MRF5035 Datasheet, PDF (7/8 Pages) Motorola, Inc – N-CHANNEL BROADBAND RF POWER FET

GAIN CONTROL
Power output of the MRF5035 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. Figure 5 is an example
of output power variation with gate–source bias voltage with
Pin held constant. This characteristic is very dependent on
frequency and load line.
AMPLIFIER DESIGN
Impedance matching networks similar to those used with
bipolar transistors are suitable for the MRF5035. For exam-
ples see Motorola Application Note AN721, “Impedance
Matching Networks Applied to RF Power Transistors.” Both
small–signal S–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 power MOSFETs.
Since RF power MOSFETs are triode devices, they are not
unilateral. This coupled with the high gain of the MRF5035
yield a device quite capable of self oscillation. Stability may
be achieved by techniques such as drain loading, input shunt
resistive loading, or output to input feedback. Different
stabilizing techniques may be required depending on the
desired gain and bandwidth of the application. The RF test
fixture implements a resistor in shunt with the gate to im-
prove stability. Two port stability analysis with the MRF5035
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.
MOTOROLA RF DEVICE DATA
MRF5035
7