MRF136Y Datasheet, PDF (8/9 Pages) Tyco Electronics – N-CHANNEL MOS BROADBAND RF POWER FET

DESIGN CONSIDERATIONS
The MRF136Y 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 MRF136Y is an enhancement mode FET and, there-
fore, does not conduct when drain voltage is applied without
gate bias. A positive gate voltage causes drain current to flow
(see Figure 8). RF power FETs require forward bias for
optimum gain and power output. A Class AB condition with
quiescent drain current (I_DQ) in the 25–100 mA range is
sufficient for many applications. For special requirements
such as linear amplification, I_DQ 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 MRF136Y 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 the MRF136Y. See
M/A-COM Application Note AN721, Impedance Matching
Networks Applied to RF Power Transistors. Large signal
impedance parameters are provided. Large signal imped-
ances 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 MRF136Y was
characterized with a resistive feedback loop around each of its
two active devices.
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.
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