
MRF1507 MRF1507T1
10
MOTOROLA RF DEVICE DATA
DC BIAS
Since the MRF1507 is an enhancement mode FET, drain
current flows only when the gate is at a higher potential than
the source. RF power FETs operate optimally with a quiescent
drain current (I
DQ
), whose value is application dependent. The
MRF1507 was characterized at I
DQ
= 150 mA, which is the
suggested value of bias current for typical applications. 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 MRF1507 may be controlled to some
degree with a low power dc control signal applied to the gate,
thus facilitating applications such as manual gain control,
ALC/AGC and modulation systems. This characteristic is very
dependent on frequency and load line.
MOUNTING
The specified maximum thermal resistance of 2
°
C/W
assumes a majority of the 0.065
″
x 0.180
″
source contact on
the back side of the package is in good contact with an
appropriate heat sink. As with all RF power devices, the goal
of the thermal design should be to minimize the temperature
at the back side of the package.
AMPLIFIER DESIGN
Impedance matching networks similar to those used with
bipolar transistors are suitable for the MRF1507. For exam-
ples see Motorola Application Note AN721, “Impedance
Matching Networks Applied to RF Power Transistors.” Large–
signal impedances are provided, and will yield a good first
pass approximation.
Since RF power MOSFETs are triode devices, they are not
unilateral. This coupled with the very high gain of the
MRF1507 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 RF
test fixture implements a parallel resistor and capacitor in
series with the gate, and has a load line selected for a higher
efficiency, lower gain, and more stable operating region.
Two–port stability analysis with the MRF1507
S–parameters provides a useful tool for selection of loading or
feedback circuitry to assure stable operation. See Motorola
Application Note AN215A, “RF Small–Signal Design Using
Two–Port Parameters” for a discussion of two port network
theory and stability.