MRF134_17 Datasheet, PDF (11/13 Pages) M/A-COM Technology Solutions, Inc. – The RF MOSFET Line: Broadband RF Power FET

MRF134
The RF MOSFET Line: Broadband RF Power FET
5.0W, to 400MHz, 28V
Rev. V1
RF POWER MOSFET CONSIDERATIONS
DESIGN CONSIDERATIONS
The MRF137 is a RF power N–Channel enhancement-
mode field–effect transistor (FET) designed especially for
VHF power amplifier applications. M/A-COM RF MOS
FETs feature a vertical structure with a planar design,
thus avoiding the processing difficulties associated with
V–groove vertical power FETs.
M/A-COM Application Note AN211A, FETs in Theory
andPractice, is suggested reading for those not familiar
with the construction and characteristics of FETs.
The major advantages of RF power FETs include high
gain, low noise, simple bias systems, relative immunity
from thermal runaway, and the ability to withstand se-
verely mismatched loads without suffering damage.
Power output can be varied over a wide range with a low
power dc control signal, thus facilitating manual gain
control, ALC and modulation.
DC BIAS
The MRF137 is an enhancement mode FET and, there-
fore, does not conduct when drain voltage is applied.
Drain current flows when a positive voltage is applied to
the gate. See Figure 10 for a typical plot of drain current
versus gate voltage. RF power FETs require forward bias
for optimum performance.
The value of quiescent drain current (IDQ) is not critical
formany applications. The MRF137 was characterized at
IDQ = 25 mA, which is the suggested minimum value of
IDQ. For special applications such as linear amplification,
IDQ may have to be selected to optimize the critical pa-
rameters.
The gate is a dc open circuit and draws no current.
Therefore, the gate bias circuit may generally be just a
simple
resistive divider network. Some special applications
may require a more elaborate bias system.
GAIN CONTROL
Power output of the MRF137 may be controlled
from its rated value down to zero (negative gain) by
varying the dc gate voltage. This feature facilitates
the design of manual gain control, AGC/ALC and
modulation systems. (See Figure 9.)
AMPLIFIER DESIGN
Impedance matching networks similar to those
used with bipolar VHF transistors are suitable for
MRF137. See M/A-COM Application Note AN721,
Impedance Matching Networks Applied to RF Power
Transistors. The higher input impedance of RF MOS
FETs helps ease the task of broadband network de-
sign. Both small signal scattering parameters and
large signal impedances are provided. While the s–
parameters will not produce an exact design solution
for high power operation, they do yield a good first
approximation. This is an additional advantage of RF
MOS power FETs.
RF power FETs are triode devices and, there-
fore, not unilateral. This, coupled with the very high
gain of the MRF137, yields a device capable of self
oscillation. Stability may be achieved by techniques
such as drain loading, input shunt resistive loading, or
output to input feedback. Two port parameter stability
analysis with the MRF137 s–parameters provides a
useful tool for selection of loading or feedback cir-
cuitry to assure stable operation. See M/A-COM Ap-
plication Note AN215A for a discussion of two port
network theory and stability.
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