MRF174 View Datasheet(PDF) - Motorola => Freescale
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MRF174 Datasheet PDF : 10 Pages
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DESIGN CONSIDERATIONS
The MRF174 is a RF power N–Channel enhancement
mode field–effect transistor (FET) designed especially for
UHF power amplifier and oscillator applications. Motorola RF
MOSFETs feature a vertical structure with a planar design,
thus avoiding the processing difficulties associated with V–
groove vertical power FETs.
Motorola 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 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
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
require 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 UHF transistors are suitable for MRF174. See
Motorola 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 approxima-
tion. This is an additional advantage of RF MOS power FETs.
MRF174
8
MOTOROLA RF DEVICE DATA
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