MRF174_17 Datasheet, PDF (10/12 Pages) M/A-COM Technology Solutions, Inc. – The RF MOSFET Line 125W, 200MHz

MRF174
The RF MOSFET Line
125W, 200MHz
DESIGN CONSIDERATIONS
The MRF174 is a RF MOSFET power N–channel en-
hancement mode field–effect transistor (FET) designed for
UHF power amplifier applications. M/A-COM RF MOSFETs
feature a vertical structure with a planar design, thus avoid-
ing 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 MRF174 is an enhancement mode FET and, there-
fore, 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 (IDQ) is
not critical for many applications. The MRF174 was charac-
terized at IDQ = 100 mA, which is the suggested minimum
Rev. V1
value of IDQ. For special applications such as linear amplifi-
cation, 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 MRF174 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 MRF174. See M/A-
COM Application Note AN721, Impedance Matching Net-
works 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.
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