MRF134 Datasheet, PDF (9/10 Pages) Motorola, Inc – N-CHANNEL MOS BROADBAND RF POWER FET

DESIGN CONSIDERATIONS
The MRF134 is a RF power N–Channel enhancement
mode field–effect transistor (FET) designed especially for
VHF power amplifier and oscillator 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 AN–211A, 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 control
signal, thus facilitating manual gain control, ALC and modula-
tion.
DC BIAS
The MRF134 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
9 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 (I_DQ) is not critical for
many applications. The MRF134 was characterized at I_DQ =
50 mA, which is the suggested minimum value of I_DQ. For
special applications such as linear amplification, I_DQ 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 MRF134 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 8.)
AMPLIFIER DESIGN
Impedance matching networks similar to those used with
bipolar VHF transistors are suitable for MRF134. 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–parame-
ters 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
MRF134, 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. The
MRF134 was characterized with a 68–ohm input shunt
loading resistor. Two port parameter stability analysis with the
MRF134 s–parameters provides a useful–tool for selection of
loading or feedback circuitry to assure stable operation. See
MA-COM Application Note AN215A for a discussion of two port
network theory and stability.
Input resistive loading is not feasible in low noise applica-
tions. The MRF134 noise figure data was generated in a circuit
with drain loading and a low loss input network.
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