A Fine-Tuned Low-Power LNA for Lower-Band UWBTransceiver

Haolu Xie and Albert Wang

Abstract - Ultra wideband (UWB) radio emergesas a very promising wireless communicationtechnology due to its many advantages: i.e., 7.5GHzspectrum bandwidth, very high data rate, very lowpower consumption, etc. This paper presents anoptimal low-power 3.1-6GHz low-noise amplifier (LNA)designed for pulse-based lower band UWBtransceivers. The LNA uses tunable shunt-seriesfeedback topology architecture to achieve desirableultra broadband gain, noise performance and tooptimize the power composition. The LNA isimplemented in a commercial 0.18im SiGe BiCMOSprocess and the measured specifications are: anadequate ultra broadband gain of 18.14dB-12.63dB, alow noise figure (NF) of 3.02dB-4.23dB across3.1-6GHz bandwidth, a very low DC powerconsumption of less than 15mW at 3V supply, Sll ofless than -6dB, and S22 of less than -5dB.

I. INTRODUCTION

Ultra wideband (UWB) is a relatively new wirelesstechnology that has been gaining momentum since theFCC opened the door for commercial development in 2002[1]. UWB technology is capable of transmitting data acrossan extremely wide bandwidth of 3.1-10.6GHz at very highdata rates [2]. Though several industrial transmissionformats have been explored recently for ultra widebandcommunications, original UWB is regarded as apulse-based, career-free, time-domain wireless technologythat transmits signals of very short pulses at the order ofnanoseconds. Since UWB transmission spreads the energyof radio signals across a very wide bandwidth up to severalGHz, its signal level can be lower than the noise floor oftraditional frequency-domain RF technologies. Hence,UWB chips consume much lower power compared withother commonly used RF transceivers [3]. Currently,several different methods of utilizing the very broadbandspectrum bandwidth allocated for UWB communicationshave been investigated and proposed [2]. In general, thetransmitted UWB signals can be carrier-free impulses thatoccupy the 3.1-6GHz spectrum (lower band) or the full

Haolu Xie and Albert Wang are with the Department of ECE,Illinois Institute of Technology, 3301 S Dearborn St., Chicago, IL60616, USA Tel.: 312-567-6912, Fax: 312-567-8976, email:awang@ece.iit.edu

7.5GHz spectrum, i.e. a single-band pulse-based UWBsystem; or they can be shaped such that different signalstreams can be transmitted through several sub-bandswithin the entire 7.5GHz band, i.e. a multi-band UWBsystem. While UWB transceivers may be different fromthe traditional RF transceivers in terms of the topologiesand typically may be simplified, some analog components,such as low-noise amplifier (LNA), are generally needed.Due to the unique natures of UWB technology, e.g., ultrawide bandwidth, extremely low signal power spectraldensity, etc, the following challenges in designing UWBLNA are to be addressed: sufficient gain to amplify the lowUWB signals and to minimize the whole transceiversystem noise figure (NF), ultra wide band (multi-GHz)gain and NF performance, etc.

This paper reports design of a 3.1-6GHz UWB LNAimplemented in a commercial 0.18pm SiGe BiCMOSprocess. The LNA performance across the 3.1-6GHzachieved are: gain of 18.14dB-12.63dB, NF of3.02dB-4.23dB, DC power dissipation of less than 15mWwith a 3V supply, SI, of less than -6dB and S22 of less than-5dB.

II. UWB LNA CIRCUIT DESIGN

Because of its ultra wideband nature (up to severalGHz spectrum), designing UWB LNA is very challengingcompared with that for relatively narrow-band LNA designfor traditional f-domain RF transceivers. One of the criticaldesign tasks in broadband LNA design is to design properresistive termination to realize across-band impedancematching between LNA input port and the output port ofthe driving source to achieve good power and noisematching. However, because the input of an LNA circuit istypically a capacitive node, due to various parasiticcapacitance of transistors used, it becomes extremelydifficult to realize good broadband impedance matchingwithout degrading the noise performance and powerdelivery. Typical LNA topologies can be grouped into thefollowing four categories based on their impedancematching methods as shown in Figure 1: Shown in Figurela is a very simple resistive termination topology, wherethe 50Q resistance is directly connected to the input nodeof a common emitter amplifier. While reasonablebroadband resistive matching is achieved, the terminatingresistor would introduce too much thermal noise to the

