New Semiconductor Components for RF Front End Applications

New Semiconductor Components for RF Front End Applications

Siemens Semiconductors
Munich, Germany

One of the fastest growing segments of consumer electronics is wireless communication. The world market for mobile radio has emerged rapidly over the last few years, presenting a challenge for semiconductor suppliers to follow the rapidly changing technical requirements of mobile radio systems. On the one hand, an ongoing supply voltage decrease is being instituted to reduce the size and weight of portables; on the other hand, more and more functions must be integrated. This article describes several new 3 V semiconductor products specifically targeted at these mobile radio requirements.

Varicap Diodes

Currently, a huge demand exists for RF varactor diodes with a high capacitance ratio at tuning voltages down to 3 V and below. Varactor diodes are typically used in the design of VCOs and phase-locked loops (PLL) such as in the new models PMB2345, -46 and -47 dual-BiCMOS frequency synthesizers. In addition, a low series resistance rs is important for a steep filter characteristic to help determine the phase noise of the VCOs.

Due to the hyperabrupt doping profile of the diode's pn junction, a maximum capacitance variation over the applied reverse voltage can be achieved. Hence, an in-house ion implantation process has been adapted to produce RF varactor diodes to meet the high capacitance vs. tuning voltage ratio and low series resistance requirements. Table 1 lists the typical characteristics of these new varicap diodes.

The new diodes are available in the very small (1.7 mm X 0.8 mm X 0.7 mm) SCD80 package. Due to the low package parasitic losses (smaller package capacity and less inductance due to the short leads compared to the SOD323), advanced tuning performance for higher frequencies is achieved and less PCB area is required.

LNA MMICs

Based on the well-established B6HF process with a transit frequency of 25 GHz, the SIEGET-25 transistors (BFP405/BFP420/BFP450/BFP490) covering a collector current range of 12 to 600 mA have been in use for some time with current production in excess of 100,000,000 per year. In particular, the BFP405 and BFP420 devices are frequently applied as low noise amplifier (LNA) transistors in well-established telephone systems worldwide. The technical performance of the LNA in the front end is decisive for the system sensitivity. A high gain in combination with a low noise figure and proper intermodulation capability at a low current consumption are required.

In order to reduce active and passive device count, space requirements and design efforts, the substitution of these current discrete solutions with fully integrated MMICs is essential for new designs. The type BGC405 and BGC420 3 V LNA MMICs with active biasing have been designed based on these SIEGET-25 transistors. Figure 1 shows the internal design of the active-biased LNA.

The single-stage amplifier consists of an active biasing network and a BFP405 or BFP420 transistor, which is designed for optimum matching over the entire bandwidth up to 3 GHz. Due to the active biasing, the device current is independent of supply voltage changes as well as hFE variations caused by production. The device current is adjusted by an external resistor between +V and Vr. Furthermore, the control pin Vc allows switching between active and standby modes.

The active-biased LNAs are produced using B6HF technology and demonstrate good gain and noise performance at 3 V, as listed in Table 2 . The devices are mounted in the new SCT-598 SMD package, which ensures the best RF performance and an easy external circuit design.

Power Amplifiers

GaAs power amplifiers (PA) have achieved a dominant position in mobile radio in the frequency range of 800 MHz and above. GaAs PAs offer a good combination of output power, efficiency and linearity performance, especially in low voltage battery environments (3.6 V and below). Considering the progress made in CMOS baseband technology (toward low power consumption), the power-added efficiency (PAE) of these GaAs devices is decisive for talk time and, hence, is one of the most critical parameters in linear systems.

The type CGY81 device is a tri-mode, single-band, two-stage amplifier covering the cellular band from 824 to 849 MHz. The CGY81 is suitable for the cellular AMPS, US-TDMA (IS136) and US-CDMA (IS95) standards. The internal device architecture is optimised for a 3 V battery environment and CW operation. It is housed in the TSSOP16 package with an integrated heat slug at the backside. This package optimizes the thermal resistance together with RF performance and, therefore, supports CW applications such as CDMA systems and driver PAs in base stations.

The PA is driven into saturation under CW conditions due to the FM of the analog cellular AMPS. Figure 2 shows the device's output power Pout and PAE as functions of the input power Pin at 836 MHz. At a maximum output power level of 31 dBm, the associated PAE (together with a power gain of 24 dB) reaches a typical value of 55 percent with an output power matching network for linear operation.

The digital AMPS (DAMPS) standard (IS136) offers a higher user capacity for the available frequency range compared to AMPS owing to TDMA. The duty cycle is approximately 33 percent of the frame (duration time is 20 ms). IS136 uses a p/4 quadrature phase-shift keying (QPSK) modulation with moderate envelope modulation. Therefore, the PA cannot be driven into saturation and must operate approximately 3 to 5 dB backed off from saturation in order to reduce adjacent-channel interference. The PAE for those systems is inherently significantly lower compared to PAs driven into saturation.

The PA's linearity performance is described by the 1 dB compression point (P1dB), third-order intermodulation (IM3) or adjacent-channel power/alternate-channel power rejection (ACPR/ALPR) measurements. P1dB and IM3 describe the PA linearity rather roughly, whereas ACPR and ALPR are specified measurement conditions for the actual system. For these measurements, a modulated RF test carrier according to the IS136 or IS95 spectral mask is fed into the input port of the PA.

