M537 Datasheet, PDF (1/4 Pages) M/A-COM Technology Solutions, Inc. – GaAs MMIC Based Control Components with Integral Drivers

Application Note
M537
GaAs MMIC Based Control Components with Integral Drivers
Rev. V5
Introduction
This application note describes the fundamental
operation and features of a new series of control
components. These switches comprise a family of
devices that use GaAs FET MMIC technology for the
RF circuitry and incorporate application specific
integrated circuit (ASIC) technology to realize an
integral TTL or CMOS compatible driver. The circuitry
is housed in ceramic surface mount packages that give
repeatable and predictable performance from DC to 3
GHz.
GaAs MMIC Switch Technology
This family of switches is based on metal
semiconductor field effect transistor (MESFET)
technology. The MESFETs are N-Channel depletion
mode devices with 1 µm Schottky gates. Depletion
mode devices are low resistance at 0 bias. When a
negative voltage is applied to the gate, the electric field
begins to narrow the channel, increasing the
resistance. The voltage that closes off the channel and
creates the highest resistance of the MESFET device is
known as the “pinch-off” voltage. Pinch-off voltages for
M/A-COM MESFETs are typically –2.5 volts.
By varying the gate voltage between 0 volts and some
value greater than pinch-off (typically –5 to –8 volts),
the MESFET acts as a variable resistor. MESFETs
can be arranged into series and/or shunt configurations
and biased to provide on and off switching
characteristics. A representative schematic for a GaAs
MMIC switch chip is shown in Figure 1.
The voltages at control inputs A and B are
complementary. From the preceding discussion, when
control input A is low (0 to –0.2 volts), the MESFETs
controlled by input A will be low resistance. Since the
inputs are complementary, control input B will be high (-
5 to -8 volts). The MESFETs controlled by this input
will be in the high resistance state. Under this set of
logic conditions, the incident RF signal will be switched
to port RF2 as shown in Figure 1.
Definition of Terms
The final design of a switch represents a trade-off
among several performance parameters. Many of
these parameters are interrelated; so improved
performance in one area comes at the expense of
degraded performance in another. This section will
define commonly used terms and provide insight into
design trade-offs.
Insertion Loss
Insertion Loss, represented in Figure 2, is a measure of
the difference between the input and output power of
the on path of a device. This loss, expressed in
decibels (dB), is composed of power dissipated in
components, reflected from mismatches and radiated
into free space. Insertion loss arises from the fact that
components used in the design of the switch are not
ideal.
ISO = 10 log ⎜⎛ ^PL ⎟⎞
⎝ P' L ⎠
Figure 1. Schematic for MASW6010
Figure 2. Insertion Loss
1
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