0-7803-9339-2/05/$20.00 0D2005 IEEE. 217

LNA circuit and attenuate the input signal to the LNAdriving transistor port, resulting in an unacceptable highnoise figure for the LNA circuit. Therefore, this resistivetermination topology is not preferred in practical ultrawideband LNA design. Figure lb illustrates a 1/gm LNAtopology where the desired matching impedance isprovided by the 1/gm of the common base transistor.While this architecture is very simple, the required biasingconditions to deliver a 1/gm = 50Q is usually differentfrom that for optimum noise figure performance [4]. Aninductive degeneration topology is shown in Figure Icwhere the desired resistive termination is realized at properresonance frequency. This inductive LNA topology canachieve excellent noise figure [5], however, itsnarrow-band nature disqualified it for broadband UWBLNA applications. Figure ld shows a closed-loopshunt-series feedback LNA topology. In contrast to theopen-loop architectures, this topology breaks-up thetroublesome global negative feedback and achieves propertrade-off between source impedance matching and noisefigure performance. Hence, this shunt-series topology isattractive to broad-band LNA design. In addition, thesimple and single-stage circuit ensures very low powerdissipation as well compared to multi-stage topologiesreported. One design challenge in using the shunt-seriestopology is to ensure circuit stability at the presence of thenegative feedback.

(a) O

RD3

(c) (d)

Fig. 1. LNA topologies: (a) Resistive-termination, (b) 1/gmtermination, (c) Inductive degeneration and (d)Shunt-series feedback.

VI, 12 V2,12

AA&& l

VI, ItRf

gm

Fig.2. Small-signal circuitfeedback amplifier

model for the shunt-series

In this design, we chose the shunt-series topology forthe 3.1-6GHz spectrum UWB LNA to achieve excellent

broad-band performance. Basic LNA circuit analysisfollows: Using the small-signal equivalent circuit modelshown in Figure 2, the two-port I-V transfer function forthe amplifier can be described by h-parameters as given inEq. 1,

gm1Rf 9____(12J(h2, k22X'2) Et R 1 BR2 1I2 1 k2 1 ~ V)

The h-parameter matrix can be further converted intothe corresponding S-parameter format as in Eq. 2,

Rf g Zo_ ~~~~2

g2(1 gmRf Rf g_z__

1 + g.R, Z0 I + g R, (2)

where the characteristic impedance Z0=50Q, and

A = 2 + R- + 0 . Using this S-parameter matrix andZo 1+ g R,

considering the ideal matching condition of S11=S22=0, theseries resistance RS can be derived as,

R =S Rf gm (3)

Substituting Eq. 3 into Eq. 2 results in S-parameterexpression,

[S]= Rf±Z1

Lzo (4)Eq. 4 reveals that the forward gain of S21 can be flat

and good matching can be achieved by choosingappropriate values for Rf and R,. Next, one needs toconsider the critical stability issue of the ultra widebandLNA. To ensure unconditional stability for the LNA, thecondition of Kf >1, where Kf is given in Eq. 5, must besatisfied.

K~ 1+jIAl-2S 2 -IS22I2Kf =- 2IS211 IS121 (5)

where, A=S11S22-S21S12.Figure 3a shows the schematic for the UWB LNA

designed in this work. The above analysis for theshunt-series feedback LNA circuit neglected all reactanceeffects in the circuit. In the real design, all the parasiticcomponents (capacitance and inductance) must be takeninto account, particularly for the 3.1-6GHz UWB circuit,because such parasitics will adversely affect LNA circuitbehaviors, such as, gain, noise figure and stability, etc.Mismatching-compensation technique was used at theinput or output port of the devices to counter for thefrequency variation problem, which leads tofrequency-dependent impedance of input or output port.As a result, one would not get maximum power transfer ofinput signals over the desired ultra wideband frequencyrange. In this schematic, the load inductance L2 replacesthe resistive load in the original shunt-series feedbackconfiguration. Since the magnitude of the equivalentimpedance of the inductor increases as frequency increases,it therefore compensates for the amplifier gain degradationthat occurs as frequency increases. With this inductor load,

218

the resonant frequency must be set outside of the operatingfrequency range to avoid the LNA becoming anarrow-bandwidth amplifier.

