MRF137 Datasheet, PDF (8/10 Pages) Motorola, Inc – N-CHANNEL MOS BROADBAND RF POWER FET

DESIGN CONSIDERATIONS
The MRF137 is a RF power N–Channel enhancement
mode field–effect transistor (FET) designed especially for
VHF power amplifier applications. Motorola RF MOS FETs
feature a vertical structure with a planar design, thus avoiding
the processing difficulties associated with V–groove vertical
power FETs.
Motorola Application Note AN211A, FETs in Theory and
Practice, 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 ther-
mal runaway, and the ability to withstand severely mis-
matched 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, therefore,
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 for
many 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 parameters.
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 bi-
polar VHF transistors are suitable for MRF137. See Motorola
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
design. Both small signal scattering parameters and large sig-
nal 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, therefore, 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
circuitry to assure stable operation. See Motorola Application
Note AN215A for a discussion of two port network theory and
stability.
MRF137
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MOTOROLA RF DEVICE DATA