MRF136 Datasheet, PDF (10/11 Pages) Motorola, Inc – N-CHANNEL MOS BROADBAND RF POWER FETs

DESIGN CONSIDERATIONS
The MRF136 is an RF power N–Channel enhancement
mode field–effect transistor (FET) designed especially for HF
and VHF power amplifier applications. M/A-COM RF MOS
FETs feature planar design for optimum manufacturability.
M/A-COM 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 MRF136 is an enhancement mode FET and, therefore,
does not conduct when drain voltage is applied without gate
bias. A positive gate voltage causes drain current to flow (see
Figure 10). RF power FETs require forward bias for optimum
gain and power output. A Class AB condition with quiescent
drain current (IDQ) in the 25 –100 mA range is sufficient for
many applications. For special requirements such as linear
amplification, IDQ may have to be adjusted to optimize the
critical parameters.
The MOS gate is a dc open circuit. Since the gate bias circuit
does not have to deliver any current to the FET, a simple
resistive divider arrangement may sometimes suffice for this
function. Special applications may require more elaborate
gate bias systems.
GAIN CONTROL
Power output of the MRF136 may be controlled from rated
values down to the milliwatt region (>20 dB reduction in power
output with constant input power) by varying the dc gate
voltage. This feature, not available in bipolar RF power
devices, facilitates the incorporation of manual gain control,
AGC/ALC and modulation schemes into system designs. A
full range of power output control may require dc gate voltage
excursions into the negative region.
AMPLIFIER DESIGN
Impedance matching networks similar to those used with
bipolar transistors are suitable for MRF136. See M/A-COM
Application Note AN721, Impedance Matching Networks
Applied to RF Power Transistors. Both small signal scattering
parameters and large signal impedance parameters are
provided. Large signal impedances should be used for
network designs wherever possible. While the s parameters
will not produce an exact design solution for high power
operation, they do yield a good first approximation. This is
particularly useful at frequencies outside those presented in
the large signal impedance plots.
RF power FETs are triode devices and are therefore not
unilateral. This, coupled with the very high gain, yields a
device capable of self oscillation. Stability may be achieved
using techniques such as drain loading, input shunt resistive
loading, or feedback. S parameter stability analysis can
provide useful information in the selection of loading and/or
feedback to insure stable operation. The MRF136 was
characterized with a 27 ohm input shunt loading resistor.
For further discussion of RF amplifier stability and the use
of two port parameters in RF amplifier design, see M/A-COM
Application Note AN215A.
LOW NOISE OPERATION
Input resistive loading will degrade noise performance, and
noise figure may vary significantly with gate driving imped-
ance. A low loss input matching network with its gate
impedance optimized for lowest noise is recommended.
REV 7
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