By virtue of the nonlinearity behaviour of the PA, intermodulation products of the carrier within one channel generate a power spectrum in the adjacent and alternate channels. The ratio of the power in the modulated carrier to the measured power level in the ACPR and ALPR in a given resolution bandwidth (30 kHz) is a figure of merit for the description of the linearity requirements. To achieve the maximum PAE, an optimised trade-off between device structure, operation point and linearity performance must be found using careful design simulations and measurements.

Simulations are carried out by means of a nonlinear Materka model using Microwave Harmonica. The devices are fabricated using the highly reliable in-house MESFET DIOM technology with an 800 nm gate length. A control circuit is integrated for adjustment of the PA's optimum operation quiescent current. Any negative voltage between -5 and -7 V must be supplied to this control circuit to bias the device to the targeted level without further adjustment during mass production. It is possible to use a silicon pnp transistor with low saturation voltage such as type BCP69 or BCP72 devices to switch the drain current off during the receive mode and nonactive time slots.

A dynamic range of 50 dB for the output power is achieved by means of an external control pin (0.5 to 2.5 V), however, linearity performance is affected due to a change in the operation point by pinching off the internal FETs. If output power control is required, Si Schottky diodes such as types BAS70, BAT62 or BAT68 in a temperature-compensated configuration (double chip in one package) are utilized.

Figure 3 shows the ACPR, ALPR and transducer gain (TG) as functions of the output power at 836.5 MHz. Up to 29 dBm (the nominal output power level) the ACPR remains constant at 30 dB beyond the carrier, meeting IS136 transmission requirements. The corresponding transducer gain decreases from 29 to 26.5 dB, and the ALPR distance increases to -50 dBc (specified -48 dBc). When used for boosting TDMA signals, the maximum PAE is 40 percent determined at -28 dBc of the ACPR at a 29 dBm maximum ouput power level. Maximum gain ripple in the transmit bandwidth is only 1.5 dB.

The CGY81 device also can be used for the IS95 CDMA standard. The linearity requirements are quite similar to the IS136 standard, however, the ACPR is determined under different measurement conditions. Moreover, the device must be able to operate in CW mode. ACPR (according to the IS95 spectral mask) is measured at 900 kHz and 1.98 MHz offset from a chosen channel. The implemented shifted QPSK scheme is for the uplink to base station (downlink is QPSK). Figure 4 shows ACPR, ALPR and TG as functions of output power at 836.5 MHz. ACPR and ALPR are -45 and -58 dBc, respectively, at +29 dBm CDMA carrier (Specified maximum values are -45 and -54 dBc). The corresponding TG decreases from 29 to 26.5 dB. A maximum PAE of 35 percent has been determined at -42 dBc at 900 kHz offset in a resolution bandwidth of 30 kHz. Maximum gain ripple at all operating frequencies is only 1.5 dB.

The type CGY191 device is a dual-mode, two-stage PA spanning the PCS frequency range from 1850 to 1910 MHz. Like the CGY81 PA, it can be used for IS136 and IS95 standard applications with a nominal maximum ouput power level of +29 dBm and associated power gain of 24 dB. As mentioned previously, power ramping with a dynamic range of 50 dB is adjusted via an extra control pin by pinching the FETs. An integrated control circuit for proper adjustment of quiescent current minimizes the external component count.

Figure 5 shows the ACPR, ALPR and TG as functions of the output power for a TDMA carrier at 1880 MHz. The ACPR increases at low output power levels from -38 to -29 dBc at the nominal 29 dBm output power. The ALPR increases from -63 dBc to -48 dBc. The corresponding TG is in 1.5 dB compression based on a small-signal gain of 22.8 dB. Maximum PAE under TDMA test conditions is typically 40 percent at an ouput power level of 29 dBm and -42 dBc ACPR. The gain ripple within the transmit bandwidth is below 1 dB.

Determination of the ACPR for the PCS CDMA standard is different compared to the cellular band. ACPR is determined at an offset frequency of 1.25 MHz off channel center frequency. Due to the larger offset frequency, PAE in the PCS band is higher than in the cellular band.

The amplifier achieves a maximum PAE of 40 percent when boosting a CDMA test signal at an ACPR of -42 dBc. Figure 6 shows ACPR, ALPR and TG as functions of the ouput power at 1880 MHz. ACPR increases from -62 to -42 dBc in an ouput power range from 14 to 29 dBm. ALPR increases from -72 to -54 dBc in the same output power range and is fully compliant to the IS95 specification. Maximum gain ripple over the total frequency range is below 2 dB.

The next step is the integration of the CGY81 and CGY191 devices on one chip in one package. The type CGY0819 device is a triple-mode, dual-band PA with two integrated PA chains for both frequency bands with separated input ports. This PA MMIC can be used for AMPS/cellular/PCS TDMA and CDMA applications. Output power is controlled via a common control pin for both paths (Vcon). Input, interstage and output matching are optimized according to the TDMA/CDMA specifications. The small structures together in one package will reduce costs and cut the required PCB area.

Conclusion

All of the results presented here have been achieved without exotic and expensive epitactically grown starting wafers. The introduced devices, fabricated with the well-matured in-house MESFET technology, rival results obtained using expensive state-of-the-art PHEMT and HBT technologies. Although the devices still require a negative bias supply, they offer an attractive low cost, high reliability solution suitable for mass production. Moreover, the performance of heterostructures is being investigated for further improvement of PAs for mobile radio applications.

Siemens Semiconductors,
Munich, Germany and

Siemens Microelectronics Inc.,
San Jose, CA
(408) 501-6000.