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ISSCC97 I SESSION 22 / COMMUNICATIONS BUILDING BLOCKS II / PAPER SP 22.6 SP 22.6: High Dynamic Range Variable-Gain Amplifier for CDMA Wireless Applications Gurkanwal (Kamal) Singh Sahota, Charles James Persico Qualcomm Inc., San Diego, CA. A variable-gain amplifier (VGA) integrated in an 0.8pm 12GHz process, has input dynamic range of 90dB with 5dB noise figure a t 45dB of gain and -3dBm IIP3 at -45dB gain. The chip occupies lmm2, uses a single 3.6V supply, achieves at least 250MHz bandwidth, and has excellent performance over temperature. In CDMA transceivers, VGAs are needed for both the receiver and transmitter automatic gain control. A transmit VGA is needed in CDMA systems to regulate each mobile unit transmit power so equal power from each user is received at the base station to optimize system capacity. The realization of the Rx VGA is in a BiCMOS process that has npn transistors with f, = 12GHz and MOS transistors with a n Leff=0.8pm. Concepts developed for the Rx VGA circuit are also used to design the TX VGA. The block diagram of the receiver AGC system is shown in Figure 2. The AGC loop forces the output of the Rx VGA to maintain a constant output power for a varying input power of the desired signal. To ease design of the receiver, it is necessary for the VGA to have the following properties: 1)Gain in dB a linear function of Veontml and a minimum gain control range of 90dB (-45dB to +45dB); 2) Input signal dynamic range of 90dB ( -102dBm < Pin < -12dBm, Rs = 500R) and a bandwidth of at least 300MHz; 3) Low-noise figure for high gains with good blocking and IM < 12mA. characteristics; 4)Single 3.6V power supply, ICC To meet conflicting system requirements such as low-noise figure and high-intercept point, with constraints of low-power dissipation, gain control is distributed in each amplifier stage. The block diagram of the Rx VGA is shown in Figure 1. A current-mode approach for the internal signal path keeps all of the node impedances low, and provides wide bandwidth for low-quiescent current. The input IF voltage is converted to a current by a variable transconductance stage. A BiCMOS process permits use of an MOS transistor as a variable resistor along with high-transconductance bipolar amplifiers. The output current of this stage is amplified or attenuated by two variable-current amplifiers. The output current ofthe second current amp. is converted to a voltage by an external resistor. To achieve 90dB gain control range, isolation of the package and the internal circuit is considered in the design and layout of the Rx VGA. To ease the requirements on isolation, the gain of the amplifier is between -45 and +45dB. In this way a minimum of only 45dB of isolation is required. The variable transconductance of the CDMA input stage shown in Figure 3 is realized by a bipolar emitter degenerated stage with an nMOS transistor used as a variable resistor. By varying the channel resistance, the nMOS transistor local feedback of the emitter degenerated stage is varied. When the input signal level is much larger than the desired output signal level, channel resistance of the nMOS is increased, resulting in a smaller transconductance and higher linearity due to the increased feedback. This feature is desirable since a smaller transcoductance of the input stage results in a lower peak current to be handled by the following stages. This allows the current amplifiers to be biased a t a lower quiescent current a t high input-signal levels.

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When the input signal level is much smaller than desired output signal level, channel resistance is reduced t o increase transconductance and decrease noise contribution from the nMOS and later stages. This also increases gain. The channel resistance of the nMOS transistor is controlled by a technique similar to the one shown in Reference 1.Amplifier A, forces its differential input voltage to zero. This implies: (I1x R,) = (Iz x Rbl); or R,, = (I, / I,) x R,. Amplifier A, forces the source voltages of M, and M, to be equal. Since the gate and source voltages of MI and M, are equal the channel resistances are equal assuming both M, and M, are the same size. Channel resistance is varied by varying the ratio (I, I,). Care must be taken in choosing transistor size to ensure that M, always remains in the triode region. The transconductance of this stage is derived in Figure 2. The current amplifier is shown in Figure 4. It consists of a differential Darlington input stage that has resistive shunt feedback providing current gain from the input to the collectors of &, and Q2.Collector currents in Q1and Qzare conveyed to the output by a translinear loop consisting of Ql, Q2,Q3,and &,. The gain of this stage is proportional to the ratio of tail currents (I, / I,) due to the translinear loop. The addition of the resistive feedback network gives the translinear stage additional current gain and reduces noise contribution from translinear circuits. Both transconductance of the input stage and gain of the current amplifier are proportional to the ratio of two currents, provided the loop gain is sufficiently large in each stage. By making this ratio an exponential function of Vwntm,,the gain of the VGA in decibels will be a linear function of Vmntm,. By scaling and level shifting the input control voltage and applying it to a bipolar differential amplifier, the ratio of collector currents of a bipolar differential amplifier is: (I, / Ic2)= exp((c x (V,,,,, - Vref)N,) where: Vrefis a positive dc voltage independent of temperature used for level shifting Vcontrol and allowing the current ratio to be less than and greater one; c is a scaling constant proportional to absolute temperature; V, is the thermal voltage; and VContm, is the external gain control voltage. The currents IC,and I, are mirrored to control gains of the current amps and transconductance of the input stage. Temperature compensation is by making the scaling constant c, proportional to absolute temperature, so the temperature effect of V, is canceled. Standard translinear techniques make c proportional to absolute temperature. Measurements are a t fixed frequencies of 85MHz and 210MHz. Figure 5 shows the measured and simulated gain versus Veontml of the amplifier over temperature. The measured and predicated performance match very well. Figure 6 shows measured and simulated IIP3 and noise figure versus gain. The Rx and Tx VGAs are in production and used phones , Both chips went into production with no iterations of the design. Acknow Eedgments:

The authors thank K. Chang for help in layout of the design and D. Watson for help in preparing the manuscript. References: [l] Meyer, R. G., W. D. Mack, “A Wideband Low-Noise Variable-Gain BiCMOS TransimpedanceAmplifier,”IEEE J. Solid State Circuits, vol. 29, pp. 701-706,June, 1994.

1997 IEEE International Solid-state Circuits Conference

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ISSCC97 / February 8,1997 / Salon 1-6 / 3:30 PM

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Figure 2: CDMA subscriber unit AGC system.

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Figure 3: Variable input transconductance stage.

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Figure 6: Rx VGA gain vs. temperature and control voltage.

DIGEST OF TECHNICAL PAPERS *

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375

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SP 22.5:. 8GHz and 12.6GHz Si Bipolar MMlCs (Continuedfrom page 373)

Figure 6: Chip micrograph.

SP 22.6: High Dynamic Range Variable-Gain Amplifier for CDMA Wireless Applications (Continuedfrom page 376)

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1997 IEEE InternationalSolid-state Circuits Conference

Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on November 2, 2008 at 07:24 from IEEE Xplore. Restrictions apply.