, tT 2

(a)

t0i' lin

tout, Iout

(b)

t

I,

(c)Fig.3. the UWB LNA (a) schematic, (b) equivalent circuitof (a), (c) equivalent representation noise generator ofcircuit (b)

However, one cannot simply use a very largeinductor to realize enough high-frequency gaincompensation to achieve the desired broadband gainflatness, because large inductance would result in a quitelow self-resonance frequency. To further improve thehigh-frequency gain, the inductor Lf is added to the circuit.From Eq. 4, the magnitude of the forward gain S21 isdetermined by the feedback resistance Rf. However, thisRf value can not be increased excessively because a largeRf causes Rs to be negative according to Eq. 3, which willresult in circuit oscillation. Yet using the Lf can increasethe required impedance without affecting the Rs, hence, itimproves the gain at high frequency without affectingcircuit stability. Another important component in thecircuit is the DC blocking capacitance Cf, which blocks theDC current coming from the output node. This blocking isnecessary to separate the input and output voltages, hence,makes the input bias tunable and, so it possible tosimultaneously achieve the optimal transistor biasing andmaximum transistor gm. It helps to reduce the circuit powerconsumption while increase the gain. This tunable circuitdesign method is very useful for RF circuit design since RFcircuit is very sensitive on the parasitic effects, which leads

to the design engineer difficultly to find real optimized DCoperating points only running the post-layout simulation.

To ensure broadband circuit stability is very criticalto UWB circuit design. Generally, the simple, yet effectiveway to stabilize an active device is to add a seriesresistance or a shunt conductance to the input. In thisdesign, an optimum shunt conductance Rs,, selected barelyadequate to ensure circuit stability, is used to achievecircuit stability without adding significant noise to thecircuit.

In term of the schematic in Figure 3, if the VBE wasproperly set, and the transistor works in mid-current region[6], we can calculate the gain:

G(s) = gmxZoxf(s) (6)where f(s) is the transfer function, and it is independent ofgi, and = I, expVBE, which valid only in mid-current

VT VTregion, also the current gain = Ic is approximately

'B

constant in the mid-current region.[6] Clearly, the higherVBE in the mid-current region, the higher gm and gain. Asthe VBE continuously increases, the transistor will moveinto the high current region. The gm and 1 will decreaseseriously, so do the gain and noise figure, which is highlyundesirable. Hence, an optimal VBE that leads to the bestgm and gain, meanwhile maintains a constant P is highlydesirable. The tunable VBE circuit design method in thiswork serves to select the optimal VBE for better LNAperformance, hence minimize LNA power consumption.From the schematic in Figure 3c, the noise figure can beestimated as:

NNoA + 2 + NoB

NF= gmNOB (7)

Where NOB is the part of the output noise due to thesource resistance, NOA is the part of the output noise due tothe transistor, and N0R/g2m is the part of the output noisedue to the output resistance. All NOB, NOA and NOR areindependent of gm [6]. Eq. 7 suggests that properly anoptimal VBE bias maximize gm and minimize the noisefigure in mid-current region.

Fig.4. Die photo of the 3.1-6GHz UWB LNA circuit

219

III. LNA MEASUREMENT AND OPTIMIZATION

This UWB LNA circuit designed in this work is afully integrated LNA that was implemented in acommercial 0.18iim SiGe BiCMOS process. Figure 4shows the die photo for the LNA circuit. Parasitics ofpassive bonding pads were considered in the LNA circuitdesign and measurements. This Section discusses thesimulated and measured LNA performance.

