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Maxim > Design Support > Technical Documents > Application Notes > Voltage References > APP 3702
Keywords: pa,power,amplifier,900mhz,ism,pcb,layout,2235,
APPLICATION NOTE 3702
MAX2235—Layout Optimization Techniques
Dec 23, 2005
Abstract: The MAX2235 is a three-stage power amplifier that operates in the 800MHz to 1000MHz range
and has been shown to produce up to 30dBm in GSM and ISM applications. Achieving maximum
performance is not a trivial matter, and the best starting point is a good PC board layout. This article
reviews proven layout practices and helps the reader understand both the nature of the PA and rationale
behind the proposed techniques.
General
The MAX2235 is a three-stage power amplifier that operates from 800MHz to 1GHz and produces up to
30dBm of output power in GSM and ISM applications. Achieving this type of performance is not a trivial
matter, and the best starting point is a good PC board layout. Conversely, and more importantly, a
poorly designed PC board layout will absolutely preclude achieving optimum performance.
This article will review proven layout practices and help the reader understand both the nature of this IC
and rationale behind the proposed techniques. Used as a guideline for new layouts, it will help anyone
designing with the MAX2235 avoid common pitfalls and get the most out of their design.
Supply Bypassing and the Interstage Match
Perhaps one of the least understood and most important aspects of this IC is the interstage match.
Because the amplifier is actually comprised of three separate stages, and each stage has slightly
different impedance characteristics, they must be matched to each other to provide maximum power
transfer.
The V
CC
pins 3, 5, 8, and 9 on the device are not purely supply inputs in need of bypassing. The lumped
element capacitance (and parasitics) applied to these pins interact with the device internally and provide
some measure of interstage matching. This allows the designer to customize the match for their
particular application.
In order to capitalize on the flexibility of this design, the bypass capacitor positions should be somewhat
adjustable along the V
CC
trace, with the cap between the V
CC
via and the device pin. The optimum
amount of trace length between the interstage cap and the IC pin can be established empirically for a
desired operating frequency by sliding the cap along the V
CC
trace (see Figure 1). Use of high-Q
capacitors at these locations generally provides the best match, and 0402 size components make the
physical process of tuning manageable (as opposed to 0603s that leave little room for adjustment, and
0201s that may be more difficult to physically manipulate). The Murata GJM1555 series (previously
GJ615) has been used in this application with good results. They have a Q factor of greater than 100 at
900MHz.
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