MRF173CQ Datasheet, PDF (8/9 Pages) Tyco Electronics – N-CHANNEL BROADBAND RF POWER MOSFET

MRF173CQ
The RF MOSFET Line
80W, 175MHz, 28V
DESIGN CONSIDERATIONS
The MRF173CQ is a RF MOSFET power N–channel
enhancement mode field–effect transistor (FET) designed
for VHF power amplifier applications. M/A-COM RF MOS-
FETs feature a vertical structure with a planar design, thus
avoiding the processing difficulties associated with V–
groove power 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
signal, thus facilitating manual gain control, ALC and modu-
lation.
DC BIAS
The MRF173CQ 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 opti-
mum performance. The value of quiescent drain current
(IDQ) is not critical for many applications. The MRF173CQ
was characterized at IDQ = 50 mA, which is the suggested
M/A-COM Products
Released - Rev. 07.07
minimum value of IDQ. For special applications such as lin-
ear amplification, IDQ 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 re-
quire 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 approximation.
This is an additional advantage of RF MOS power FETs.
8
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