 40(1):156–167, January 2005. [3] T.O. Dickson, M.-A. LaCroix, S. Boret, D. Gloria, R. Beerkens, and S.P. Voinigescu. 30-100GHz inductors and transformers for millimeter-wave (Bi)CMOS integrated circuits. IEEE Transactions on Microwave Theory and Techniques, 53(1):123–133, January 2005. [4] I. Aoki, S.D. Kee, D.B. Rutledge, and A. Hajimiri. A fully-integrated 1.8-v, 2.8-w, 1.9-ghz, cmos power amplifier. Radio Frequency Integrated Circuits (RFIC) Symposium, pages 199–202, June 2003. [5] Cheung T.S.D., Long J.R., Tretiakov Y.V., and Harame D.L. A 21-27ghz self-shielded 4-way power-combining pa balun. IEEE Custom Integrated Circuits Conference, October 2004. [6] T. Biondi, A. Scuderi, E. Ragonese, and G. Palmisano. Wideband lumped scalable modeling of monolithic stacked transformers on silicon. In Proceedings of the IEEE Bipolar/BiCMOS Circuits

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[12]

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Units

V mA GHz dBm dB dBm %

and Technology Meeting, pages 265–268, September 2004. Ansoft Corporation. High frequency structure simulator HFSS, version 9. Pittsburgh. B. Jagannathan et al. Self-aligned SiGe NPN transistors with 285 GHz fmax and 207 GHz fT in a manufacturable technology. IEEE Electron Device Ltrs., 23(5), 2002. T. Zwick and U. R. Pfeiffer. Pure-mode network analyzer concept for on-wafer measurements of differential circuits at millimeter wave frequencies. IEEE Transactions on Microwave Theory and Techniques, 53(3):934–937, March 2005. Jun Pan, Guofu Niu, Alvin Joseph, and David L. Harame. Impact of profile design and scaling on large signal performance of SiGe HBTs. In Proceedings of the IEEE Bipolar/BiCMOS Circuits and Technology Meeting, pages 209–212, September 2004. D.L. Harame, J.H. Comfort, J.D. Cressler, E.F. Crabbe, J.Y.-C. Sun, B.S. Meyerson, and T. Tice. Si/SiGe epitaxial-base transistors – part I: Materials, physics, and circuits. IEEE Transactions on Electron Devices, 42(3):455–468, March 1995. U. Pfeiffer, S. Reynolds, and B. Floyd. A 77 GHz SiGe power amplifier for potential applications in automotive radar systems. Radio Frequency Integrated Circuits (RFIC) Symposium, pages 91–94, June 2004. Hao Li, Hans-Martin Rein, Thomas Suttorp, and Josef Bck. Fully integrated SiGe VCOs with powerful output buffer for 77-GHz automotive radar systems and applications around 100 GHz. IEEE Journal of Solid-State Circuits, 39(10):1650–1658, October 2004.