2.0-22.0 GHz GaAs MMIC
Power Amplifier
September 2009 - Rev 14-Sep-09
CMM0014-BD
App Note [1] Biasing
- As shown in the bonding diagram, this device operates
using a self-biased architecture and only requires one drain bias. Bias is
nominally Vd=8V, Id=300 mA. For additional assistance in setting current via
source resistor, see source resistance table below.
App Note [2] Bias Arrangement
- Each DC pad (Vd) needs to have DC bypass
capacitance (~100-200 pF) as close to the device as possible. Additional DC
bypass capacitance (~0.01 uF) is also recommended. Additionally, to achieve
the required broadband decoupling network a high-Q Drain bias inductor with
high-Q bypass capacitor is needed. The proper network is necessary in order to
bring Drain bias into the device with minimal impact on RF performance. The
high-Q inductor is typically an air coil that can be purchased from an air coil manufacturer (Microwave Components or Piconics for example).
The air coil needs to have minimum current handling capability, thus planned operating current needs to be defined and considered before
defining actual air coil to be used. Mimix recommends 1.4 mil diameter gold wire and 4 turns as a starting point and may need to be
optimized based on the actual application. Self-resonance of the bias inductor causes degradation in performance at both the low and high
ends of the band. The self resonance is sensitive to spacing between turns and number of turns used. For example, the more turns in the
Drain bias inductor the lower the self-resonant frequency of the inductor creating high end RF performance degradation. The opposite is
true for a smaller number of turns. In terms of coil attachment to MMIC device (wedge bond tool method), cut coil leads to desired length,
use tweezers or wedge bond tip (press on wire to pick up) to place coil for bonding. Make first bond on MMIC die bond pad using wedge
bonder tool. Move coil lead as necessary and make second and final bond to bypass capacitor with wedge bond tool using same method as
first bond.
Current Select
- At times the need to balance performance against system
power budgets forces a trade off between bias current, gain, P1dB, or other
parameters. This note includes information on how to use the built-in binary
bias ladder to adjust the currents enabling this trade off. The bias is controlled
by the self bias resistor network in the bottom right corner of the die. These
resistors have binary relative values so that you can step the current from a
minimum to a maximum with multiple different bias options available along
the way. The infinity option is not useful as there is no current flow with all
resistors open. Using the information from the current select table shown here
allows the user to set the resistors adjusting the current up or down from a
nominal value. In addition, the table can be used to estimate how to make a
change with minimum trial and error. The net result is that the current can be
adjusted over a wide range with incremental control.
Bonding Substrate
- If you are concerned about dialing in the exact current or
making fine adjustments to the bias point it is recommended that a bonding
substrate, like the one shown here, be used. The purpose is to allow the chip to
substrate wire bonds to be left intact and not to be used for adjustments. The
bond wires that go from the substrate to ground are then added or subtracted
to tune the bias as necessary.
App Note [3] Material Stack-Up –
In addition to the practical aspects of bias and
bias arrangement, device base material stack-up also must be considered for best
thermal performance. A well thought out thermal path solution will improve overall
device reliability, RF performance and power added efficiency. The photo shows a
typical high power amplifier carrier assembly. The material stack-up for this carrier
is shown below. This stack-up is highly recommended for most reliable performance
however, other materials (i.e. eutectic solder vs epoxy, copper tungsten/copper
moly rib, etc.) can be considered/possibly used but only after careful review of
material thermal properties, material availability and end application performance requirements.
MMIC, 3mil
Diemat DM6030HK Epoxy, ~1mil
MOLY Rib, 5mil, Au plated
MOLY Carrier, 25mil
Au plated
Copper Block
Alumina Substrate
AuSn Eutectic Solder
CMM0014 - Source Resistance Table
Left
3.3
0
1
0
1
0
1
0
1
Center
2.1
0
0
1
1
0
0
1
1
Corner
1.2
0
0
0
0
1
1
1
1
Net R
Infinity
3.30
2.10
1.28
1.20
0.88
0.76
0.62
Delta Current
mA
NA
-200
-175
-150
-75
-50
-25
Max
Mimix Broadband, Inc., 10795 Rockley Rd., Houston, Texas 77099
Tel: 281.988.4600 Fax: 281.988.4615 mimixbroadband.com
Page 7 of 9
Characteristic Data and Specifications are subject to change without notice.
©2009
Mimix Broadband, Inc.
Export of this item may require appropriate export licensing from the U.S. Government. In purchasing these parts, U.S. Domestic customers accept
their obligation to be compliant with U.S. Export Laws.
MIMIX [ MIMIX BROADBAND ]