The LNA circuit performance can be improved byfine-tuning the VBE bias. The VBE was initially set to 0.84V.Increase of the VBE bias leads to better gain and noisefigure, however, the power consumption increasessignificantly too. For a VBE>O.87V, the transistor movesinto high-current region, which leads to the degradations ingm and P. The LNA is tuned to an optimal VBE=0.87V,which results in a very low power consumption 14.8mW.Figures 5 show the gain, noise figure performance of theUWB LNA circuit from both simulation andmeasurements for VBE=0.87V. Good broadband gain wasachieved across the 3.1-6GHz ultra wideband with a peakgain of 18.14dB at 3.1GHz and an adequate gain of12.63dB at 6GHz, a low NF of 3.02dB at 3.1GHz to4.23dB at 6GHz. Figure 6 shows the measured results forSll, S22 and Kf factor. A low SI, loss of better than -6dBacross the 3.1-6GHz UWB frequency band and a S22 ofless than -5dB are achieved. In general, the measured gaindata agrees well with the simulation results. The circuitstability is ensured by designing the Kf >1 for the entire3.1-6GHz, which is confirmed in testing as shown inFigure 6. This is done by selecting an adequate R,t withoutintroducing any significant extra thermal noise to the LNA,therefore guarantees the unconditional stability of thisUWB LNA circuit.

m

.:co(5

20-

16-

12-

8-

4.

U Simulation Gain---o Measured Gain

Measured Noise Figure-0-- Simulation Noise Figure

t-T 7ww0 . .

3G 4G 5GFrequency (Hz)

6G

Fig.5. Simulated and measured gain and noise figureresults across the 3.1-6GHz spectrum with VBE=0.87V

O-

mco70cJ0aoC.)

cc

-4-

-8-

-12-

-16-

0 Measured S11. 0 Measured S22

* Kf -

I*3G 4G 5G

Frequency (GHz)6G

6

-5

4

3

2

Fig.6. Measured reflection data at input and output and thestability of the LNA with VBE=0.87V.

IV. CONCLUSION

We report design of an optimal low-power 3.1-6GHzLNA circuit for pulse-based lower band UWB transceivers.The LNA was designed and implemented in a commercial0.18,m SiGe BiCMOS. The LNA uses tunableshunt-series feedback topology to ensure low powerconsumption while achieve optimized ultra broadbandgain and noise performance. For the optimal bias ofVBE=0.87V, the measured LNA results achieves adequateperformance across the ultra wide UWB spectrum of3.1-6GHz, featuring: a broadband gain of 18.14dB-12.63dB, a low NF of 3.02dB-4.23dB across the 2.9GHzspectrum, a very low power consumption of less than15mW at 3V supply, low input reflection of less than -6dB,and S22 of less than -5dB.

ACKNOWLEDGEMENT

The authors wish to thank the Skyworks Solutions,Inc., for IC fabrication.

REFERENCES

[1] FCC, First Report and Order, FCC 02-48, Feb. 14, 2002.[2] IEEE Task Group 15a, http://www.ieee802.org/l15/pub/

TG3a.html.[3] M. Win and R. Scholtz, "Impulse radio: How it works,"

IEEE Communications Letters, vol. 2, no. 2, pp. 36-38,Feb. 1998.

[4] K. Kobayashi and A. Oki, "A low-noise baseband 5GHzdirect-coupled HBT amplifier with common-base activeinput match," IEEE Microwave and Guided Wave Letters,vol. 4, no. 1 1, pp. 373-375, Nov. 1994.

[51 Q. Huang and F. Piazza, "The impact of scaling down todeep submicron on CMOS RF circuits," IEEE Journal ofSolid State Circuits. vol.33, no. 7, pp. 1023-1036 July1998.

[6] Paul Gary, Analysis and Design of Analog IntegratedCircuits, JOHN WILEY & SONS, INC., 2000, ISBN:0-471-32168-0.

220