MRF173CQ Datasheet, PDF (5/6 Pages) Tyco Electronics – N-CHANNEL BROADBAND RF POWER MOSFET

DESIGN CONSIDERATIONS
The MRF173CQ is a RF MOSFET power N–channel en-
hancement mode field–effect transistor (FET) designed for
VHF power amplifier applications. M/A-COM's RF MOSFETs
feature a vertical structure with a planar design, thus avoid-
ing the processing difficulties associated with V–groove pow-
er 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 mis-
matched loads without suffering damage. Power output can
be varied over a wide range with a low power dc control sig-
nal, thus facilitating manual gain control, ALC and modula-
tion.
DC BIAS
The MRF173CQ is an enhancement mode FET and,
therefore, does not conduct when drain voltage is ap-
plied. Drain current flows when a positive voltage is ap-
plied to the gate. See Figure 9 for a typical plot of drain
current versus gate voltage. RF power FETs require for-
ward bias for optimum performance. The value of quies-
cent drain current (I_DQ) is not critical for many
applications. The MRF173CQ 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. There-
fore, the gate bias circuit may generally be just a simple re-
sistive divider network. Some special applications may
require a more elaborate bias system.
GAIN CONTROL
Power output of the MRF173CQ 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 MRF173CQ. See
M/A-COM Application Note AN721, Impedance Matching
Networks Applied to RF Power Transistors. The higher input
impedance of RF MOSFETs helps ease the task of broad-
band network design. Both small–signal scattering parame-
ters 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 approxima-
tion. This is an additional advantage of RF MOS power FETs.
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