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Half Watt Driver Amplifier with Dynamically Adjustable Bias

May 15, 2012
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Half Watt Driver Amplifer With Dynamically Adjustable Bias

Today’s radio designs, and other RF communication equipment, are being made to consume less power while occupying less physical space, which leads to less available board area for heat sinking. In addition, these systems are being deployed in more extreme environmental conditions where only passive cooling is being used, which leads to the need for very reliable ICs that can withstand greater temperature variations. When the factor of system output power scalability gets added in, then the need emerges for a new equally scalable RF driver amplifier. To satisfy all of these goals, Analog Devices introduces the ADL5324 ½ W SOT-89 RF driver amplifier.

The ADL5324 incorporates a dynamically adjustable biasing circuit that allows for the customization of OIP3 and P1dB performance from 3.3 to 5 V, without the need for an external bias resistor. This feature gives the designer the ability to tailor driver amplifier performance to the specific needs of the design. The adjustable bias also allows for the driver to be dynamically biased in order to conserve power consumption when the full performance of the driver amplifier is not required, such as when a system is in stand-by mode. This scalability reduces the need to evaluate and inventory multiple driver amplifiers for different output power requirements, from 25 to 29 dBm output power levels. The ADL5324 is also rated to operate across the widest temperature range of -40° to +105°C for reliable performance in designs that will experience higher temperatures, such as power amplifiers. The ½ W driver amplifier also covers the wide frequency range of 400 to 4000 MHz, and only requires a few external components to be tuned to a specific band within that wide range. This high performance broadband RF driver amplifier is well-suited for a variety of wired and wireless applications including cellular infrastructure, ISM band power amplifiers, defense equipment and instrumentation equipment.

The ADL5324 GaAs HBT ½ W driver amplifier consumes a low 5 V current of 133 mA and delivers best-in-class performance with an  OIP3 of 43.1 dBm, a P1dB of 29.1 dBm, a gain of 14.6 dB and a low noise figure of 3.8 dB at 2140 MHz. When the bias voltage is reduced to 3.3 V the driver only consumes 62 mA and delivers an OIP3 of 34.4 dBm, a P1dB of 25.3 dBm, a gain of 13.6 dB and a lower noise figure of 3.2 dB at 2140 MHz. The driver amplifier can also be biased anywhere from 3.3 to 5 V to meet system needs that are in between the performance stated at 3.3 or 5 V. This feature creates the opportunity for dynamic biasing of the driver amplifier where a variable supply is used to allow for full 5 V biasing under large signal conditions, and then reduced supply voltage when signal levels are smaller and lower power consumption is desirable (see Figure 1).

Figure 1

Fig. 1 ADL5324 OIP3 and P1dB vs. Vcc and temperature showing that 3.3 to 5 V bias meets system needs.

Figure 2

Fig. 2 ADL5324 ACPR vs. output power and Vcc (3GPP 3.5 TM1-64 at 2.14 GHz) showing a minimal reduction to 15 dBm in the -55 dBc ACPR at 3.3 V bias.

The ADL5324 also delivers excellent ACPR versus output power and bias voltage. The driver can deliver greater than 17 dBm of output power at 2140 MHz, while achieving an ACPR of −55 dBc at 5 V. If the bias is reduced to 3.3 V, the −55 dBc ACPR output power only minimally reduces to 15 dBm (see Figure 2).

The ADL5324 further simplifies RF design by eliminating the need for complicated external tuning. The driver only uses 50 V lines at the input and output of the amplifier and does not require a bias resistor. The ADL5324 only needs one shunt capacitor at its input and one shunt capacitor at its output for frequency tuning. The usual AC coupling capacitors and DC bias choke inductor are also required, along with the standard bypass capacitance on the DC bias trace.

The ADL5324 was also designed and packaged to simplify thermal considerations. The driver has best-in-class current consumption of 133 mA with 5 V, which reduces the amount of heat that is created. The 3.3 V current consumption is reduced to 62 mA, which further reduces the amount of heat created. The standard SOT-89 package also has a large backside ground paddle that allows for an efficient thermal transfer path from the driver amplifier. The data sheet shows the recommended circuit board land pattern with added thermal transfer vias to further improve thermal transfer from the driver amplifier. This allows the driver to be kept within safe operating temperatures without the need for forced air cooling, and the driver is fully specified over the widest operating temperature range of −40° to +105°C. The driver amplifier is also rated to an ESD rating of ±3 kV (HBM, Class 2), which makes it equally robust in high volume manufacturing environments.

Another way Analog Devices helps improve RF design is with the amount of information provided in the ADL5324 data sheet. Data provided such as critical parameter variation versus temperature, from −40° to +105°C, voltage supply, from 3.3 to 5 V, and operating frequency, from 400 to 4000 MHz, reduces the amount of qualification time a designer needs to spend. That reduction in qualification time can significantly improve a projects’ time-to-market. The comprehensive data sheet also allows the designer to accurately determine the least amount of power consumption achievable to meet the performance goals for their given application.

Analog Devices improves RF design in multiple ways; through innovative circuit design, simplified tuning requirements and detailed data sheet information. These attributes allow RF designers to go to market quickly with solutions that meet their needs for smaller, lower power consuming systems.

Analog Devices,
Norwood, MA,
www.analog.com/rf.

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