MRF136Y_17 Datasheet, PDF (9/11 Pages) M/A-COM Technology Solutions, Inc. – The RF MOSFET Line

MRF136Y
The RF MOSFET Line
30W, to 400MHz, 28V
Rev. V2
RF POWER MOSFET CONSIDERATIONS
DESIGN CONSIDERATIONS
The MRF136Y 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 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 thermal runaway, and the ability to withstand severely
mismatched loads without suffering damage. Power output
can be varied over a wide range with a low power dc con-
trol signal, thus facilitating manual gain control, ALC and
modulation.
DC BIAS
The MRF136Y 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 MRF136Y 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. There-
fore, 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 MRF136Y may be controlled from
its rated value down to zero (negative gain) by varying the
dc gate voltage. This feature facilitates the design of man-
ual 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 MRF136Y. 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 design. 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, therefore, not
unilateral. This, coupled with the very high gain of the
MRF136Y, yields a device capable of self oscillation. Sta-
bility 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 MRF136Y s–
parameters provides a useful tool for selection of loading or
feedback circuitry to assure stable operation. See M/A-
COM Application Note AN215A for a discussion of two port
network theory and stability.
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 im-
pedance optimized for lowest noise is recommended.
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